Apparatus for stimulating muscles of a subject and method of performing the same

Abstract

Apparatus 2 for the electrostimulation of the muscles of a subject comprises a system 12 configured to generate a stimulation signal comprising a plurality of electrical pulses, and means 14a,b for ap

Description

The present invention relates to apparatus for stimulating the muscles of a subject and a method of performing the same. More particularly, the present invention provides apparatus and methods for increasing blood flow in a subject by inducing contraction of at least one muscle of the subject. The electrical stimulation of the muscles of a subject is known in the art. Electrical muscle stimulation (EMS) is achieved by applying one or more electrodes to the subject, typically to the skin in the region of the muscles to be stimulated. Electrical pulses are applied to the subject through the electrodes, which in turn stimulate the muscles to elicit muscle contraction. It is known to apply electrical stimulation by means of a train of pulses having a range of different patterns, including a uniform pulse length and pulse separation. In addition, electrical stimulation of the muscles has been found to be effective in improving blood flow in subjects. Electrical stimulation is widely used in applications such as strength training, functional rehabilitation and pain management. In addition, various types of electrical stimulation devices have been found to be effective in improving blood flow in patients affected by a variety of clinical conditions such as chronic limb ischaemia and those affected by peripheral arterial and venous diseases. Evidence also suggests they can accelerate wound healing and can facilitate complex sensorimotor processes such as voluntary swallowing. Electrical stimulation to increase blood flow also has non-therapeutic benefits, for example promoting relaxation and improving athletic performance and recovery. Apparatus and devices for such electrical stimulation are known and commercially available. One such apparatus is the Revitive™ Medic Circulation Booster™ available from Actegy Limited. The apparatus comprises an electrically conductive pad, onto which the user places a foot. Pulses of electrical current are applied to the pad, which induce the muscles in the foot and leg of the subject to contract. The pulses of electrical current may be applied in a number of different patterns and at different current levels, according to the treatment required and the condition of the subject. Methods and apparatus for the electrical stimulation of the muscles of a subject are also known in the literature. US 4,528,984 discloses an autoprogrammable functional electrical stimulation apparatus and a method of operating the same. The apparatus provides the functional electrical stimulation (FES) of muscles or muscle groups of a subject by applying electrical pulses. The apparatus is manually operable to control the amplitude of the electrical pulses to provide a desired FES regime. This regime is stored in the apparatus and is reproduced by a processor, to apply the desired pulse regime to the subject. US 4,712,558 concerns an investigative method and apparatus for the electrical stimulation of the muscles of a subject. US 4,712,558 discusses recording the electromyographic responses of a muscle when stimulated, for example by neural or electrical stimulation. The electromyographic responses typically include so-called motor unit action potentials or MUAPs. US 4,712,558 reports on the investigation of MUAP discharge sequences and their analysis, in particular that the MUAP responses comprise two elements of information: first the information necessary to generate the stimulus needed for contraction of the muscle and resulting movement of joints to which the muscle is attached; and second the information for generating an intracellular environment within the muscle tissue for biosynthesis. This information is employed in US 4,712,558 to generate an electrical signal for the artificial stimulation of the muscles of a subject. In the method of US 4,712,558, electrical pulses are generated and applied to the subject, in particular by means of electrodes overlying the skin of the subject, the intervals between respective stimulating pulses being capable of being individually defined or varied. In a further development of US 4,712,558, US 5,350,415 describes a method and apparatus for the trophic stimulation of muscles. The stimulation is achieved by applying a pattern of electrical pulses to the muscles of the subject, as in US 4,712,558. US 5,350,415 describes the application of sampled predictive running average analysis of the MUAP signals to generate a pulse sequence for electrical stimulation of muscle tissue. In particular, US 5,350,415 discloses the generation of a pulse sequence having a continuous low-rate firing activity, described mathematically as a delta function continuously pulsing at a slow base rate with interpulse intervals in the range of from 120 to 200 milliseconds. This pattern has superimposed thereon a rectangular-like modulation with an exponential rise and fall, which quickly (that is within 20 to 40 milliseconds) changes the low base rate into a burst of high rate short pulses having a short interpulse interval (IPI) of from 20 to 40 milliseconds. The decrease in the IPI is maintained for about 40 milliseconds, that is for about three to four pulses, after which it is reduced exponentially to an IPI of from 60 to 80 milliseconds. Finally, after about 500 milliseconds, the IPI is further decreased to a base slow rate of 120 to 200 milliseconds. The entire cycle has a recurrence period of about 1 second. The train of electrical pulses having the aforementioned pattern is applied to the subject to stimulate the muscle activity. WO 2008 / 086411 discloses an electro-stimulation device to provoke the venous pumping of blood from the legs of a subject. The venous blood pumping is induced by applying to the subject short bursts of electrical impulses of high energy at a predetermined durational range, in which each burst has an associated modulated intensity and / or time duration. Particular pulse regimes are disclosed in which pulses are spaced by intervals of 4 milliseconds. Superimposed on these pulses is a modulation pattern, in which the width of the pulses is first increased, to provide increasing energy applied to the subject, and thereafter decreased to reduce the energy being applied in each pulse. The impulse width of the high energy pulses is indicated to range from 26 to 240 microseconds. A so-called ‘minimal energy’ phase then follows, during which low energy pulses are applied, the low energy pulses having insufficient energy to stimulate contractions of the muscles. The pulse width of the low energy pulses is from 400 to 900 milliseconds. The low energy pulses are indicated to provide an electroanesthetization of the subject. More recently, GB2500641B and GB2500642B disclose methods of electrostimulation of the muscles of a subject. To induce venous blood flow, in particular the venous return, in the legs of the subject, the electrical pulses are applied to one or both feet of the subject, preferably to the soles of one or both feet. As described above, electrostimulation is known to be applied to subjects to induce muscle contractions, in particular to promote the flow of blood through the vascular system of the subject. However, while much work has been devoted to developing systems and methods for the electrostimulation of a subject’s muscles, there is still a need for an improved system and method, in particular for promoting significant increases in the flow of blood through the vascular system of a subject, while avoiding rapid muscle fatigue. Accordingly, in a first aspect, the present invention provides an apparatus for the electrostimulation of the muscles of a subject, the apparatus comprising: a system configured to generate a stimulation signal comprising a plurality of electrical pulses; and means for applying the stimulation signal to the subject to induce contraction of at least one muscle of the subject; wherein the stimulation signal is configured to induce in the subject an increase in peak blood flow of at least 50% above baseline when applied to the subject. The present invention provides apparatus and methods of delivering EMS to a subject. A stimulation signal is applied to the subject comprising a plurality of electrical pulses to induce one or more contractions in one or more muscles of the subject. Contraction of the one or more muscles induces in the subject an increase in blood flow in vascular vessels, including the large blood vessels (arteries and veins) and the small blood vessels (tissue capillaries) of the subject, in the region of the muscle contractions. In particular, contraction of the one or more muscles constricts the vascular vessels, increasing pressure within the vessels to induce an increase in the movement or flow of blood contained within the vessels. By inducing an increase in blood flow, symptoms associated with poor blood flow, including but not limited to pain, swelling, and muscle weakness, may be reduced or relieved. Inducing an increase in blood flow may also promote relaxation of the subject, and improve athletic performance and recovery. In addition to promoting an increase in blood flow in a subject, the present invention may produce an analgesic effect, for example by transcutaneous