Effect of phase of stimulation on acute damage caused by eccentric contractions in mouse soleus muscle

Eccentric contractions (activation during muscle lengthening) can cause muscle damage. The effect of phase of stimulation on the extent of muscle damage was studied by using the work-loop method. For the work-loop method, the muscle was subjected to sinusoidal length changes at 2 Hz. The muscle was activated at different times during the imposed length-change cycle; this time is called the phase of stimulation. Work was calculated from the loop formed when force was plotted against length. Work done was positive when the muscle was shortening and was negative when the muscle was lengthening; net work was the difference. One complete length-change cycle was 100 (i.e., given as a percentage of the cycle); shortening occurred from 25 to 75. The muscle did the most net work when stimulated at phase 20, that is, when activation started just before shortening. Damage was defined as a decrease in work. Significant damage occurred after a single trial of three consecutive eccentric contractions; the muscle did less positive and less net work because of the damage. Maximal damage occurred at phases 90 and 0, the center of the lengthening part of the length-change cycle (work decreased 10%). Negligible damage occurred at phases 20-40. Negative work (work required to lengthen the muscle) also decreased because of the damage. Eccentric contractions caused much more damage than concentric contractions during oscillatory work.