The effects of passive warming on muscle injury.

This study investigated the effects of passive warming on the biomechanical properties of the musculotendinous unit. Paired tibialis anterior (TA) and extensor digitorum longus (EDL) muscles in the rabbit hindlimb were passively heated to different temperatures and then subjected to controlled strain injury. The parameters examined were: 1) percent increase in length to failure, 2) force to failure, 3) energy absorbed by the musculotendinous unit to failure, and 4) site of failure. Warmed (39 degrees C +/- 0.5 degrees C) TA (P less than or equal to 0.01) and EDL (P less than or equal to 0.05) muscles achieved a greater increase in length from rest before failing than did their contralateral controls at 35 degrees C +/- 0.5 degrees C. In both the TA and EDL the force at failure was greater at 35 degrees C than at 39 degrees C, although the difference was significant for only the EDL (P less than or equal to 0.05). The energy absorbed (area beneath the length-tension curve) by both the TA and EDL was greater at 39 degrees C, but these differences were not significant. All muscles failed at the distal musculotendinous junction. These data suggest that passive warming increases the extensibility of the musculotendinous unit and may thereby reduce its susceptibility to strain injury.