Operating length and velocity of human vastus lateralis muscle during walking and running

According to the force-length-velocity relationships, the muscle force potential during locomotion is determined by the operating fibre length and velocity. We measured fascicle and muscle-tendon unit length and velocity as well as the activity of the human vastus lateralis muscle (VL) during walking and running. Furthermore, we determined the VL force-length relationship experimentally and calculated the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) for both gaits. During the active state of the stance phase, fascicles showed significantly (p < 0.05) smaller length changes (walking: 9.2 ± 4.7% of optimal length (L0); running: 9.0 ± 8.4%L0) and lower velocities (0.46 ± 0.36 L0/s; 0.03 ± 0.83 L0/s) compared to the muscle-tendon unit (walking: 19.7 ± 5.3%L0, −0.94 ± 0.32 L0/s; running: 34.5 ± 5.8%L0, −2.59 ± 0.41 L0/s). The VL fascicles operated close to optimum length (L0 = 9.4 ± 0.11 cm) in both walking (8.6 ± 0.14 cm) and running (10.1 ± 0.19 cm), resulting in high force-length (walking: 0.92 ± 0.08; running: 0.91 ± 0.14) and force-velocity (0.91 ± 0.08; 0.97 ± 0.13) potentials. For the first time we demonstrated that, in contrast to the current general conception, the VL fascicles operate almost isometrically and close to L0 during the active state of the stance phase of walking and running. The findings further verify an important contribution of the series-elastic element to VL fascicle dynamics.