Injury to skeletal muscles of mice by forced lengthening during contractions.

Our purpose was to determine the amount of injury to extensor digitorum longus (EDL) and anterior tibial (ATB) muscles of mice from 1 h to 30 days following passive shortening and lengthening, shortening contractions and lengthening contractions. The shortening-lengthening cycle was of 600 ms duration and was repeated every 5 s for 30 min. Contractions were produced during either the shortening or lengthening phase with stimulation at 100 Hz for 300 ms. The amount of injury was evaluated by the decrease in the number of fibres in a cross-section of the muscle and in the maximum isometric tetanic force (Po). Passively shortened and lengthened and actively shortened EDL and ATB muscles showed no evidence of injury. At 3-5 days after lengthening contractions, EDL and ATB muscles showed an inflammatory response. At 3-5 days, the number of fibres in the cross-section of the injured EDL muscles was 65% of that of the contralateral control EDL muscle, whereas no change was observed in the number of fibres in the injured ATB muscles. By 3 days, the presence of myoblasts, myotubes and central nuclei indicated regeneration of fibres. One hour after shortening contractions, the Po of EDL and ATB muscles decreased to about 75% of the contralateral control value, but recovered completely by 3 days. In contrast, 1 h after lengthening contractions the Po of EDL and ATB muscles decreased to 52 and 40% respectively of the contralateral control value and then both recovered to about 65% within 3 h. For the ATB muscle, the Po showed a secondary decrease to 49% of the contralateral control value at 1 day and for the EDL muscle, to 47% at 3 days. Each muscle recovered gradually and by 30 days the number of fibres and the Po of the injured muscles were not significantly different from values for contralateral control muscles.