Biomechanical comparison of stimulated and nonstimulated skeletal muscle pulled to failure

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Abstract
We compared the biomechanical properties of passive and stimulated muscle rapidly lengthened to failure in an experimental animal model. The mechanical param eters compared were force to tear, change in length to tear, site of failure, and energy absorbed by the muscle- tendon unit before failure. Paired comparisons were made between 1) muscles stimulated at 64 Hz (tetanic stimulation) and passive (no stimulation) muscles, 2) muscles stimulated at 16 Hz (wave-summated stimu lation) and passive muscles, and 3) muscles stimulated at 64 Hz and at 16 Hz. Both tetanically stimulated and wave-summation con tracted muscles required a greater force to tear (at 64 Hz, 12.86 N more, P < 0.0004; and at 16 Hz, 17.79 N more, P < 0.003) than their nonstimulated controls, while there was no statistical difference in failure force between muscles stimulated at 16 Hz and 64 Hz. The energy absorbed was statistically greater for the stim ulated muscles than for the passive muscles in Groups 1 and 2 (at 64 Hz, 100% more, P < 0.0003; and 16 Hz, 88% more, P < 0.0002). In Group 3, the tetanically contracted muscle-tendon units absorbed 18% more energy than the wave-summated stimulated muscles (P < 0.01). All muscles tore at the distal musculotendi nous junction, and there was no difference in the length increase at tear between muscles in each group. These findings may lead to enhanced understanding of the mechanism and physiology of muscle strain injuries.

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