[Effects of fatigue on the characteristics of cat gastrocnemius muscle contraction and elongation in response to changes of the controlled load].

Spreading of fatigue-related effects from active to inactive parts in the cat gastrocnemius muscle was studied. Fatiguing contractions of a part of gastrocnemius muscle were evoked through stimulation of the cut ventral root (VR) S1 with rate 50 s(-1) during 20 s followed by 10 s of rest, over 10-30 min (CFS). To test activation of a non-active part of this muscle the distributed stimulation of dissected VR L7 with rate of 10 s(-1) per filament was applied. After achieving by the muscle a steady-state level of isometric contraction, a quick transition to isotonic condition was produced. Transient responses of shortening and lengthening were analyzed in terms of the two-component model including the fast inertialess and slow aperiodic components. Test duration was 12 s, time intervals between tests consisted of 3 min. CFS reduced isometric force of the tested part of muscle as well as the velocity of shortening and lengthening. The muscle lengthening had been slowing down with development of the fatigue effects; the final steady value of the muscle length in transients was reduced, while weight of the inertialess component increased. Slowing of shortening was accompanied by reduction of the final length amplitude and the time constants of aperiodic component were increased. We suppose that diffusion of metabolites from active to inactive muscle fibers was the major reason for contractility deterioration.