Subject-specific estimates of tendon slack length: A numerical method

Tendon develops force proportional to the distance it is stretched beyond its slack length. Tendon slack length is an important parameter for musculoskeletal models because it can greatly affect estimations of muscle force. Unfortunately, tendon slack length is a difficult parameter to measure, and therefore values for it are not often reported in the literature. In this paper we present a numerical method for estimating tendon slack length from architectural parameters of the muscle. Specifically, tendon slack length is computed iteratively from musculotendon lengths determined when a corresponding joint is held at two angles, and from knowledge of the muscle's optimal fiber length. Idealized data generated using SIMM were used to test the tendon slack length algorithm. The method converged to within 1% of the true tendon slack length specified in the SIMM model. The advantage of the method outlined in this paper is that it yields subject-specific estimates of tendon slack length, given subject-specific input parameters.