A support structure having a seat surface (2), particularly for bicycles and other pedal operated machines, comprises a substantially rigid or semirigid frame (3), means for connecting the frame (3) to the bicycle or a pedal operated machine, a yieldable padding (7) secured to the top face of the frame (3), a covering layer (8) superimposed to the yieldable pad (7), characterized in that the frame (3) has one or more differential rigidity portions (11, 11') adapted to facilitate the pedaling motion. The differential rigidity portions (11, 11') are situated at an outer peripheral edge (13) of the frame (3), along which at least one recess (12) is provided. Each differential rigidity portion (11, 11') comprises a plurality of projections (14) extending outwardly of recesses (12) formed in the frame (3).