Effect of pre-activation force on active force generation in skeletal muscle

Cross bridges play a central role in skeletal muscle force generation. The level of force per cross bridge and the number of attached cross bridges are thought to determine muscle performance. Recent studies propose the so-called myosin-activation hypothesis, which suggests that stress exerted on myosin filaments increases the number of attached cross bridges and, hence, active force. This study was aimed at investigating the influence of passive stress magnitude exerted at the onset of activation on active force in a whole muscle preparation. The tibialis anterior (TA) muscle-tendon unit (MTU) of mice (N = 8) was stretched uniaxially in situ to long lengths where substantial viscoelastic passive force relaxation occurs. Muscle stress upon activation was varied by activating the TA either immediately at the end of the passive stretch (high passive force), or following nearly complete passive force relaxation (low passive force). Total forces with and without activation were measured from every MTU. Active forces were calculated by subtracting the passive force relaxation curve from the total force measured over a 1.13-s activation. We found that active force generated by the TA at low passive stress was 5-13 % higher than that at high passive stress. While the results seem contradictory to the myosin-activation hypothesis, we speculate that the results arose either from length adjustments between muscle and tendon during passive force relaxation, from excessive lattice spacing compression, or from unfavourable alterations of myosin conformation by high passive stress. Further research is required to improve our understanding of active force generation under the influence of viscoelasticity of muscle and tendon.