The force-length-velocity potential of the human soleus muscle is related to the energetic cost of running

According to the force-length-velocity relationships, the muscle force potential is determined by the operating length and velocity, which affects the energetic cost of contraction. During running, the human soleus muscle produces mechanical work through active shortening and provides the majority of propulsion. The trade-off between work production and alterations of the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) might mediate the energetic cost of running. By mapping the operating length and velocity of the soleus fascicles onto the experimentally assessed force-length and force-velocity curves, we investigated the association between the energetic cost and the force-length-velocity potentials during running. The fascicles operated close to optimal length (0.90 +/- 0.10 L-0) with moderate velocity (0.118 +/- 0.039 V-max [maximum shortening velocity]) and, thus, with a force-length potential of 0.92 +/- 0.07 and a force-velocity potential of 0.63 +/- 0.09. The overall force-length-velocity potential was inversely related (r = -0.52, p = 0.02) to the energetic cost, mainly determined by a reduced shortening velocity. Lower shortening velocity was largely explained (p < 0.001, R-2 = 0.928) by greater tendon gearing, shorter Achilles tendon lever arm, greater muscle belly gearing and smaller ankle angle velocity. Here, we provide the first experimental evidence that lower shortening velocities of the soleus muscle improve running economy.