A support structure having a seat surface, particularly for pedal operated machines, such as bicycles, comprises a substantially rigid or semi-rigid frame, means for connecting the frame to the pedal operated machine, a yieldable padding secured to the top face of the frame, a covering layer superimposed to the yieldable pad, wherein the frame has one or more differential rigidity portions, preferably adapted to facilitate the pedaling motion. The differential rigidity portions are preferably situated at an outer peripheral edge of the frame, along which at least one recess is provided. Each differential rigidity portion comprises a plurality of outwardly extending projections.