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Ankle morphology amplifies calcaneus movement relative to triceps surae muscle shortening. J Appl Physiol. 2013 Jun 6.
The present study investigated the mechanical role of the dorsoventral curvature of the Achilles tendon in the conversion of the shortening of the plantarflexor muscles into ankle joint rotation. Dynamic, sagittal-plane magnetic resonance spin-tagged images of the ankle joint were acquired in six healthy subjects during both passive and active plantarflexion movements driven by an MR compatible servo-motor controlled foot-pedal device. Several points on these images were tracked to determine the (i) path and deformation of the Achilles tendon, (ii) ankle's center of rotation and (iii) tendon moment arms. The degree of mechanical amplification of joint movement was calculated as the ratio of the displacements of the calcaneus and myotendinous junction. In plantarflexion, significant deflection of the Achilles tendon was evident in both the passive (165.7°±7.4°; 180° representing a straight tendon) and active trials (166.9°±8.8°). This bend in the dorsoventral direction acts to move the Achilles tendon closer to the ankle's center of rotation, resulting in a ~5% reduction of moment arm length. Over the entire range of movement, the overall displacement of the calcaneus exceeded the displacement of the MTJ by ~37%, with the mechanical gains being smaller in dorsi- and larger in plantarflexed joint positions. This is the first study to assess non-invasively and in-vivo using MRI the curvature of the Achilles tendon during both passive and active plantarflexion movements. The dorsoventral tendon curvature amplifies the shortening of the plantarflexor muscles, resulting in a greater displacement of the tendon's insertion into the calcaneus compared to its origin.
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