Compliance, activation and the force-length relationship in skeletal muscle

The force-length relationship in skeletal muscle is attributed to the requirement for overlap between the contractile proteins actin and myosin. However, this relationship shifts with changes in muscle activation. Here we present data suggesting that this activation-dependent shift occurs not only because of a length dependence of calcium sensitivity, but also because of series compliance within muscles. We attribute this effect to strain-dependent inhibition of crossbridge binding and thus suggest a common mechanism for the activation-dependent shift in optimum length and shortening depression. This has major implications for both our understanding of the mechanisms of muscle contraction, and our interpretation of in vivo muscle performance.