Passive forces in mammalian skeletal muscle: a freely-jointed and worm-like chain

The passive mechanical properties of whole muscle in active and nonactive states are compared. The experimental results are presented as stress-strain curves, which are analyzed in the framework of the current theoretical background [viz. the freely-jointed chain model (FJCM) and the worm-like chain model (WLCM)] in a semi-quantitative fashion. This analysis shows that both models can explain the mechanical behavior of whole muscle in non-active state. In the active state, the presence of crossbridges alters the mechanical response, leading to a markedly different behavior, as expected. A discussion of the mechanisms involved and the interpretation of the parameters required for the fitting of the stress-strain curves is also presented.