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

The present study measured passive resistance to stretch in the hamstring m uscles during a standardized stretch maneuver to estimate tensile forces an d energy of the individual hamstring muscles in 7 flexible and 6 inflexible persons defined according to joint range of motion. Using a dynamometer, k nee joint moment was measured during slow passive knee extension to a maxim al angle (dynamic phase) followed by a 90-s static phase. Cross-sectional a reas (CSA) of the separate hamstring muscles were obtained with magnetic re sonance (MR) imaging. Mathematical modeling was used to calculate instantan eous muscle length and joint moment arm for each muscle. Subsequently, pass ive muscle tension (N/cm(2)) was calculated based on moment arm lengths, kn ee joint moments, and CSA. Maximal tolerated joint angle was greater in fle xible (Delta1.30+/-0.06 rad) than inflexible (Delta0.84+/-0.06 rad) subject s, P<0.01. The peak tension at maximal angle was greater in flexible (81.8/-12.5 N/cm(2)) than inflexible subjects (29.3+/-4.1 N/cm(2)), P<0.001. For the separate muscles the overall change in muscle length (Delta cm) and mo ment arm (Delta cm) differed between groups, P<0.01. Similarly, muscle stif fness (<Delta> tension/Delta muscle length) was greater in flexible than in flexible subjects in the final 3 cm, P<0.01, and in the final 20% of length change, P<0.01. Absorbed energy (mJ/cm(2))mas greater in flexible than inf lexible 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 accomp anying greater tensile stress and energy than inflexible persons due to an apparant greater tolerance to the externally applied load, and larger chang e 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 th erefore be more appropriate to analyze during the stretch procedure.