The Relationship Between Passive Ankle Plantar Flexion Joint Torque and Gastrocnemius Muscle and Achilles Tendon Stiffness: Implications for Flexibility

Abstract

Experimental laboratory study. We tested the hypothesis that the muscle fibers and the connective tendinous structures, combined in series, provide the resistance to passive joint movement at the ankle. We also determined the relative association between passive joint torque and each of these 2 elements. The reason for individual variation in joint flexibility or tightness is not clearly understood, but the influence of musculotendinous stiffness has been inferred. Each of the subjects (6 women and 6 men) was seated with the right knee extended and right ankle positioned at a 30 degrees , 20 degrees , 10 degrees , 0 degrees , -10 degrees , -20 degrees , and -30 degrees (0, neutral position, positive values reflecting plantar flexion) angle while passive plantar flexion torque was measured. The distal muscle-tendon junction of the medial gastrocnemius was visualized by ultrasonography, and its positional change was defined as muscle belly length change. The whole muscle-tendon unit length change was estimated from joint angle changes, from which Achilles tendon length change was estimated. Both the muscle belly and tendon were significantly elongated as the ankle was dorsiflexed (at 0 degrees the mean +/- SD muscle belly elongation was 10.3% +/- 1.8 %, and the tendon elongation was 2.8% +/- 1.2 %, of the initial length at 30 degrees of ankle plantar flexion), from which stiffness indices were determined both for muscle belly and tendon. The passive torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees was significantly correlated with the stiffness indices of the Achilles tendon (at 0 degrees , r2 = 0.70 and 0.62 for overall and specific stiffness, respectively; P<.05). A tendon stiffness index, separately obtained from tendon lengthening during maximal isometric contraction, was also correlated with passive ankle plantar flexion torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees (at 0 degrees , r2 = 0.76; P<.05). The specific stiffness index of the muscle belly was correlated (r2 = 0.47, P<.05) with the passive ankle plantar flexion torque at 0 degrees , but its overall stiffness index was not (r2 = 0.32, P>.05). Results suggest that extensibility of the muscle-tendon unit of the Achilles tendon for the most part is related to passive ankle plantar flexion joint torque.