Determinants of length range of active force exertion by muscle

A comparison was made between the relative lengths of sarcomeres at which no active force can be generated and normalized fiber lengths found experimentally in muscles positioned at such lengths of zero force exertion. The relevant sarcomere lengths were estimated on the basis of electron microscopic data on filament lengths, and the relevant fiber lengths were estimated using macrophotography of muscles during isometric tetanic contractions. Any substantial difference between these sarcomere-length and fiber-length estimates were taken as indicators for variation of muscle architecture. For rat gastrocnemius muscle no such differences were found, but for soleus and extensor digitorum muscles they did occur. For semimembranosus muscle they are expected.<>