Abstract
Changes in muscle shape could play an important role during contraction allowing to circumvent some limits imposed by the fascicle force–velocity (F–V) and power–velocity (P–V) relationships. Indeed, during low-force high-velocity contractions, muscle belly shortening velocity could exceed muscle fascicles shortening velocity, allowing the muscles to operate at higher F–V and P–V potentials (i.e., at a higher fraction of maximal force/power in accordance to the F–V and P–V relationships). By using an ultrafast ultrasound, we investigated the role of muscle shape changes (vastus lateralis) in determining belly gearing (muscle belly velocity/fascicle velocity) and the explosive torque during explosive dynamic contractions (EDC) at angular accelerations ranging from 1000 to 4000°.s–2. By means of ultrasound and dynamometric data, the F–V and P–V relationships both for fascicles and for the muscle belly were assessed. During EDC, fascicle velocity, belly velocity, belly gearing, and knee extensors torque data were analysed from 0 to 150 ms after torque onset; the fascicles and belly F–V and P–V potentials were thus calculated for each EDC. Absolute torque decreased as a function of angular acceleration (from 80 to 71 Nm, for EDC at 1000 and 4000°.s–1, respectively), whereas fascicle velocity and belly velocity increased with angular acceleration (P < 0.001). Belly gearing increased from 1.11 to 1.23 (or EDC at 1000 and 4000°.s–1, respectively) and was positively corelated with the changes in muscle thickness and pennation angle (the changes in latter two equally contributing to belly gearing changes). For the same amount of muscle’s mechanical output (force or power), the fascicles operated at higher F–V and P–V potential than the muscle belly (e.g., P–V potential from 0.70 to 0.56 for fascicles and from 0.65 to 0.41 for the muscle belly, respectively). The present results experimentally demonstrate that belly gearing could play an important role during explosive contractions, accommodating the largest part of changes in contraction velocity and allowing the fascicle to operate at higher F–V and P–V potentials.
Highlights
The ability of the human skeletal muscle to produce high values of force as quickly as possible is important for movements that involve explosive tasks or rapid postural adjustments such as those encountered in sport performance or during balance recovery (Aagaard et al, 2002; Maffiuletti et al, 2016)
Hahn et al (2017) investigated torque production in order to provide insight into the influence of angular acceleration on maximal torque at matched angular position and velocity. They showed that the absolute torque generated during dynamic explosive contractions was lower than that obtained during fixed-end contractions but, in accordance with Tillin et al (2018), that the normalised torque generated during dynamic explosive contractions was larger than in fixed-end conditions
We found that belly gearing could allow to circumvent the limits imposed by the F–V and power– velocity (P–V) relationships during low-force high-velocity contractions, allowing the muscle fascicles to operate at higher F–V and P–V potential compared to the muscle belly
Summary
The ability of the human skeletal muscle to produce high values of force as quickly as possible is important for movements that involve explosive tasks or rapid postural adjustments such as those encountered in sport performance or during balance recovery (Aagaard et al, 2002; Maffiuletti et al, 2016). Tillin et al (2018) investigated the effects of contraction speed on torque production during the rising phase of the torque–time curve (from start to 150 ms after torque onset) These authors provided evidence that the normalised EMG activity and neural efficacy during explosive concentric contraction were similar among contraction speeds, suggesting that the effect of speed on normalised explosive torque is not dependent on neuromuscular activation but could be an intrinsic property of contracting myofibers. Hahn et al (2017) investigated torque production in order to provide insight into the influence of angular acceleration on maximal torque at matched angular position and velocity They showed that the absolute torque generated during dynamic explosive contractions (from 10 to 4000◦/s2) was lower than that obtained during fixed-end contractions but, in accordance with Tillin et al (2018), that the normalised torque generated during dynamic explosive contractions was larger than in fixed-end conditions (and increased with increasing angular acceleration). They showed that the absolute torque generated during dynamic explosive contractions (from 10 to 4000◦/s2) was lower than that obtained during fixed-end contractions but, in accordance with Tillin et al (2018), that the normalised torque generated during dynamic explosive contractions was larger than in fixed-end conditions (and increased with increasing angular acceleration). Hahn et al (2017) argued that one possible source of difference between the two types of contractions could be attributed to the behaviour of the elastic components or to the decoupling behaviour between muscle belly and muscle fascicles
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