Effects of prior hamstring injury on morphology, function, and mechanics

Hamstring strain injuries are common in sprinting related sports, and have a high re-injury rate. The majority these injuries occur along the proximal musculotendon junction (MTJ) of the biceps femoris long head (BFLH). The purpose of this dissertation was to investigate the long term effects of a prior hamstring strain injury on morphology and biomechanical function. Magnetic resonance (MR) imaging was used to quantify scar tissue and atrophy in the previously injured musculotendon many months following injury. To understand the influence of morphological changes on hamstring function, we tested the hypothesis that scar tissue would induce shifts in the active and passive force-length relationship, such that forces would be produced at effectively shorter lengths, when compared to the un-injured limb. A passive joint stiffness model was developed and used to measure bilateral differences in hamstring musculotendon stiffness in subjects with a prior injury. These same subjects participated in a sprinting assessment during which bilateral differences in muscle activities and musculotendon mechanics were measured. Finally, isokinetic knee flexion-extension strength was measured for each subject. Results revealed no significant differences in running kinematics or muscle activation patterns. A consistent shift in angle of peak torque during strength testing was not revealed. However, the five subjects with the largest amount of scarring exhibited a 10° increase in the angle of peak torque compared to the un-injured limb, suggesting the degree of scarring may influence functional changes that take place. Thus, to understand the influence of prior injury on in-vivo hamstring muscle mechanics, a MR compatible device was designed and built to impose either elastic or inertial loads on the hamstrings, thereby enabling a comparison active-lengthening and relaxing-lengthening contractions. Cine phase contrast imaging was used to obtain hamstring muscle tissue velocities from eleven healthy and four subjects with prior hamstring injuries. Displacements and mechanical strains were estimated within a region that enclosed the BFLH. Peak first principal strains were greatest adjacent to the proximal MTJ and significantly greater in the previously injured subjects. We conclude that scar tissue may adversely affect local tissue mechanics in such a way that increases the risk for re-injury.