electrical nerve stimulation. The apparatus comprises a system for generating a stimulation signal comprising a plurality of electrical pulses. Examples of suitable systems are known in the art. Such systems may comprise a power supply unit for providing a supply of electricity. The power supply unit may be any suitable supply unit, preferably one connectable to a domestic electrical supply. The system may further comprise a processor for operating control electronics, for example a control unit, for providing a voltage signal. This in turn is provided to a transformer to step the voltage up to a level suitable for administering to the subject. The processor further operates a pulse control circuit, for generating a signal comprising electrical pulses of the required shape and duration. The apparatus comprises means for applying the stimulation signal to the subject. The electrical pulses may be applied to the subject in any suitable manner. The pulses may be applied percutaneously, by way of one or more electrodes extending into the skin of the subject. More preferably, the apparatus comprises one or more contact members having a contact surface for location on the skin of the subject. In this way, the electrical pulses are applied transcutaneously, that is delivered to the surface of the skin of the subject in the region of the muscles to be stimulated. In a preferred embodiment, application of the signal induces an increase in blood flow in the feet and legs of the subject. In some embodiments, application of the signal can induce in a subject an increase in the venous blood flow within the feet and legs of the subject, to promote venous return of blood to the heart. In particularly preferred embodiments, in order to induce an increase in blood flow in the feet and legs of the subject, the electrical pulses are applied to one or both feet of the subject, more preferably to the sole or plantar aspect of one or both feet, by electrical contact members placed in contact with the skin of the feet. The stimulation signal comprises a plurality of electrical pulses. A ‘pulse’ refers to an electrical signal with a current and / or voltage varying from a first level, at which contraction of the one or more muscles of the subject is not induced, to a second level at which muscle contraction occurs, before returning to the first level. The term ‘level’ refers to any parameter of the signal that can be varied between a first value and a second value, such as the amplitude of the signal. The first level is any suitable level that does not induce muscle contraction, for example zero. The second level is any level that induces muscle contraction, irrespective of the polarity of the signal, for example a positive or negative voltage signal, as described in more detail below. The electrical pulses preferably have a pulse duration, or pulse width, of from 100 ps to 2000 ps. The pulse duration refers to the period of time during which the signal remains at a second level sufficient to induce muscle contraction in the subject. Preferably, the pulses have a pulse duration of at least 200 ps, preferably at least 300 ps, more preferably at least 400 ps, for example at least 500 ps. Preferably, the pulses have a pulse duration of up to 1750 ps, more preferably up to 1500 ps, still more preferably up to 1250 ps, most preferably up to 1000 ps. For example, the electrical pulses may have a pulse width of from 200 ps to 1750 ps, more preferably from 300 ps to 1500 ps, still more preferably from 400 ps to 1250 ps, for example from 500 ps to 1000 ps. The electrical pulses are separated by a first inter pulse interval, during which the signal remains at a first level insufficient to induce muscle contraction in the subject. The first inter pulse interval may have a duration of from 50 ps to 500 ps, preferably from 60 ps to 400 ps, more preferably from 70 ps to 300 ps, for example from 80 ps to 200 ps. A first pulse interval of at least 90 ps is particularly preferred, for example 100 ps. Accordingly, each pulse may have a phase duration (that is the time elapsed between the start of successive pulses, including the pulse width and first inter pulse interval) of from 150 ps to 2500 ps, preferably from 250 ps to 2000 ps, more preferably from 350 ps to 1500 ps, still more preferably from 450 ps to 1000 ps. The electrical pulses are preferably applied in groups, each group comprising two or more pulses separated by a first inter-pulse interval, the groups being separated by a second inter-pulse interval, during which the signal remains at a first level insufficient to induce muscle contraction in the subject. The second inter-pulse interval is preferably longer than the first interpulse interval. The second inter-pulse interval may have a duration of from 10 ms to 50 ms, more preferably from 15 ms to 45 ms, still more preferably from 20 ms to 40 ms, most preferably from 25 ms to 35 ms, for example 30 ms. The second inter-pulse interval represents a period of rest between successive groups of pulses. This rest period is preferably a sufficient length of time to allow the vasculature in the region of the stimulated muscle or muscles to refill with blood, in advance of the next muscle contraction. By allowing a sufficient rest period, the vasculature substantially or wholly refills with oxygenated blood. Consequently, muscle fatigue is delayed and, as a result, the efficiency of the treatment in expelling and pumping blood is increased. The groups of pulses may contain the same number of pulses or a different number of pulses. Preferably, the pulses are applied in a plurality of groups, each group having the same number of pulses therein. Each group of pulses comprises at least two pulses, more preferably at least three pulses. Higher numbers of pulses may be applied in each group, for example four pulses, five pulses, up to ten pulses. However, a high number of pulses in a given group can reduce the efficiency of the electrostimulation in inducing an increase in blood flow, as prolonged stimulation with insufficient rest periods to allow the vasculature to refill with oxygenated blood, leads to muscle fatigue. Accordingly, it is preferred that the number of pulses in each group is less than ten, more preferably less than eight, still more preferably less than six. A group of less than five pulses is preferred, with pulses arranged in groups of three or triplets being particularly preferred. Preferably, the electrical pulses are arranged in groups having a group frequency, that is the number of groups of pulses delivered per unit time, of 10 Hz to 100 Hz, more preferably from 20 Hz to 90 Hz, for example from 30 Hz to 80 Hz. The stimulation signal has parameters that are tolerable to the subject. In particular, suitable stimulation signals are those that do not cause pain or excessive, intolerable levels of discomfort to the subject which may cause harm to the subject or prevent the application of the signal to the subject, for example if the level of discomfort is so great as to cause the subject to recoil. The maximum current level of the electrical pulses may be varied and selected to provide the desired level of muscle contraction, without being painful or unacceptably uncomfortable to the subject. Factors affecting the current level to be applied include the condition of the subject and their muscles and the resistance of the skin or tissue of the subject to which the electrical pulses are being applied. The current level will also determine the stimulation intensity of the treatment. Preferably, peak current levels are in the range of from 1 mA to 50 mA, more preferably from 5 mA to 40 mA, still more preferably from 10 mA to 30 mA. However, any suitable peak current level may be employed, depending on the desired level of muscle stimulation and the electrical properties of the skin or tissue of the subject. Similarly, the voltage level to be applied will depend upon the prevailing conditions, such as the condition of the subject, the resistance of the skin or tissue of the subject, which is in turn dependant on the separation of the electrodes, and the stimulation intensity to be achieved. For a healthy subject, a typical skin resistance is from 100 kOhm / cm to 500 kOhm / cm for dry skin, and may be lower if the skin is wet, for example from 1 kOhm / cm to 10 kOhm / cm. The resistance of injured skin may be lower still, for example from 100 Ohm / cm to 1 kOhm / cm. Accordingly, a typical peak voltage for the pulses is from 20 to 160 V, more preferably from 30 to 150 V, still more preferably from 40 to 140 V, most preferably from 50 to 130 V. A peak voltage of up to 250 V may be applied, depending upon the intensity of the stimulation to be delivered to the muscles of the subject, with peak voltages of up to 150 V, for example up to 100 V, being preferred for many subjects. In some embodiments, peak voltages of up to 80 V, for example up to 70 V are most preferred. However, any suitable peak voltage level may be employed, depending on the desired level of muscle stimulation and the electrical properties of the skin or tissue of the subject. The pulses may be applied with voltage of a single polarity. Alternatively, pulses may be applied with an alternating polarity. The polarity of the voltage may alternate between consecutive single pulses, for example consecutive pulses in each group of pulses, or between consecutive groups of pulses. In further embodiments the polarity of the stimulation signal may