Stress-strain Relationship Of Each Hamstring Muscles: A Human Cadaver Study

PURPOSE: Hamstring strains are associated with increased mechanical stress during muscle elongation in hip flexion and knee extension. The long head of the biceps femoris (BF) experienced the most frequent injuries, followed by the semimembranosus muscle (SM) and semitendinosus muscle (ST). Based on the strain measurements of each hamstring muscle, we suppose that the mechanical stress associated with elongation leads to varying injury frequencies between muscles; however, the quantitative assessment of the stresses involved in the hamstring strain mechanism remains pending. This study aimed to clarify the stress-strain relationship of hamstring muscles using human cadavers. METHODS: The BF, ST, and SM muscles were dissected from eight cadavers. The proximal and distal hamstring tendons were affixed to a mechanical testing machine. The slack length was defined as the muscle length at the initial loading point detected when a tensile load was applied. Muscle length was measured using a tape measure, and the anatomical cross-sectional area (ACSA) of the muscle was measured at two sites—the proximal and distal sites using B-mode ultrasonography. For the loading protocol, the muscle was elongated from its slack length to a maximum of 8% strain at a rate of 20 mm/min, and the amount of displacement and tensile load were measured for each muscle. Strain (%, displacement/slack muscle length) and stress (kPa, tensile load/ACSA) were calculated to evaluate mechanical properties. A two-way repeated-measures analysis of variance was used to compare the stress changes with increasing strain. RESULTS: A significant interaction between muscle and strain factors was observed concerning stress. Post-hoc tests revealed higher stresses in BF and SM than in ST after 3% strain (P < 0.01). No significant difference was observed between BF and SM. At 8% strain, BF exhibited a stress of 63.7 ± 12.1 kPa, SM showed 53.7 ± 23.2 kPa, whereas ST registered 21.0 ± 11.9 kPa. CONCLUSIONS: The results indicate that the stress changes associated with muscle strain are different among muscles. Particularly, the stresses applied to the three muscles at the same strain may be higher in BF and SM. These findings suggest that the increased mechanical stress during elongation might contribute to the frequent occurrence of muscle strain in BF and SM.