http://www.jospt.org/tetsuofukunaga Wed, 30 Apr 2008 09:05:25 EST 0 http://www.jospt.org/issues/articleID.1388/article_detail.asp STUDY DESIGN: Experimental laboratory study. OBJECTIVES: 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. BACKGROUND: The reason for individual variation in joint flexibility or tightness is not clearly understood, but the influence of musculotendinous stiffness has been inferred. METHODS AND MEASURES: Each of the subjects (6 women and 6 men) was seated with the right knee extended and right ankle positioned at a 30°, 20°, 10°, 0°, –10°, –20°, and –30° (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. RESULTS: Both the muscle belly and tendon were significantly elongated as the ankle was dorsiflexed (at 0° 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° of ankle plantar flexion), from which stiffness indices were determined both for muscle belly and tendon. The passive torque at 0°, –10°, –20°, and –30° was significantly correlated with the stiffness indices of the Achilles tendon (at 0°, r2 = 0.70 and 0.62 for overall and specific stiffness, respectively; Pr2 = 0.76; Pr2 = 0.47, Pr2 = 0.32, P>.05). CONCLUSION: 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. J Orthop Sports Phys Ther. 2008;38(5):269-276, published online 22 January 2008. doi:10.2519/jospt.2008.2632KEY TERMS: dorsiflexion, flexibility, plantar flexors, stretching http://www.jospt.org/issues/articleID.1388/article_detail.asp