be periodically inverted, so that consecutive groups of pulses are applied at a first polarity for a first duration, and at a second polarity for a second duration. Accordingly, in preferred embodiments, the electrical pulses are arranged as a series of repeating cycles, each cycle comprising at least two pulses having different pulse parameters selected from peak voltage, peak current, pulse width, and / or polarity. For example, a cycle of pulses may comprise two groups of pulses, such as triplets, the first group of pulses having a positive voltage and the second group of pulses having a negative voltage. Each cycle may comprise more than two groups of pulses, for example at least three groups of pulses, at least four groups of pulses, or at least five groups of pulses. Each group of pulses within a cycle of pulses may vary in one or more of its parameters, for example pulse number, peak voltage, peak current, pulse duration, first inter-pulse interval duration, and second inter pulse interval duration, as described hereinbefore. Each pulse or cycle of pulses is preferably repeated to deliver a series or train of pulses to the subject for any required duration. The train of stimulation pulses may be considered to have a duty cycle, that is the fraction of the total elapsed time of the train during which the pulses have an intensity sufficient to stimulate muscle contraction. The duty cycle may range from 5% to 75%, more preferably from 10% to 60%, still more preferably from 20% to 50%. The electrical pulses, or trains of pulses, are preferably applied to a subject in bursts with a rest period between successive bursts. Each burst of pulses preferably comprises one or more cycles of electrostimulation pulses, repeated for a desired burst duration. Each burst preferably has a duration or ‘burst on’ duration of from 1 s to 10 s, preferably from 2 s to 5 s. Each burst is separated by a rest period or ‘burst off’ period. The burst off period preferably has a duration of from 0.5 s to 3 s, preferably from 1 s to 2 s. However, in some embodiments, the rest period between successive bursts of pulses may be up to 4 s. The stimulation signal may be applied to a subject for any duration, as required, for example from 1 minute to 60 minutes, preferably from 5 minutes to 55 minutes, more preferably from 10 minutes to 50 minutes, still more preferably from 15 minutes to 45 minutes, for example 30 minutes. The desired stimulation duration may be selected based on the condition of the subject. For example, stimulation may be applied for as long as can be tolerated before muscle fatigue sets in, to provide the maximum benefit associated with increased circulation. For example, stimulation may be applied for up to 5 hours, preferably up to 4 hours, more preferably up to 3 hours, still more preferably up to 2 hours, most preferably up to 1 hour, for example for up to 30 minutes. Stimulation may be applied for a period of from 5 minutes, preferably from 10 minutes, more preferably from 15 minutes, still more preferably from 20 minutes, more preferably still from 25 minutes. A stimulation period of from 5 minutes to 5 hours is very suitable, preferably from 10 minutes to 4 hours, still more preferably from 15 minutes to 3 hours, more preferably still from 20 minutes to 2 hours, especially from 25 minutes to 1 hour. In many embodiments, a stimulation period of 1 hour or less is preferred, for example up to 45 minutes, preferably up to 40 minutes, more preferably up to 35 minutes. In many embodiments, a stimulation period of from 20 to 30 minutes is preferred, especially about 30 minutes. The electrostimulation may be applied by way of pulses of any suitable shape. For example, the variation of the voltage with time of each pulse may be in the form of a square wave. More preferably, the amplitude of the pulses is modulated by an amplitude modulation signal. Any suitable amplitude modulation signal may be applied to the pulses to modify the shape of the pulse train. Preferably, the modulation is achieved by applying one or more of a square wave signal, a triangular or sawtooth signal, and a sine wave signal to the train of electrical pulses. In further preferred embodiments, the modulation signal may comprise a gradual ramp or increase in voltage, followed by a rapid decay or decrease in voltage. Conversely, the modulation signal may comprise a rapid ramp or increase in voltage, followed by a gradual decay or decrease in voltage. The frequency of the amplitude modulation signal may vary, but is preferably from 0 to 10 Hz, more preferably from 1 to 8 Hz, still more preferably from 2 to 6 Hz, more preferably still from 3 to 4 Hz. In some embodiments, amplitude modulation frequencies of from 0 Hz are employed, more preferably from 0.5 Hz, still more preferably from 1 Hz, for example from 1.5 Hz. In some embodiments, amplitude modulation frequencies of up to 10 Hz are employed, more preferably up to 8 Hz, still more preferably up to 6 Hz, most preferably up to 4 Hz, for example up to 2 Hz. In some embodiments, the modulation signal may have a frequency of from 0.5 to 4 Hz, preferably from 1 to 3 Hz. The depth of the modulation signal, that is the difference between the peak voltage of the modulation waveform and the lowest voltage point, as a percentage of peak pulse voltage, is preferably from 0% to 90%, more preferably from 10% to 80%, still more preferably from 20% to 70%, more preferably still, from 30% to 60%, for example from 40% to 50%. In some embodiments, modulation depths of from 0% are employed, preferably from 5%, more preferably from 10%, still more preferably from 15%, for example from 20%. In some embodiments, modulation depths of up to 100% are employed, preferably up to 90%, more preferably up to 80%, still more preferably up to 70%, for example up to 60%. In some embodiments, the depth of the modulation signal may be from 10% to 70%, preferably from 20% to 50%. Where the amplitude modulation signal has a frequency and / or depth of zero, the pulse train may be considered to be unmodulated. Different pulse trains having a variety of pulse parameters may be combined with different modulation signals to create stimulation signals comprising a wide variety of EMS waveforms. Examples of waveforms for the electrostimulation of the muscles of a subject, and configured to induce in the subject an increase in peak blood flow, are characterised and described in more detail hereinbelow. A range of EMS waveforms are known to induce in a subject an increase in peak blood flow above baseline, for example up to 40% above baseline, in some cases up to 30% above baseline, for example up to 20% above baseline. However, it has now surprisingly been found that stimulation signals comprising certain EMS waveforms are particularly effective in inducing an increase in peak blood flow in a subject, without causing pain or excessive discomfort to the subject. Accordingly, the stimulation signal employed in the present invention is configured to induce in the subject an increase in peak blood flow of at least 50% above baseline when applied to a subject, for example from 50% to 100% above baseline. In preferred embodiments, an increase in peak blood flow of from 55% to 100% above baseline is achieved, preferably from 60% to 100%, more preferably from 65% to 100%, still more preferably from 70% to 100%, more preferably still from 75% to 100%, especially from 80% to 100%, more especially from 85% to 100%, still more especially from 90% to 100%. Increases in peak blood flow of up to 75% above baseline can be achieved, preferably up to 80%, more preferably up to 85%, still more preferably up to 90%, more preferably still up to 95%, especially up to 100%. Increases of greater than 100% above baseline can be achieved, for example up to 110%, up to 120%, up to 150%, or up to 200%. In particularly preferred embodiments, the stimulation signal is configured to induce in the subject an increase in peak blood flow in excess of 100% above baseline when applied to the subject. Baseline peak blood flow is determined while a subject is at rest. References herein to ‘baseline peak blood flow’ are to the peak blood flow measured while the subject is at rest, in particular before stimulation according to the present invention is applied. Peak blood flow may be measured in any suitable blood vessel of the subject, preferably a blood vessel in the region of interest, that is the region of the subject where blood flow is to be stimulated, for example a limb of the subject, such as a leg, for example in the lower leg. Techniques for measuring and obtaining an indication of blood flow are known in the art and include ultrasound scanning of one or more blood vessels in the region of interest. In the present invention, it is not essential that an absolute measurement of the blood flow in the region of interest is obtained. Rather, it is sufficient that a technique is used to provide an indication of the peak blood flow before and after the stimulation has been applied, allowing the relative change in blood flow to be determined. An increase in peak blood flow refers to a rise in one or more measurable aspects of peak blood flow, for example the volume flow (volume per unit time), velocity (average velocity of blood flow), and / or blood vessel diameter or volumetric capacity, compared to baseline. Increases in each of these aspects of peak blood flow may be measured directly, or may be inferred based on mathematical evaluation of