Mechanical properties within and beyond the physiological length of the semitendinosus muscle of knee-immobilized rats.

Background: The effects of immobilization on passive muscle mechanical properties are inconsistent between studies. Here, we investigated the mechanical properties of immobilized muscle by obtaining length-force curves within and beyond the physiological muscle-tendon length in a knee-contractured rat model.

Methods: Unilateral rat knee joints were immobilized using an external fixator for up to 21 days. Length-passive force relationships in the immobilized and contralateral semitendinosus muscles were determined by tensile testing.

Findings: The semitendinosus muscle-tendon length at end physiologic length in vivo was approximately at 5% strain of the slack length. Dynamic, elastic, and viscous force (three aspects of muscle mechanical properties) evoked by instantaneous constant stretch were higher than contralateral side within the physiological muscle-tendon length limit (strains of 5% or 5%-7.5% slack length). When beyond muscle-tendon length corresponding to the maximum knee extension (strains of 7.5% or 10%-20% slack length), there was no difference between the two sides. Dynamic and elastic stiffness were also larger, as estimated by tangent angles of length-force curves, at strains of 5% slack length, and matched contralateral levels at strains of 7.5% slack length on day 21. There were no differences in semitendinosus muscle-tendon lengths overtime. Despite significantly reductions in knee extension range of motion, collagen content only showed slight changes and correlation was hardly detected between collagen and mechanical properties on day 21.

Interpretation: Viscoelasticity in immobilized semitendinosus muscle increased within the physiological muscle-tendon length. Collagen content may have little effect on passive force and stiffness.