Isotonic lengthening and shortening movements of cat soleus muscle.

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to ;step' changes in tension after which it lengthened or shortened isotonically.3. Isotonic lengthening movements usually began relatively slowly but proceeded with increasing velocity; this acceleration was most conspicuous when low rates of stimulation were used.4. At low rates of stimulation the isotonic lengthening movement often continued beyond the length at which the muscle could have generated that tension in an isometric contraction. The muscle then shortened slowly back toward that length.5. Isotonic shortening movements began relatively rapidly, but as shortening continued the movement became slower, and often had an irregular oscillatory course.6. The isotonic movements are discussed in relation to the sliding filament theory of muscle contraction.7. The compliance of the series elastic elements was calculated from the relative amplitudes of the ;step' changes in tension and length. The stiffness of this component increased with increasing muscle tension.