other measurable features. For example, information on the peak blood flow may be inferred from Doppler ultrasound images of a blood vessel of the subject. In one method, as described in more detail hereinbelow, peak blood flow volume (arbitrary units) and peak blood flow intensity (arbitrary units) may be inferred by analysing the respective number and colour intensity of pixels within the imaged blood vessel. Peak blood flow volume can be linked to the peak volumetric capacity of the blood vessel. Peak blood flow intensity can be linked to the peak volume flow. Increases in other parameters, such as peak blood flow velocity may also be inferred based on mathematical evaluation of blood vessel images. An increase in peak blood flow may occur anywhere in the subject upon delivery of a stimulation signal. For example, application of the signal may induce in the subject an increase in peak blood flow throughout the entire vascular system of the subject. However, in preferred embodiments, an increase in peak blood flow is induced within a particular region of interest, by the delivery of a targeted stimulation signal. For example, in preferred embodiments, increases in peak blood flow may be induced only in the feet and / or legs of a subject. As described hereinbefore, it has surprisingly been found that certain EMS waveforms of a first type, referred to hereinafter as ‘Type T waveforms, induce in a subject an exceptionally large increase in peak blood flow in excess of 100% above baseline, while other EMS waveforms of a second type, referred to hereinafter as ‘Type 2’ waveforms, induce in a subject a lower, but albeit greatly improved, increase in peak blood flow of from 50% to 100% above baseline. Accordingly, EMS waveforms configured to induce in the subject an increase in peak blood flow of at least 50% above baseline can be grouped into two categories, denoted herein as ‘Type T and ‘Type 2’ waveforms. Type 1 waveforms induce increases in peak blood flow in excess of 100% above baseline, for example at least 110% above baseline, preferably at least 120% above baseline, for example at least 125%, such as at least 130%, for example at least 140%, such as at least 150%. In preferred embodiments, an increase in peak blood flow of greater than 100% to 200% above baseline is achieved, preferably from 110% to 200%, more preferably from 120% to 200%, still more preferably from 130% to 200%, more preferably still from 140% to 200%, especially from 150% to 200%, more especially from 160% to 200%, still more especially from 170% to 200%, for example from 180% to 200%, such as 190% to 200%. Increases in peak blood flow of up to 200% or higher may be achieved. Type 2 waveforms induce increases in peak blood flow of at least 50% but not more than 100% above baseline, for example at least 60% above baseline, more preferably at least 70% above baseline, for example up to 95%, such as up to 90%. In preferred embodiments, an increase in peak blood flow of from 55% to 100% above baseline is achieved, preferably from 60% to 100%, more preferably from 65% to 100%, still more preferably from 70% to 100%, more preferably still from 75% to 100%, especially from 80% to 100%, more especially from 85% to 100%, still more especially from 90% to 100%. Increases in peak blood flow of up to 75% above baseline can be achieved, preferably up to 80%, more preferably up to 85%, still more preferably up to 90%, more preferably still up to 95%, especially up to 100%. The stimulation signal preferably comprises two or more EMS waveforms applied in sequence to provide an EMS program. Preferably the EMS program comprises at least three EMS waveforms, more preferably at least four EMS waveforms, most preferably at least five EMS waveforms. It has been found that the application of an alternating sequence of Type 1 and Type 2 waveforms reduces muscle fatigue whilst maintaining significantly increased blood flow in a subject. It is thought that the application of a Type 2 waveform, following the application of a Type 1 waveform, provides a period of lower stimulation intensity, whilst maintaining an increase in peak blood flow of at least 50% above baseline. This affords the muscles of the subject in the stimulation region a chance to recover from the more intense stimulation provided by Type 1 waveforms. These periods of recovery, whilst nevertheless maintaining a significant increase in peak blood flow, increase the tolerance of the subject to the EMS program, and allow for the application of a stimulation signal for longer periods before the muscles of the subject become fatigued. Accordingly, the stimulation signal preferably comprises a waveform of a first type configured to induce in the subject an increase in peak blood flow in excess of 100% above baseline when applied to the subject and a waveform of a second type configured to induce in the subject an increase in peak blood flow of from 50% to 100% above baseline when applied to the subject. In particularly preferred embodiments, the stimulation signal comprises an alternating sequence of one or more waveforms of the first type and one or more waveforms of the second type. For example, the stimulation signal may comprise an alternating sequence of one or more Type 1 waveforms followed by one or more Type 2 waveforms. Alternatively, the stimulation signal may comprise an alternating sequence of one or more Type 2 waveforms followed by one or more Type 1 waveforms. In particularly preferred embodiments, the stimulation signal comprises a sequence of waveforms alternating between a single Type 1 waveform and a single Type 2 waveform. Examples of Type 1 and Type 2 waveforms are characterised and described in more detail hereinbelow, with reference to the Figures. For example, the stimulation signal preferably comprises at least one of waveforms W4, W5, W6, W7, W8, and W11, as described hereinbelow and illustrated in Figures 9, 10, 11, 12, 13, and 16, respectively. More preferably, the stimulation signal comprises waveforms W4, W5, W6, W7, \N8, and W11, applied in the following sequence: W5, W4, W8, W6, W7, W11, W5, W4, W8, W6. As discussed hereinbefore, the waveforms W4, W5, W6, W7, W8, and W11 may have any suitable amplitude or peak voltage, depending on the condition, and hence resistance, of the subject’s skin, so as to induce contraction of at least one muscle of the subject. An experimental methodology is also described hereinbelow which may be employed to test and categorise waveforms having a variety of parameters, to identify further suitable Type 1 and Type 2 waveforms. In a second aspect, the present invention provides a method of electrostimulation of the muscles of a subject, the method comprising applying to the subject a stimulation signal comprising a plurality of electrical pulses; wherein the application of the signal induces in the subject an increase in peak blood flow of at least 50% above baseline. The stimulation signal employed in the second aspect of the invention may comprise any of the features of the stimulation signal employed in the first aspect, as hereinbefore described and / or any of the waveforms according the present invention, as described and characterised hereinbelow. In a third aspect, the present invention provides a method of operating an electrical stimulation device, the method comprising generating a stimulation signal comprising a plurality of electrical pulses; wherein the stimulation signal is configured to induce an increase in peak blood flow of at least 50% above baseline when applied to a subject. The stimulation signal generated in the third aspect of the invention may comprise any of the features of the stimulation signal employed in the first aspect, as hereinbefore described and / or any of the waveforms according the present invention, as described and characterised hereinbelow. In a fourth aspect, the present invention provides an apparatus for the electrostimulation of the muscles of a subject, the apparatus comprising: a system configured to generate a stimulation signal comprising a plurality of electrical pulses; and means for applying the stimulation signal to the subject to induce contraction of at least one muscle of the subject; wherein the stimulation signal comprises at least one of waveforms W1, W2, W3, W4, W5, W6, W7, W8, W9, W10, W11, W12, W13, W14, and W15, as described hereinbelow and illustrated in Figures 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20, respectively. The waveforms may have any suitable amplitude. For example, peak voltage for the waveforms is preferably from 20 to 160 V, more preferably from 30 to 150 V, still more preferably from 40 to 140 V, most preferably from 50 to 130 V. A peak voltage of up to 250 V may be applied, depending upon the intensity of the stimulation to be delivered to the muscles of the subject, with peak voltages of up to 150 V, for example up to 100 V, being preferred for many subjects. In some embodiments, peak voltages of up to 80 V, for example up to 70 V are most preferred. However, any suitable peak voltage level may be employed, depending on the desired level of muscle stimulation and the electrical properties of the skin or tissue of the subject. The waveforms may be applied in any suitable sequence. For example, the stimulation signal may comprise the following sequence of waveforms: W1, W2, W3, W4, W5, W6, W7, W8, W9, W10, W11, W12, W13, W14, W15. Any one or more of the waveforms may be omitted, for example the following