Clinical Biomechanics 1990; 5: 17-22 Electrically evoked knee flexion torque increases with increased pelvifemoral angles R W Bohannon PT, EdD’ M L Reed PT, BS R L Gajdosik PT PhD* ‘School of Allied Health, University of Connecticut and *Department of Physical Therapy, University of Montana, USA Summary This study was designed to determine the extent to which knee flexion torques would differ when submaximal hamstring muscle contractions were evoked by constant levels of eksctrical stimulation and Men the pelviiemoral angle was increased. Nineteen healthy subjects (ten women and nine men) underwent electrical stimulation of the hamstring muscles while randomly positioned either supine, sitting upright, or sitting leaning forward. The pelvifemoral angle for each position was measured from lateral photographs: the knee flexion torque was calculated from the knee flexion force, and lever arms measured directly at a constant knee angle. A repeated measures AWVA demonstrated significant differences for pelviiemoral angles (F = 48590, P < 0901) and knee flexion torques (F = 21.97, P c 0901) among the positions. The mean torques in the upright and leaning forward positions were 2.2 and 3.7 times greater, respectively, than mean torques in the supine poskion. The increase between the supine and leaning forward positiins exceeded the Increase previously reported in the literature for subjects performing maximal voluntary knee flexion efforts. Rdwance Because knee flexion torques increased in the presence of constant levels of electrical stimulation but increased pelvifemoral angles, the results elucidate the potential contribution of hamstring muscle zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA le ng th to force production in the muscles when activated submaximally. Key words: Muscles, p&is, knee, bimechanics zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Introduction Muscle force production is the summed effect of active and passivecomponents, both of which are affected by the length of the muscle. The active component, which depends in part on the amount of overlap of actin and myosin filaments, is classically demonstrated by maximum isometric force production at the resting length of muscle, and by decreases in isometric force production as the muscle is either shortened or lengthened relative to the resting lengthlT2. The passive Received: 31 May 1989 Accepted: 29 September 1989 Correspondence and reprinr requests IO: Dr Richard W Bohannon, School of Allied Health, U-101. University of Connecticut, Stows, CT 06269, USA @ 1990 Butterworth & Co (Publishers) Ltd u268-0033I9cNJ1017-06 mechanical component is brought about by stable cross- links between actin and myosin filaments that bend as the muscle is lengthened3-‘, and also by the connective tissues within and around the muscle belly6-‘. Unlike the influence of the active component that decreases beyond the resting length of the muscle, the influence of the passive component increases as the muscle is lengthened beyond its resting length. thus contributing to increases in total muscle force production. The specific in vivo effect of muscle length on torque production about a joint has been the subject of several investigations that have focused on the hamstring muscles’0-‘3. In these investigations, healthy persons or patients voluntarily performed knee flexion efforts at different trunk-thigh angles: varying the trunk- thigh angles resulted in changes in hamstring muscle length ‘O-12. Lunnen et al.“’ reported mean maximal knee