It is known that recumbent bicycles, when compared to normal ones, benefit from aerodynamic drag reduction, but could suffer from higher mechanical friction (small wheels and long chain). Recumbent posture could also alter the operative range of propulsive muscles and cardiovascular condition. Here we infer length change of the muscle-tendon units during the pedaling cycle of both bicycles with the aim to investigate, in future papers, which of the two allows to work near to optimal muscle length. Muscle lengths were estimated with OpenSim 2.4 from 3D kinematic data sampled with a motion capture system in two healthy men during pedaling on recumbent and normal bicycle at 60RPM. In recumbent bicycling, when compared to the "classic" one, some muscles are more shortened during pedaling: lateral (-2.26±0.43%) and medial (-2.24±0.36%) gastrocnemius, biceps femoris (-2.85±0.02%), semimembranosus (-3.02±0.22%), semitendinosus (-3.34±0.26%) and soleus (-1.15±0.14%). Other muscles are more stretched: tibialis anterior (+2.67±0.99%), rectus femoris (+3.39±0.13%), sartorius (+4.09±0.18%), iliacus (+6.40±0.69%) and psoas (+4.49±0.68%). Although these results suggest that muscles are working at slightly different operative ranges of their length, the final propulsive effectiveness is difficult to assess. To try to infer whether muscles in the recumbent posture move towards a more or less favorable portion of the force/length relationship, EMG of the same muscles will be used next.