Stress relaxation of human ankles is only minimally affected by knee and ankle angle.

Comprehensive characterization of stress relaxation in musculotendinous structures is needed to create robust models of viscoelastic behavior. The commonly used quasi-linear viscoelastic (QLV) theory requires that the relaxation response be independent of tissue strain (length). This study aims to characterize stress relaxation in the musculotendinous and ligamentous structures crossing the human ankle (ankle-only structures and the gastrocnemius muscle-tendon unit, which crosses the ankle and knee), and to determine whether stress relaxation is independent of the length of these structures. Two experiments were conducted on 8 healthy subjects. The first experiment compared stress relaxation over 10 min at different gastrocnemius muscle-tendon unit lengths keeping the length of ankle-joint only structures fixed. The second experiment compared stress relaxation at different lengths of ankle-joint only structures keeping gastrocnemius muscle-tendon unit length fixed. Stress relaxation data were fitted with a two-term exponential function (T=G(0)+G(1)e(-lambda(1))(t)+G(2)e(-lambda(2))(t)). The first experiment demonstrated a significant effect of gastrocnemius muscle-tendon unit length on G(1), and the second experiment demonstrated an effect of the length of ankle-joint only structures on G(2), lambda(1) and lambda(2) (p<0.05). Nonetheless, the size of effects on stress relaxation was small (DeltaG/G<10%), similar to experimental variability. We conclude that stress relaxation in the relaxed human ankle is minimally affected by changing gastrocnemius muscle-tendon unit length or by changing the lengths of ankle-joint only structures. Consequently quasi-linear viscoelastic models of the relaxed human ankle can use a common stress relaxation modulus at different knee and ankle angles with minimal error.