sequence, omitting waveform W7, may be applied: W1, W2, W3, W4, W5, W6, W8, W9, W10, W11, W12, W13, W14, W15; It has surprisingly been found that waveforms W4, W5, W6, W7, W8, and W11 induce in the subject an increase in peak blood flow of at least 50% above baseline when applied to the subject, and are therefore particularly effective in inducing an increase in peak blood flow in a subject. Accordingly, the stimulation signal preferably comprises at least one of waveforms W4, W5, W6, W7, W8, and W11, as described hereinbelow and illustrated in Figures 9, 10, 11, 12, 13, and 16, respectively. The stimulation signal preferably comprises one or more of waveforms W4, W5, W6, W7, W8, and W11, applied in one or more of the following sequences: W5, W4, W8, W6, W7, W11, W5, W4, W8, W6; W5, W4, W8, W6, W11, W5, W4, W8, W6; and W5, W4, W8, W6, W7, W11, W5, W4, W8, W6, W6, W8, W4, W5, W11, W7, W6, W8, W4, W5, W5, W4, W8, W6, W7, W11, W5, W4, W8, W6. Optionally, the stimulation signal comprises one or more of waveforms W4, W5, W6, W7, W8, and W11, in combination with one or more of waveforms W1, W2, W3, W9, W10, W12, W13, W14, and W15, applied in one or more of the following sequences: W3, W4, W5, W6, W7, W8, W9, W5, W11, W8; W3, W4, W5, W6, W7, W9, W8, W11, W5, W14; W2, W3, W4, W5, W6, W7, W9, W8, W11, W14; W1, W13, W2, W14, W3, W4, W11, W7, W6, W5, W11, W8, W9, W7, W4, W11, W8, W7, W5, W6, W11, W8, W7, W5, W4, W3, W14, W2, W13, W1; W3, W4, W5, W7, W8, W2, W5, W7, W8, W1, W8, W7, W5, W14, W8, W7, W5, W12, W5, W7, W8, W15, W5, W7, W8, W9, W5, W7, W8, W6; W1, W2, W3, W4, W5, W6, W7, W8, W9, W10; W1, W3, W4, W5, W6, W7, W8, W10, W2, W11, W4, W5, W8, W7, W1, W12, W3, W4, W5, W6, W7, W8, W10, W2, W11, W4, W5, W8, W7, W1; W2, W11, W4, W5, W8, W7, W1, W2, W11, W4, W5, W8, W7, W1, W2, W11, W4, W5, W8, W7, W1, W2, W11, W4, W5, W8, W7, W1, W9, W13; W1, W2, W8, W7, W11, W5, W6, W3, W8, W7, W11, W5, W6, W3, W4, W9, W8, W7, W11, W5, W6, W3, W8, W7, W11, W5, W6, W3, W2, W1; W1, W2, W8, W4, W11, W4, W6, W3, W8, W4, W11, W5, W4, W3, W7, W6, W8, W9, W11, W5, W9, W3, W8, W9, W11, W5, W9, W3, W2, W1; and W1, W2, W3, W4, W5, W6, W7, W8, W9, W10, W11, W12, W13, W14, W15, W15, W14, W13, W12, W11, W10, W9, W8, W7, W6, W5, W4, W3, W2, W1. In a fifth aspect, the present invention provides a method of operating an electrical stimulation device, the method comprising generating a stimulation signal comprising a plurality of electrical pulses; wherein the stimulation signal comprises at least one of waveforms W1, W2, W3, W4, W5, W6, W7, W8, W9, W10, W11, W12, W13, W14, and W15, as described hereinbelow and illustrated in Figures 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20, respectively. The stimulation signal generated in the fifth aspect of the invention may comprise any of the features of the stimulation signal employed in the first aspect, as hereinbefore described and / or any of the sequences of waveforms employed in the fourth aspect, as hereinbefore described. Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a system for generating electrical pulses for use in methods and apparatus according to the present invention; Figure 2 is a graphical representation of a first cycle of electrostimulation pulses; Figure 3 is a graphical representation of a second cycle of electrostimulation pulses; Figure 4 is a graphical representation of an EMS waveform comprising a train of the electrostimulation pulses, for example the pulses of Figures 2 or 3; Figure 5 is a graphical representation of an EMS waveform comprising a train of electrostimulation pulses, for example the pulses of Figures 2 or 3, with sine wave amplitude modulation; Figure 6 is a graphical representation of an EMS waveform, W1; Figure 7 is a graphical representation of an EMS waveform, W2; Figure 8 is a graphical representation of an EMS waveform, W3; Figure 9 is a graphical representation of an EMS waveform, W4; Figure 10 is a graphical representation of an EMS waveform, W5; Figure 11 is a graphical representation of an EMS waveform, W6; Figure 12 is a graphical representation of an EMS waveform, W7; Figure 13 is a graphical representation of an EMS waveform, W8; Figure 14 is a graphical representation of an EMS waveform, W9; Figure 15 is a graphical representation of an EMS waveform, W10; Figure 16 is a graphical representation of an EMS waveform, W11; Figure 17 is a graphical representation of an EMS waveform, W12; Figure 18 is a graphical representation of an EMS waveform, W13; Figure 19 is a graphical representation of an EMS waveform, W14; Figure 20 is a graphical representation of an EMS waveform, W15; Figure 21 is a sample of ultrasound image data obtained from a subject before and after the application of the EMS waveform of Figure 10; Figure 22 is a plot of the percentage change in mean waveform peak blood flow volume from mean baseline peak blood flow volume for each of the EMS waveforms of Figures 6 to 20; and Figure 23 is a plot of the percentage change in mean waveform peak blood flow intensity from mean baseline peak blood flow intensity for each of the EMS waveforms of Figures 6 to 20. Referring to Figure 1, there is shown a schematic diagram of one embodiment of a system for generating electrical pulses for the stimulation of the muscles of a subject. The system, generally indicated as 2, is suitable for use in the methods and apparatus of the present invention. The system 2 is arranged for providing electrical stimulation to the plantar surfaces of the feet of a subject. However, the general aspects of the system may be used to provide electrical stimulation to muscles in other parts of the subject in like manner as described below. The system 2 comprises a power supply unit 4, for example connectable to an electrical power supply, such as a domestic electrical supply. The power supply unit 4 outputs an electric current, the voltage of which is adjusted, as required, by a control unit 6, under the action of a processor 8. The adjusted voltage is stepped up by a transformer 10, before being fed to a pulse control unit 12, also operated by the processor 8. The pulse control unit generates a pulsed electrical signal having the desired pulse shape and duration, under the action of the processor. The output of the pulse control unit 12 is connected to first and second contact members 14a, 14b of the system. A control panel 14 provides a user interface for controlling the processor 8. In use, by placing their feet on respective contact surfaces of the contact members 14a, 14b, the user completes an electrical circuit, allowing the pulsed electrical signal to travel from one foot to the other and stimulate muscle contraction in the feet and legs of the user. In this manner, the system 2 may be used to deliver Electrical Muscle Stimulation (EMS) to a subject. Referring to Figure 2, there is shown a graphical representation of a signal comprising a first cycle of electrostimulation pulses for applying to a subject to stimulate contraction of a target group of muscles, such as those of the lower leg. The pulses are represented as plots of voltage against time. The cycle of pulses represented in Figure 2 comprises a first group of pulses, indicated generally as 102, comprising three pulses 102a, 102b and 102c, and a second group of pulses, indicated generally as 104, comprising three pulses 104a, 104b and 104c. Accordingly, the first and second groups of pulses 102, 104 each comprise a triplet of pulses. As can be seen, the pulses in the first group 102 are in the opposite polarity to those of the second group 104. The pulses in the first and second groups 102, 104 are generally square in form, having a rapid increase in voltage from zero to a single nominal voltage, for example between 50 and 150 V, which is held for substantially the entire duration of the pulse, after which the voltage is rapidly decreased to zero. Peak voltages of up to 250 V may be applied, depending upon the intensity of the stimulation to be applied to the subject, more typically in many cases up to 150 V, and preferably up to 100V. As indicated in Figure 2, each pulse has a width, that is a duration, of 1000 ps. The pulses in each group are separated by a first inter-pulse interval of 100 ps. Accordingly, each pulse has a period or phase duration of 1100 ps. Each triplet 102, 104 has a duration of approximately 3.3 ms. The groups or triplets of pulses 102, 104 are arranged with alternating polarity, in particular each triplet of pulses is followed by a second inter-pulse interval or inter-triplet interval of 25 ms, as indicated in Figure 2, after which the triplet of pulses is repeated with the opposite polarity. Accordingly, the cycle of pulses has a triplet frequency of approximately 35 Hz. The cycle of pulses represented in Figure 2 are preferably operated at a duty cycle of from 20% to 50%. The pulse cycle shown in Figure 2 is repeatedly applied to the subject for any required duration as a train of pulses. The parameters of the pulse train can be varied to deliver alternative EMS waveforms. For example, in Figure 3 there is shown a graphical representation of a signal comprising a second, alternative, cycle of electrostimulation pulses for applying to a subject to stimulate contraction of a target group of muscles, such as those of the lower leg. The pulses are represented as plots of voltage against time. The cycle of pulses represented in Figure 3 comprises a first group of pulses, indicated generally as 202, comprising three pulses 202a, 202b and 202c, and