Determining the force-length-velocity relations of the quadriceps muscles: II. Maximum muscle stress

The purpose of this study was to determine the maximum muscle stress (sigma), defined as the maximum isometric force divided by the physiological cross-sectional area, of the quadriceps muscles for a pilot study involving musculoskeletal modeling. One female subject performed maximum effort isometric knee extension exercises on an isokinetic dynamometer at different attachment arm angles. The gravitational effect was taken into consideration when determining the isometric resultant knee torques at different knee flexion angles. The anatomical and geometric parameters of the quadriceps muscles were obtained from radiography and magnetic resonance imaging taken from the subject. The sigma value was computed using the measured knee torques, musculoskeletal parameters data, and information reported in the literature. The computation procedures used in this study represented the first attempt to incorporate the concept of optimal muscle length in the determination of maximum muscle stress. The sigma values obtained from the data for nine different knee flexion angles ranged from 21.4 to 30.5 N/cm(2). The average value of 25.6 +/- 2.6 N/cm(2) is notably smaller than the human a values reported in the literature, but is comparable to the a values obtained from isolated muscles.