Extensibility of the hamstrings is best explained by mechanical components of muscle contraction, not behavioral measures in individuals with chronic low back pain

Objective: To examine the relationship between hamstring extensibility by use of the instrumented straight leg raise; mechanical components of muscle contraction, including muscle recruitment, passive torque measures of tissue stiffness, and eccentric strength; and self-reported measures of pain and disability. Design: Cross-sectional study. Setting: University laboratory. Participants: Twenty-one individuals with chronic nonspecific axial lower back pain and 15 healthy control subjects. Assessment: Instrumented straight leg raise, concentric and eccentric hamstring strength, self-reported measures of pain, disability, fear avoidance, general health and well-being. Main Outcome Measurements: Objective measures included hamstring extensibility, hamstring muscle stiffness, absolute and relative concentric/eccentric strength, concentric/eccentric strength ratios. Self-reported measures included Oswestry disability index, visual analog pain scale, fear avoidance beliefs, and general health and well being. Results: Patients with lower back pain had lower range of motion, greater changes in muscle stiffness, and impaired concentric-to-eccentric strength levels. Stepwise regression identified measures of stiffness as significantly predicting hamstring extensibility (adjusted r2 = 0.58, F = 23.76, P < .001). Self-reported measures were not associated with extensibility. Gender differences were noted for passive stiffness and absolute strength. For women, later onset of the medial hamstrings also was associated with greater hamstring extensibility. Conclusions: Decreased extensibility of the hamstrings was associated with increased passive stiffness during the common range of motion (20 to 50°). Impaired stretch tolerance is associated with actual mechanical restriction, not behavioral measures indicating increased pain or fear-avoidant behavior. With no relationship to actual disability and contradictory findings in the literature for the relationship of the hamstrings to the mechanics of the low back, it is unclear whether decreased hamstring extensibility should be targeted in rehabilitation programs for axial lower back pain.