a second group of pulses, indicated generally as 204, comprising three pulses 204a, 204b and 204c. Accordingly, the first and second groups of pulses 202, 204 each comprise a triplet of pulses. As can be seen, the pulses in the first group 202 are in the opposite polarity to those of the second group 204. The pulses in the first and second groups 202, 204 are generally square in form, having a rapid increase in voltage from zero to a single nominal voltage, for example between 50 and 150 V, which is held for substantially the entire duration of the pulse, after which the voltage is rapidly decreased to zero. Peak voltages of up to 250 V may be applied, depending upon the intensity of the stimulation to be applied to the subject, more typically in many cases up to 150 V, and preferably up to 100V. As indicated in Figure 3, each pulse has a width, that is a duration, of 422 ps. The pulses in each group are separated by a first inter-pulse interval of 259 ps. Accordingly, each pulse has a period or phase duration of 681 ps. Each triplet 202, 204 has a duration of approximately 1.79 ms. The groups or triplets of pulses 202, 204 are arranged with alternating polarity, in particular each triplet of pulses is followed by a second inter-pulse interval or inter-triplet interval of 30 ms, as indicated in Figure 3, after which the triplet of pulses is repeated with the opposite polarity. Accordingly, the cycle of pulses has a triplet frequency of approximately 31 Hz. The cycle of pulses represented in Figure 3 are preferably operated at a duty cycle of from 20% to 50%. The pulse cycle shown in Figure 3 is repeatedly applied to the subject for any required duration as a train of pulses. Each pulse train is preferably applied to a subject in bursts, as described in more detail below. Turning to Figure 4, there is shown a graphical representation of a train of electrostimulation pulses for applying to a subject to stimulate contraction of a target group of muscles, such as those of the lower leg. The pulse train is represented as a plot of voltage against time. The train of pulses represented in Figure 4 comprises a first burst of pulses, indicated generally as 302 and a second burst of pulses, indicated generally as 304. The pulses in each of the first and second bursts 302, 304 are arranged into groups, each group having a plurality of individual pulses therein, for example three pulses to form a triplet of pulses. As can be seen, the triplets of pulses are arranged with alternating polarity. Each burst of pulses preferably comprises a cycle of electrostimulation pulses, for example as shown in Figures 2 and 3, which is repeated for a desired burst duration. As shown in Figure 4, the first burst of pulses 302 and the second burst of pulses 304 have a burst duration or ‘burst on’ duration of 2-5 s. Each burst is separated by a rest period or ‘burst off’ duration of approximately 1 s. In some embodiments, the rest period between successive bursts of pulses may be up to 4 s. In some embodiments, the bursts of pulses may be modulated to deliver alternative EMS waveforms. For example, turning to Figure 5, there is shown a graphical representation of a train of electrostimulation pulses for applying to a subject to stimulate contraction of a target group of muscles, such as those of the lower leg. The pulse train is represented as a plot of voltage against time. The train of pulses represented in Figure 5 comprises a first burst of pulses, indicated generally as 402, a second burst of pulses, indicated generally as 404, and a third burst of pulses, indicated generally as 406. The pulses in each of the first, second, and third bursts 402, 404, 406 are arranged into groups, each group having a plurality of individual pulses therein, for example three pulses to form a triplet of pulses. As can be seen, the triplets of pulses are arranged with alternating polarity. Each burst of pulses preferably comprises a cycle of electrostimulation pulses, for example as shown in Figures 2 and 3, which is repeated for a desired burst duration. A sine wave voltage signal, the frequency of which may vary, for example from 0.5 to 4.0 Hz, is applied to modulate the peak voltage of the individual pulses in each burst 402, 404, 406, as shown in Figure 5. The modulation depth in the amplitude of the peak voltage of each pulse as shown in Figure 5 is about 40%. The modulation depth may, for example, be from 15% to 50%. Alternative voltage signals may be applied to modulate the pulse train, for example a square wave or triangle wave modulation signal. As shown in Figure 5, each burst of pulses 402, 404, 406 may have a burst on duration of 2-9 s. Each burst is separated by a burst off period of approximately 1 s. In this embodiment, the burst off period between successive bursts of pulses may be up to 4 s. It has been found that some EMS waveforms are particularly effective in promoting the flow of blood through the vascular system of a subject. Details of experiments conducted to determine the best performing waveforms are described in more detail below. Experimental procedure Stimulation signals comprising various EMS waveforms were delivered to asymptomatic adults comprising 15 men and 15 women. EMS was delivered to the plantar aspect of the feet of each subject by a Revitive™ Medic Circulation Booster™ available from Actegy Limited, and comprising a system 2 as shown in Figure 1. The unit was programmed to deliver waveforms in patterns according to the present invention. The participants received 15 EMS waveforms, W1-W15, represented graphically as Figures 6-20, respectively, each differing in its critical pulse parameters such as phase duration, triplet frequency, burst on duration, burst off duration, amplitude modulation waveform, amplitude modulation frequency, and modulation depth. Accordingly, in Figure 6 there is shown a graphical representation of an EMS waveform, indicated generally as W1. In Figure 7 there is shown a graphical representation of an EMS waveform, indicated generally as W2. In Figure 8 there is shown a graphical representation of an EMS waveform, indicated generally as W3. In Figure 9 there is shown a graphical representation of an EMS waveform, indicated generally as W4. In Figure 10 there is shown a graphical representation of an EMS waveform, indicated generally as W5. In Figure 11 there is shown a graphical representation of an EMS waveform, indicated generally as W6. In Figure 12 there is shown a graphical representation of an EMS waveform, indicated generally as W7. In Figure 13 there is shown a graphical representation of an EMS waveform, indicated generally as W8. In Figure 14 there is shown a graphical representation of an EMS waveform, indicated generally as W9. In Figure 15 there is shown a graphical representation of an EMS waveform, indicated generally as W10. In Figure 16 there is shown a graphical representation of an EMS waveform, indicated generally as W11. In Figure 17 there is shown a graphical representation of an EMS waveform, indicated generally as W12. In Figure 18 there is shown a graphical representation of an EMS waveform, indicated generally as W13. Figure 19 there is shown a graphical representation of an EMS waveform, indicated generally as W14. Figure 20 there is shown a graphical representation of an EMS waveform, indicated generally as W15. As can be seen from Figures 6 to 20, waveforms W1, W2, W3, W5, W6, W8, W10, W12, W13, W14, and W15 comprise sequences of pulse triplets having alternating polarity. In other words, each triplet of pulses is 5 followed by a second inter-pulse interval, after which the triplet of pulses is repeated with the opposite polarity. In contrast, the polarity of waveforms W4, W7, W9, and W11, is periodically inverted, such that consecutive pulse triplets are applied at a first polarity for a first duration, and a second polarity for a second duration. 0 Details of waveforms W1-W15 are provided in Table 1 below. Table 1 Waveform Phase duration (MS) Triplet Frequency (Hz) Burst On duration (s) Burst Off duration (s) Peak voltage (V) (impedance = 500Q) Amplitude modulation waveform Amplitude modulation Frequency (Hz) Modulation depth (percent of peak voltage) W1 450 20 3.2 1.0 70 Slow ramp, fast decay 0.91 48.5 % W2 450 25 6.9 1.0 69 Sine wave 0.55 67.0 % W3 450 38.8 4.4 1.0 66 Sine wave 3.8 35.6 % W4 450 35.7 7.1 1.0 67 Sine wave 2.1 35.5 % W5 450 32.5 4.4 1.5 67 No modulation 0 0 W6 450 32.4 4.6 1.0 67 Sine wave 1.2 44.0 % W7 970 35.6 1.9 1.5 52 No modulation 0 0 W8 450 25.3 4.3 1.5 70 No modulation 0 0 W9 450 32.5 7.8 1.0 68 Sine Wave 1.9 33.8 % W10 450 43.7 8.3 1.0 63 Sine Wave 1.7 48.4 % W11 450 31.3 2.2 1.5 68 No modulation 0 0 W12 450 43.7 5.5 1.0 63 Slow ramp, fast decay 2.0 43.3 % W13 450 32.4 4.6 1.0 67 Sine Wave 1.1 44.9 % W14 450 37.6 4.2 1.0 66 Sine Wave 3.8 27.9 % W15 450 43.7 5.0 1.0 62 Sine Wave 1.0 63.4 % The participants were asked to avoid food, beverages and strenuous exercises before the start of the sessions to minimise physiological inconsistencies. They were asked to attend in clothing that is loose on the legs. The participants each attended two sessions, with a minimum gap of 48 hours between the two. In the first session screening for EMS safety was performed. Then the participants familiarised themselves with the 15 waveforms W1-W15 in a 30-minute EMS session. In the second session, initially the participants rested on a highchair for the first 20 minutes to acclimatise with the laboratory conditions. The