Recent developments in understanding the length dependence of contractile response of skeletal muscle

Maximal active force of skeletal muscle contraction occurs at a sarcomere length where overlap of thick and thin filaments is optimal. However, the interaction of muscle length and active force is complicated. Active force, is the force generated by energy-requiring processes. To calculate active force, passive force provided by in-parallel structures must be subtracted from total force. Sarcomere length will change during a contraction with constant muscle-tendon length, due to tendon stretch. Passive force therefore changes during the contraction. Taking this into account, it has been demonstrated that there is less length dependence of fatigue than previously thought. The remaining difference may be associated with length dependence of activation, a property that is evident with submaximal activation. The sarcomere length at which peak contraction occurs is longer than the length that gives optimal overlap of the filaments and this shift of optimal length appears to be due to increased Ca²⁺ sensitivity. The increased Ca²⁺ sensitivity occurs because at longer lengths, the myofilaments are closer together allowing greater force than expected. However, the potential for length-dependent activation has been challenged. Submaximal contractions obtained by recruitment of fewer motor units but with maximal stimulation across different muscle lengths still demonstrate length-dependent activation. In contrast, contractions with similar absolute electromyographic signal magnitude at different lengths do not demonstrate length-dependent activation. Recent work has improved our understanding of how sarcomere length impacts the force of contraction but also reveals inadequacies in our knowledge that need to be addressed by additional research.