Abstract
Background: Acute hamstring strains are common injuries in sports, responsible for excluding athletes for a considerable amount of time from participation and competition. This places a substantial burden on professional sporting organisations and healthcare systems.
Aims: To describe patterns in the site and mechanism of hamstring strains and examine the morphology of the proximal tendon and musculotendinous junctions (MTJs) of these muscles using dissection and magnetic resonance imaging (MRI).
Methods: Two systematic literature reviews were undertaken, firstly to ascertain the site and mechanism of hamstring strains in the medical imaging literature and secondly to review the anatomy of the hamstring muscles relevant to acute strains.
The morphology of the proximal MTJs of biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) was investigated using two methods: (i) individual muscles in ten thighs from five male cadavers (aged 64-85 years) were dissected and measured in situ and then resected, embedded in jelly and serially sectioned for scaled photography; (ii) the hamstring muscles of 11 physically active men (aged 18-30 years) were imaged bilaterally using MRI. In each study group, the length, volume and cross-sectional area of the proximal tendon and muscle belly were measured. In addition, novel measurements of the muscle-tendon interface area at the proximal MTJs were taken in cadavers and the MTJs in both groups were reconstructed three-dimensionally (3-D) using Amira 4 software. The relative force of muscle contraction at the proximal MTJ was estimated by calculating a muscle belly volume to muscle-tendon interface area ratio. Comparisons were made between the three hamstring muscles and between study groups.
Results: The literature review of hamstring strains indicated that the most frequently injured hamstring muscle is BFlh (65.8% of all acute strains). The MTJ was the site of injury in 73% of strains, usually the proximal MTJ. The site and mechanism of hamstring strains appear to be related: powerful eccentric contraction as occurs in sprinting is commonly associated with BFlh strains with or without additional pathology in ST, and, slow-speed stretching injuries predominantly affect SM but can also involve adjacent muscles such as quadratus femoris. Review of the anatomical literature showed that the morphology of these proximal structures is not well understood.
Three-dimensional reconstruction showed a similar proximal tendon and MTJ morphology in elderly cadavers and young active men. In both groups SM consistently had the longest proximal tendon in all specimens and, on average, MTJ, the latter extending over 20 cm2 in both groups. In cadavers the MTJ of SM also had the greatest muscle-tendon interface area (84.6 ±31.5 cm2). Muscle belly volume was more than three times greater in young active men than in elderly male cadavers. Muscle belly volume/MTJ interface area ratios suggested that BFlh experiences the greatest forces at its proximal MTJ, with a ratio of 1.68, compared to 1.65 for the proximal region of ST, and 1.24 for SM. ST muscle fibres inserted proximally into both the common proximal tendon of BFlh and directly into the ischial tuberosity. The hierarchy of hamstring muscle peak cross-sectional area varied among the young active men but was constant in elderly cadavers.
Conclusions: These results suggest that the morphology of the proximal MTJ is relatively consistent. However, key differences were evident between hamstring muscles. Estimation of force transfer at the proximal MTJ suggests that the architecture of the proximal MTJ in BFlh and ST may render them vulnerable to acute strains during powerful muscle contraction. In contrast, the larger muscle-tendon interface found in SM suggests less concentration of force at the MTJ and its longer proximal free tendon may explain its vulnerability to slow speed stretching injuries.