Magnetic resonance imaging (MRI) of in vivo human triceps surae motion during contraction after atrophy and achilles tendon rupture

A muscle tendon unit complex (MTC) is often viewed as an arrangement of contractile and passive elements arranged in series and parallel with the contractile elements moving homogeneously. However, the actual in vivo measurement of the movement of the contractile elements has been limited. The purpose of this study was to measure the in vivo movement of the medial gastrocnemius (MG) and soleus (Sol) of the triceps surae MTC during a voluntary submaximal isometric contraction so that a model of normal and abnormal in vivo triceps surae motion could be developed.;The motion of the triceps surae MTC was examined in three conditions: (1) uninjured; (2) atrophied and (3) injured following an Achilles tendon rupture. Subjects performed cyclic submaximal isometric contractions while in a magnetic resonance imaging (MRI) scanner. A velocity encoded cine phase contrast MRI pulse sequence obtained measures of tissue motion at proximal and distal regions of the muscle compartment so that contraction velocities could be calculated. MG and Sol contraction velocities were determined in all three conditions. Deep plantar flexor (DPF) contraction velocities were investigated in the injured condition.;In the control condition, MG and Sol moved heterogeneously. During the force development phase of the contraction, the MG shortened while the Sol lengthened. In the atrophied condition, the in vivo dynamics of the triceps surae changed such that the Sol no longer lengthened but instead shortened during force development. After 6 weeks of rehabilitation, Sol motion returned to normal. MG movement did not change from control in the atrophied condition. In the Achilles tendon ruptured condition, the most consistent finding was an increase in DPF motion. After 6 weeks of rehabilitation, Sol and DPF motion did not normalize and the MG motion began to change.;These data demonstrate that normal and abnormal in vivo triceps surae motion cannot be explained by a simple in series/parallel arrangement of contractile and passive elements acting homogeneously. Models of motion must take into account the neural input into the MG and Sol, the compliance of the contractile and passive tissues, as well as the architectural arrangement of these tissues.