Quadriceps force and myoelectric activity during flexed knee stance.

The purpose of this study was to determine the quadriceps demand during single limb stance with the knee in five positions of flexion (0 degree -60 degrees). Two variables were used to estimate the quadriceps demands: the integrated electromyogram (EMG) of three vasti and the torque about the knee joint. Ten normal subjects 23-29 years of age were tested. Myoelectric activity was recorded with intramuscular electrodes and knee joint angle with an electrogoniometer. The EMG data were integrated and normalized. A visible vector system was used to determine knee flexion torque. A significant linear correlation was found between values of patellar ligament force and knee angle (R2 = 0.86). The patellar ligament force increased 4.16% of body weight per degree of knee flexion between 0 degree and 60 degrees. Both vector-estimated quadriceps force and normalized EMG showed significant correlations with knee angle (R2 = 0.91 and R2 = 0.88, respectively). Both exhibited a slower rate of rise below 30 degrees flexion and a higher rate of rise above 30 degrees. The authors' findings suggest that the stabilizing effects of plantar flexors (except gastrocnemius) on the knee joint, changing muscle moment arm length, and force ratio between the patellar ligament and quadriceps muscle with respect to knee angle are primary reasons for the quadratic normalized integrated EMG (NIEMG)-force nonlineality during flexed stance. This might indicate the existence of a critical angle of knee flexion contracture beyond which patients cope poorly with standing and functional ambulation.