Revitive™ Medic Circulation Booster™ was placed on a non-slippery surface below the highchair. Movement of the device was prevented to avoid excessive movement of the participant’s legs while EMS was applied. The participants placed their feet comfortably on the foot plate of the Revitive™ Medic Circulation Booster™, in contact with electrodes 14a, 14b. Then the baseline blood flow was recorded. Subsequently, blood flow data was collected during the application of each of the 15 waveforms W1-W15. Blood flow was recorded from each participant’s right leg using Doppler ultrasound that provided information on the volume and intensity of blood flow. An Esaote MyLab70 XVG’ (Esaote S.p.A, Genoa, Italy) ultrasound scanner was used alongside a linear array probe LA5231 that supports a frequency range of 4-13 MHz (Esaote S.p.A, Genoa, Italy) to collect the data. Blood flow was identified by manoeuvring the ultrasound probe over the ‘region of interest’ (ROI), from the posterior tibial vessels at the medial aspect of the right ankle posterior to the medial malleolus, until the most prominent pulsatile (arterial) flow was identified. Once the most prominent pulsatile (arterial) flow was identified, skin markings were used to establish the accuracy of probe placement and ensure repeatability. Recordings were made in the form of sequential power Doppler images of the blood flow. In Figure 21, there is shown sample ultrasound image data obtained from one of the study participants. The left hand frame, indicated as F1, shows the baseline ultrasound image data. The right hand frame, indicated as F2, shows the ultrasound image data obtained while receiving treatment with waveform W5. For testing, the device was set to the required waveform and the intensity increased gradually in increments of 10, reaching the optimum level in about 20-30 seconds. At the optimum intensity level the participant was required to feel a strong stimulation causing the muscles of the feet and leg to contract but not strong enough to cause pain or discomfort. Blood flow was recorded immediately, with the level of stimulation maintained. Once the necessary images were captured the treatment intensity was immediately reduced to zero. This brought the blood flow instantly back to baseline. A five-minute rest period was allowed before moving on to studying the next waveform. In this study separate sequential orders were followed for male and female participants, so that the application of waveforms was done in a chronological order while making sure each participant in either category started the study on dissimilar waveforms. For example, participant number one started the study with waveform W1 and ended with W15, while participant number fifteen started with W15 and ended with W14. This ensured that all waveforms were studied with the same priority. Ultrasound images were computationally analysed using MATLAB (MathWorks, Massachusetts) algorithms to process the colour image data into numerical data. For every image frame the MATLAB algorithms generated two types of data: the ‘pixel count’ (total number of coloured pixels) in the ROI and the ‘colour intensity’ (sum of colour intensity values of all the coloured pixels in the ROI) indicating the blood flow information. The pixel count represented the overall ‘blood flow volume’ and the colour intensity index represented the ‘blood flow intensity’. Results All participants completed the study. All procedures were well tolerated and there were no reports of any adverse events. Table 2, below, provides the mean peak blood flow data (volume and intensity) for each of waveforms W1-W15 across all 30 participants. Table 2 Waveform Mean baseline peak blood flow volume (arbitrary units) Mean waveform peak blood flow volume (arbitrary units) Percentage change to peak blood flow volume Mean baseline peak blood flow intensity (arbitrary units) Mean waveform peak blood flow intensity (arbitrary units) Percentage change to peak blood flow intensity W1 2.05 2.59 26.3 173.2 247.1 42.7 W2 2.78 35.6 265.4 53.2 W3 2.98 45.4 284.1 64.0 W4 3.48 69.8 335.9 93.9 W5 4.88 138.0 492.3 184.2 W6 3.45 68.3 326.0 88.2 W7 4.55 122.0 430.2 148.4 W8 4.83 135.6 500.5 189.0 W9 3.01 46.8 275.0 58.8 W10 2.45 19.5 226.5 30.8 W11 3.31 61.5 305.2 76.2 W12 2.32 13.2 216.7 25.1 W13 2.72 32.7 256.1 47.9 W14 2.88 40.5 268.5 55.0 W15 2.64 28.8 272.3 57.2 For ease of comparison, Figure 22 shows a plot of the percentage change in mean waveform peak blood flow volume from mean baseline peak 5 blood flow volume for each of waveforms W1 -W15 and Figure 23 shows a plot of the percentage change in mean waveform peak blood flow intensity from mean baseline peak blood flow intensity for each of waveforms W1-W15. As can be seen from Figures 22 and 23, a significant overall difference 10 in peak blood flow (both volume and intensity) during EMS was noted for each of waveforms W1-W15 when compared to the baseline flow. When analysed individually, waveforms W5, W7 and W8 produced the highest percentage increase in peak blood flow volume and intensity, with increases in excess of 100% from baseline in each case, more particularly at least 120% from baseline in each case. Waveforms W4, W6 and W11 produced the second highest increase in peak blood flow volume and intensity, with increases of at least 50% from baseline in each case, more particularly at least 60% from baseline in each case. The remaining waveforms produced only lower levels of increase in peak blood flow volume and intensity from baseline. Waveforms W4, W5, W6, W7, W8, and W11 can thus be applied singly or as a sequence forming an EMS program to induce a significant increase in blood flow in a subject. It has been found that EMS programs comprising one or more of waveforms W4, W5, W6, W7, W8, and W11 are particularly effective in promoting blood flow through the vascular system of a subject, and therefore produce a particularly advantageous circulation boosting effect. In preferred embodiments, an EMS program comprising a variety of different waveforms, for example a combination of two or more of waveforms W4, W5, W6, W7, W8, and W11 is applied to a subject. The application of a sequence of different waveforms is advantageous, as it reduces the tendency for the muscles of a subject to become acclimatised or adapted to a particular stimulation pattern, and therefore prevents any reduction in the efficacy of each waveform throughout the duration of the EMS program. The waveforms W4, W5, W6, W7, W8, and W11 can be divided into two groups, according to the relative increase in blood flow volume and intensity induced upon application of the waveform to a subject. In particular, those waveforms inducing increases in peak blood flow volume and intensity in excess of 100% are categorised as ‘Type T waveforms. Those waveforms inducing increases in peak blood flow volume and intensity of at least 50% but not more than 100% are categorised as ‘Type 2’ waveforms. Type 1 waveforms therefore include waveforms W5, W7 and W8, while Type 2 waveforms include waveforms W4, W6 and W11. It has been found that the application of an alternating sequence of Type 1 and Type 2 waveforms is particularly beneficial in promoting increased blood flow in a subject, while avoiding muscle fatigue. In this respect, the application of one or more Type 2 waveforms following the application of one or more Type 1 waveforms provides a period of ‘rest’ during which muscle stimulation is reduced, while nevertheless maintaining an increase in blood flow volume and intensity of at least 50% above baseline. The application of Type 1 and Type 2 waveforms in an alternating sequence therefore reduces muscle fatigue whilst continuing to induce significantly increased blood flow in a subject. EMS programs comprising an alternating sequence of Type 1 and Type 2 waveforms can therefore be applied to a subject for a greater duration than would be possible with the application of Type 1 waveforms only, whilst nevertheless maintaining significant increases in peak blood flow. This increases the length of time for which a subject can tolerate an EMS program inducing such significant increases in blood flow before muscle fatigue and associated discomfort sets in. Accordingly, in particularly preferred embodiments of apparatus or methods according to the present invention, a stimulation signal is provided comprising at least one of waveforms W4, W5, W6, W7, W8, and W11, preferably a sequence of two or more of waveforms W4, W5, W6, W7, W8, and W11. More preferably, the waveforms are applied in a sequence which alternates between one or more Type 1 waveforms and one or more Type 2 waveforms. In particularly preferred embodiments, the EMS program comprises a sequence of waveforms alternating between a single Type 1 waveform and a single Type 2 waveform. For example, in particularly preferred embodiments of apparatus or methods according to the present invention, the stimulation signal comprises the following sequence of 10 waveforms: W5, W4, \N8, W6, W7, W11, W5, W4, W8, \N6. The sequence may be repeated for any desired duration. For example, the sequence may be repeated twice for a duration of approximately 30 minutes. The sequence may preferably be repeated in reverse order, to mitigate adaptation of the subject to a particular waveform sequence. Alternative EMS programs comprising Type 1 and Type 2 waveforms, including but not limited to waveforms W4, W5, W6, W7, W8, and W11, may be provided, as described hereinbefore.

