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1 Department of Radiological Sciences, University of California , Los Angeles, Los Angeles, CA, USA; Department of Physiological Sciences, University of California , Los Angeles, Los Angeles, CA, USA
2 Department of Biology of Physical Activity, University of Jyvaskyla, Jyvaskyla, Finland
3 Department of Physiological Sciences, University of California , Los Angeles, Los Angeles, CA, USA
4 Department of Radiological Sciences, University of California , Los Angeles, Los Angeles, CA, USA
5 Department of Physiological Sciences, University of California , Los Angeles, Los Angeles, CA, USA; Department of Neurobiology, University of California , Los Angeles, Los Angeles, CA, USA
* To whom correspondence should be addressed. E-mail: SSinha{at}mednet.ucla.edu.
The in vivo strain properties of human skeletal muscle-tendon complexes are poorly understood, particularly following chronic periods of reduced load bearing. We studied eight healthy volunteers that underwent 4 weeks of unilateral lower limb suspension (ULLS) to induce chronic unloading. Before and after the ULLS, maximum isometric ankle plantarflexion torque was determined using an MR-compatible dynamometry. Volumes of the triceps surae muscles and strain distribution of the soleus aponeurosis and the Achilles tendon at a constant submaximal plantarflexion (20% MVCPre) were measured using magnetic resonance imaging (MRI) and velocity-encoded, phase-contrast (VE-PC) MRI techniques. Following ULLS, volumes of the soleus and the medial gastrocnemius and the maximum isometric ankle plantarflexion (MVC) decreased by 5.5±1.9 %, 7.5±2.7 % and 48.1±6.1 %, respectively. The strain of the aponeurosis along the length of the muscle before the ULLS was 0.3±0.3 %, ranging from -1.5 to 2.7 % in different locations of the aponeurosis. Following ULLS the mean strain was -6.4±0.3 %, ranging from -1.6 to 1.3%. The strain distribution of the mid region of the aponeurosis was significantly influenced by the ULLS while the more distal component showed no consistent changes. Achilles tendon strain was not affected by the ULLS. These results raise the issue as to whether these changes in strain distribution affect the functional properties of the triceps surae and whether the probability of strain injuries within the triceps surae increases following chronic unloading in those regions of this muscle complex in which unusual strains occur.
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