Acute muscle strain injuries typically occur during active lengthening contractions, and tend to present along or near a musculotendon junction (MTJ) [1]. For example, among sprinters, the majority of hamstring strain injuries involve the proximal MTJ of the biceps femoris long head [1,2]. We have previously shown that post-injury scarring at the MTJ can persist for many months after return to sport [2], which may contribute to the high risk for re-injury associated with these injuries. Thus, it is pertinent to understand why the MTJ is susceptible to injury, and ultimately how post-injury remodeling may alter in-vivo mechanics. The purpose of the current study was to use dynamic magnetic resonance (MR) imaging to visualize muscle tissue displacements and mechanical strains during active lengthening contractions in the biceps femoris. We hypothesized that active lengthening contractions would induce greater mechanical strain along the proximal MTJ, when compared to shortening contractions of similar loads and ranges of motion.
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