Claims

1. An apparatus for the electrostimulation of the muscles of a subject, the apparatus comprising:a system configured to generate a stimulation signal comprising a plurality of electrical pulses; andmeans for applying the stimulation signal to the subject to induce contraction of at least one muscle of the subject;wherein the stimulation signal is configured to induce in the subject an increase in peak blood flow of at least 50% above baseline when applied to the subject.

2. The apparatus according to claim 1, wherein the stimulation signal is configured to induce in the subject an increase in peak blood flow in excess of 100% above baseline when applied to the subject.

3. The apparatus according to claim 1 or claim 2, wherein the electrical pulses are arranged in groups, each group comprising two or more pulses separated by a first inter-pulse interval, the groups being separated by a second inter-pulse interval.

4. The apparatus according to claim 3, wherein the electrical pulses have a group frequency of 10 Hz to 100 Hz, preferably from 20 Hz to 90 Hz, more preferably from 30 Hz to 80 Hz.

5. The apparatus according to claim 3 or claim 4, wherein each group comprises a triplet of pulses.

6. The apparatus according to any preceding claim, wherein the electrical pulses are arranged as a series of repeating cycles, each cycle comprising atleast two pulses having different pulse parameters selected from peak voltage, peak current, pulse width, and / or polarity.

7. The apparatus according to any preceding claim, wherein the electrical pulses are arranged in bursts with a rest period between successive bursts.

8. The apparatus according to claim 7, wherein each burst has a duration of from 1 s to 10 s, preferably from 2 s to 5 s9. The apparatus according to claim 7 or claim 8, wherein the rest period between successive bursts has a duration of from 0.5 s to 3 s, preferably from 1 s to 2 s10. The apparatus according to any preceding claim, wherein the amplitude of the pulses is modulated by a modulation signal.

11. The apparatus according to claim 10, wherein the modulation signal comprises one or more of a sine wave, a triangular wave, and a square wave.

12. The apparatus according to claim 10 or claim 11, wherein the modulation signal has a frequency of from 0.5 to 4 Hz, preferably from 1 to 3 Hz.

13. The apparatus according to any of claims 10 to 12, wherein the depth of the modulation signal is from 10% to 70%, preferably from 20% to 50%.

14. The apparatus according to any preceding claim, wherein the peak voltage of the electrical pulses is from 20 to 160 V, preferably from 30 to 150V, more preferably from 40 to 140 V, most preferably from 50 to 130 V.

15. The apparatus according to any preceding claim wherein the stimulation signal comprises a waveform of a first type configured to induce in the subject an increase in peak blood flow in excess of 100% above baseline when applied to the subject and a waveform of a second type configured to induce in the subject an increase in peak blood flow of from 50% to 100% above baseline when applied to the subject.

16. The apparatus according to claim 15 wherein the stimulation signal comprises an alternating sequence of one or more waveforms of the first type and one or more waveforms of the second type.

17. The apparatus according to any preceding claim, wherein the stimulation signal comprises at least one of waveforms W4, W5, W6, W7, W8, and W11, as described herein and illustrated in Figures 9, 10, 11, 12, 13, and 16, respectively.

18. The apparatus according to claim 17, wherein the stimulation signal comprises waveforms W4, W5, W6, W7, W8, and W11, applied in the following sequence: W5, W4, W8, W6, W7, W11, W5, W4, W8, W6.

19. A method of electrostimulation of the muscles of a subject, the method comprising applying to the subject a stimulation signal comprising a plurality of electrical pulses;wherein the application of the signal induces in the subject an increase in peak blood flow of at least 50% above baseline.45

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