Passive tensile stress and energy of the human hamstring muscles in vivo.

The present study measured passive resistance to stretch in the hamstring muscles during a standardized stretch maneuver to estimate tensile forces and energy of the individual hamstring muscles in 7 flexible and 6 inflexible persons defined according to joint range of motion. Using a dynamometer, knee joint moment was measured during slow passive knee extension to a maximal angle (dynamic phase) followed by a 90-s static phase. Cross-sectional areas (CSA) of the separate hamstring muscles were obtained with magnetic resonance (MR) imaging. Mathematical modeling was used to calculate instantaneous muscle length and joint moment arm for each muscle. Subsequently, passive muscle tension (N/cm2) was calculated based on moment arm lengths, knee joint moments, and CSA. Maximal tolerated joint angle was greater in flexible (delta1.30+/-0.06 rad) than inflexible (delta0.84+/-0.06 rad) subjects, P<0.01. The peak tension at maximal angle was greater in flexible (81.8+/-12.5 N/cm2) than inflexible subjects (29.3+/-4.1 N/ cm2), P<0.001. For the separate muscles the overall change in muscle length (delta cm) and moment arm (delta cm) differed between groups, P<0.01. Similarly, muscle stiffness (delta tension/delta muscle length) was greater in flexible than inflexible subjects in the final 3 cm, P<0.01, and in the final 20% of length change, P<0.01. Absorbed energy (mJ/cm2) was greater in flexible than inflexible subjects in the final 40% of length change, P<0.05. These data show that flexible persons can attain a greater angle of stretch with an accompanying greater tensile stress and energy than inflexible persons due to an apparent greater tolerance to the externally applied load, and larger change in moment arm. The obtained stress data appear to be in the toe region of a 'classical' stress-strain curve, and energy rather than stiffness may therefore be more appropriate to analyze during the stretch procedure.