Muscle stiffness and locomotion

Summary

Recent measurements of tension in the soleus and medial gastrocnemius muscles of the walking cat show that soleus undergoes very rapid tension changes and at most speeds of walking and running develops more tension than gastrocnemius. Since in locomotion both muscles are stretched during their periods of activity, we have tried to account for the observed changes by measuring the stiffness of muscle fibres and of the tendon during stretch of contracting muscle. We found the tendon stiffness of both muscles to be about the same, while, for a similar level of isometric tension, muscle fibres of medial gastrocnemius were twice as stiff as in soleus. This could be accounted for by soleus fibres having twice as many sarcomeres. The peak level of force during the rapid stretch, using amplitudes typical for locomotion, was found to occur when the stretch was applied during the rising phase of a tetanic contraction, rather than during the subsequent plateau. It was concluded that soleus was a better choice than gastrocnemius for most forms of locomotion because it could maintain a relatively high stiffness over large distances of stretch and because it could operate over a wide range of joint angles, developing tension at relatively low energy cost. Medial gastrocnemius really only becomes important when very large, rapidly rising forces are required, such as occurs during jumping.