Note: This work is currently in progress, but premilinary data was presented at the 25th Congress of the International Society of Biomechanics. Data from this project was combined with data from another research group for another study, which was presented at the same conference.
Overview: Hamstring strain injuries are common injuries but their mechanism is still unknown. It’s likely that muscle strain (i.e. length changes), instead of force, is the culprit. However, when measured at the time of injury, neither seem sufficiently high enough to cause injury. It is possible that microdamage occurs in the muscle and may lead to injury, similar to fractures in bone. The purpose of this study was to determine if hamstring strain injuries are related to changes in hamstring stiffness, as quantified by shear modulus using ultrasound elastography. Hmastring stiffness was prospectively tracked throughout the season in collegiate sprinters and jumpers, with subjects coming into the lab once per month. Nine sites were identified on each hamstring of both legs: a proximal, middle, and distal portion of each of the biceps femoris long head, semitendinosus, and semimembranosus. On each visit, three shear modulus measurements were taken at each of the 18 sites to get a mean value for each region of each muscle.
While none of the athletes were injured at the beginning of the study, only six were free of any history of hamstring injuries. Throughout the course of the study, only one athlete (male) experienced a hamstring strain, which occurred one week post-initial scans. The athlete indicated that they felt the strain in the middle portion of their semimembranosus on their right leg near ultrasound site. Using this athlete as a case study, we found that in the injured semimembranosus, the midbelly showed the highest shear modulus values, which were roughly 50% higher than the proximal portion and 3x higher than the distal portion. The injured semimembranosus mid-belly also had shear modulus values 2.5-3x higher than the semimembranosus mid-bellies (right leg) of three non-injured males. Distal portions, meanwhile, showed similar values across the injured and non-injured males. While this is a case study and more work is needed, these data provide evidence that hamstring stiffness may be altered prior to strain injury. Possible explanations for increased stiffness include: 1. edema at the injury site leading to increased stiffness or 2. higher tension in areas adjacent to injury due to damage at the injury.
