Short-range elasticity and resting tension of relaxed human lower leg muscles.

1. Tension responses of relaxed human lower leg muscles were recorded during applied triangular stretch cycles at a range of constant angular velocities which were below the threshold for inducing stretch reflexes. 2. Length-tension curves so obtained begin with a steep initial rising phase which is demonstrated to be equivalent to the 'short-range elastic component' (s.r.e.c.) observed in resting frog muscle (Hill, 1968). The s.r.e.c. develops only gradually following a stretch-release cycle. The time course can be described by a saturation function. It is suggested that this can be explained as resulting from a slow shift of the kinetic equilibrium between attached and detached cross-bridges towards attached cross-bridges. A kinetic model is presented, by which the sum of the cross-bridge formation and decay rate constants can be calculated from the saturation curves. 3. The recovery of resting tension following a stretch-release cycle proceeds at a significantly higher rate than the development of the s.r.e.c. Resting tension is also selectively influenced by passive movements of very small amplitude. This further suggests that the s.r.e.c. and resting tension are generated by different sources.