Estimation of active force-length characteristics of human vastus lateralis muscle

The length and angles of fascicles were determined for the vastus lateralis muscle (VL) using ultrasonography in 6 subjects performing ramp isometric knee extension. The subject increased torque from zero (relax) to maximum (MVC) with the knee positioned every 15°, from 10° to 100° flexion (0° = full extension). As the knee was positioned closer to extension, fascicle length was shorter [116+4.7 (mean ± SEM) mm at 100° vs. 88+4.1 mm at 10° (relax)]. The fascicle length of the VL decreased with increasing torque at each knee position [116+4.7 (relax) to 92 ± 4.3 mm (MVC) at 100°]. On the other hand, fascicle angles increased with an increase in torque. These changes reflected the compliance of the muscle-tendon complex which increased as the knee reached a straight position. The estimated muscle force of the VL was maximal (2,052 ± 125 N) for a fascicle length of 78 ± 2.7 mm (i.e. optimum length) with the knee positioned at 70° of flexion. The relationship between muscle force and fascicle length indicated that the VL uses the ascending (knee < 70°), plateau (70 °), and descending regions (>70°) of the force-length curve.