Abstract
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.
Highlights
A muscle’s potential to generate force depends on intrinsic muscle properties as the force-length and force-velocity relationships[1,2]
A specific stimulation onset timing condition led to zero net mechanical work of the muscle-tendon unit (MTU), which compares to terrestrial locomotion, featuring relatively small length changes of the fascicles and lower fascicle shortening velocity compared to the MTU18
The findings provide evidence that the fascicles of the vastus lateralis (VL) operate close to their optimum working length with very small length changes in the stance phase, where the muscle is active and generates force
Summary
A muscle’s potential to generate force depends on intrinsic muscle properties as the force-length and force-velocity relationships[1,2]. It has been suggested that proximal muscles feature less compliant tendons and, MTU’s length changes would be accompanied by major changes in fascicle length[8,9,10] In this regard, modelling studies predicted a stretch-shortening cycle of the VL muscle during the stance phase with notable fascicle length changes of up to ≈25% of optimal length during running[11] and ≈20% during walking[11,12], covering a wide portion around the plateau of the force-length curve. We expected that the muscle intrinsic mechanics (i.e. force-length-velocity relationships) are governed by adjusted muscle activation in a way that facilitates the VL force generation potential during the different locomotion modes, i.e. walking and running. We evaluated the reliability of the ultrasound-based VL’s fascicle length determination during walking and running by a systematic test-retest analysis, since such methodologically essential information has not been provided yet
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