Hamstring Strain Reinjury Nearly one third of hamstring strain injuries recur within the first year following rehabilitation and a return to athletic activity. Research suggests several factors contribute to the high rate of reinjury: Persistent weakness in the injured muscle Reduced extensibility of the musculotendon unit due to scar tissue Compensatory changes in the biomechanics of movement due to the original injury Lack of strength and control of the lumbopelvic muscles Quadriceps tightness