Hamstring injuries in elite rugby union: Identifying biomechanical mechanisms of injury during running

Abstract

Hamstring injuries are a problem in many running-based sports, resulting in significant time lost from training and competition, as well as large financial burdens on sporting clubs and organisations. Hamstring injuries most often occur during high-speed running and typically affect the biceps femoris long head (BFlh) muscle, however, the exact mechanism for injury is not well understood. A narrative review of the proposed mechanisms underpinning hamstring injuries during high-speed running indicated that injuries are most likely to take place during late swing phase, due to peak musculotendon strain, force and negative work. Therefore, this phase was the focus of the biomechanical analyses within this thesis. The first study in this thesis examined the epidemiology of hamstring injuries in elite rugby union. Hamstring injuries across five years at a single elite rugby union team were analysed, and magnetic resonance imaging (MRI) was used to examine intramuscular injury location. In line with previous research, running was the most common activity at the time of injury, and the BFlh was the most commonly injured muscle. However, in contrast to reports in other running-based sports, the most common intramuscular injury location was the distal myofascial junction of BFlh. The second study was a biomechanical analysis of maximum velocity sprinting and was performed on ten elite rugby union players during the Super Rugby pre-season. Three players sustained a running-based hamstring injury in the following Super Rugby season, allowing the relationship between running mechanics and injury to be analysed prospectively. Functional principal component analyses were used to identify patterns of variability in biomechanical variables during the late swing phase which distinguished between prospectively injured and uninjured athletes. Prospectively injured athletes displayed a tendency for greater thoracic lateral flexion, greater hip extension moments and greater knee power absorption compared to uninjured athletes during the late swing phase. Therefore, these altered mechanics may place rugby athletes at greatest risk of running-based hamstring injury. The third study sought to understand the mechanics at the musculotendon level that may predispose an athlete to hamstring injury. High-speed running data from two athletes who were prospectively injured was used in a musculoskeletal model to predict hamstring muscle-tendon unit (MTU) mechanics. Hamstring MTU strain, velocity, force and power were calculated across the late swing phase. MRI data were used to determine subject-specific muscle volumes and to subsequently calculate physiological cross sectional area (PCSA) for each hamstring muscle. This allowed MTU force to be expressed relative to its PCSA, which is a measure of force production capacity. The BFlh muscle experienced the greatest strain of all hamstring muscles, suggesting this may be a key contributor to its propensity for injury. While semimembranosus (SM) produced the largest MTU forces when normalised to body mass, when normalised to PCSA, BFlh loads were equal to or greater than SM forces. This suggests that BFlh loads are very large relative to the muscle's maximum force generating capacity. These results indicate that both strain and force, when expressed relative to load capacity, may be key mechanisms for hamstring injury. Show more