Study of structure and function in the human triceps surae muscle -tendon complex under normal and atrophic states using magnetic resonance phase contrast imaging

Velocity-encoded phase contrast MRI (VE-PC-MRI) and MR-compatible dynamometry were used to study the mechanics of the active and passive elements of the muscle-tendon system.;A novel analytical technique was developed for measurement of in vivo tendon mechanical properties. Its reliability was assessed based on repeated measurements from the Achilles tendon of healthy humans. The data demonstrated excellent within-session and day-to-day repeatability and were found to be in good agreement with ultrasonography data from other groups. This validated methodology was then used to measure the Achilles tendon elastic properties before/after atrophy induced by 4-week limb suspension and during 6-week physical rehabilitation. Maximum force and tendon cross sectional area were also monitored. Atrophy significantly altered the tendon elastic properties and decreased muscle force output while tendon size remained unchanged. These changes fully recovered to their normal levels at the end of the rehabilitation.;In order to elucidate the relationship between the mechanics of muscle shortening to ankle excursion, a computer-controlled hydraulic-based MR-compatible ankle dynamometer was developed, and tested. Water was used as the hydraulic medium that transmitted the power generated by a remote linear motor outside the scanner room to a foot pedal device located inside the scanner. The motor provided an EKG-like trigger for the scanner to allow gated image acquisition. Three modes of muscle contraction could be investigated with this device, i.e. passive plantarflexions (no-load), active eccentric and concentric contractions. The foot pedal device, VE-PC-MRI, and developed image processing algorithms were used to estimate fiber length and pennation angle during dynamic ankle rotation. Architectural gear ratio and the intra-fiber strain were also measured at different proximo-distal locations along the human medial gastrocnemius muscle. Unexpected and novel findings of inhomogeneous intra-fiber strain and spatially varying strain and the gear ratio were observed. Regional changes in these parameters agreed well with the theoretical output of geometrical models of contracting muscle. This work has demonstrated that muscle architecture & function are intricately interrelated and the techniques developed herein can be used to elucidate the structure-function relationship of the various elements of the musculoskeletal system.