Deformation along human medial gastrocnemius muscle fibers in vivo during low-level plantar flexion activity

Abstract

Mechanical behavior of skeletal muscle has been previously shown to be determined by the interactions between contractile elements of the muscle and the extracellular matrix. Moreover, in the level of a whole limb, interactions between muscles and non-muscular structures through connective tissues have been shown to affect muscular mechanics causing varying force and movement production. The central determinant of those effects is sarcomere length changes and their heterogeneity along the muscle fibers. Quantification of that for human muscles in vivo is lacking. This study utilized magnetic resonance imaging (MRI) in combination with non-rigid demons image registration method to quantify local muscle tissue deformations. Additionally, diffusion tensor imaging (DTI) was used in order to determine muscle fiber direction. Combination of those methods allowed quantifying length changes along muscle fibers, in vivo. Presently, this method was applied to the medial gastrocnemius muscle (GM) of female subjects (n = 4) following a transition from 15% to 5% maximal voluntary contraction (MVC) in sustained isometric plantar flexion activity. The aim was to test the hypothesis that low-level plantar flexion activity tested results in non-uniform distribution of strain along the muscle fibers. The results show sizable simultaneous lengthening (by up to 32.7%) and shortening (by up to 15.5%) occurring along the same muscle fiber tracts. In addition, average distribution of strain across fiber tracts was also found to be heterogeneous. Therefore, the posed hypothesis was confirmed. The findings are explained through the effects of myofascial force transmission on skeletal muscle function. Insight from this work can find practical application in surgical interventions such as aponeurotomy, tendon transfer surgery, or procedures like botulinum toxin treatment.|Keywords : Diffusion tensor imaging, Magnetic resonance imaging, Tractography, Gastrocnemius, in vivo, Myofascial force transmission.