A reduction in compliance or activation level reduces residual force depression in human tibialis anterior

Abstract

Aim: We investigated if residual force depression (rFD) is present during voluntary fixed-end contractions of human tibialis anterior (TA) and whether reducing TA's activation level after active shortening could reduce rFD. Methods: Ten participants performed fixed-end dorsiflexion contractions to a low, moderate or high level while electromyography (EMG), dorsiflexion force and TA ultrasound images were recorded. Contractions were force-or EMGmatched and after the low or high contraction level was attained, participants respectively increased or decreased their force/EMG to a moderate level. Participants also performed moderate level contractions while the TA muscle-tendon unit (MTU) was lengthened during the force/EMG rise to the reference MTU length. Results: Equivalent fascicle shortening over moderate and low to moderate level contractions did not alter EMG (P = 0.45) or dorsiflexion force (P = 0.47) at the moderate level. Greater initial fascicle shortening magnitudes (1.7 mm; P ≤ 0.01) to the high contraction level did not alter EMG (P = 0.45) or dorsiflexion force (P = 0.30) at the subsequent moderate level compared with moderate level contractions. TA MTU lengthening during the initial force/EMG rise reduced TA fascicle shortening (−2.5 mm; P ≤ 0.01), which reduced EMG (−3.9% MVC; P < 0.01) and increased dorsiflexion force (3.7% MVC; P < 0.01) at the moderate level compared with fixed-end moderate level contractions. Conclusion: rFD is present during fixed-end dorsiflexion contractions because fascicles actively shorten as force/EMG increases and rFD can be reduced by reducing the effective MTU compliance. A reduction in muscle activation level also reduces rFD by potentially triggering residual force enhancement-related mechanisms as force drops and some fascicles actively lengthen. K E Y W O R D Selectromyography, fascicle, fixed-end contraction, force enhancement, history dependence, isometric

Triceps surae torque-length relationships relevant for walking activity levels with and without an ankle exoskeleton

“…Participants received real-time biofeedback of their SOL activity level via a computer monitor so that target activity levels (22% and 30% MVC) could be achieved voluntarily for a 10 second test period, similar to experiments on the tibialis anterior (Raiteri and Hahn, 2019). The procedure required participants to match their real◻time SOL EMG (moving 0.25◻ second RMS amplitude) to within ± 5% of a predefined EMG RMS amplitude (Fig.…”

Google maps for tissues: Multiscale imaging of biological systems and disease

“…Applied to the kidney, it provides information about renal perfusion and glomerular filtration rate (GFR). 6 For the assessment of muscle architecture changes including fascicle length and pennation angle, B-mode US was employed using a temporal resolution of 16 ms (framerate: 60 Hz). 3 Increased renal sympathetic nervous activity depressed GFR within 30 seconds upon stimulation of intranasal CaSR.…”

“…This MR renography approach was recently applied to study the response to stimulation of olfactory calcium-sensing receptors (CaSR) and to detail the pathways and mechanisms of this response in rats. 6 Trevino et al used US to probe muscle cross-sectional area and muscle tissue composition (contractile vs non-contractile tissue), thereby revealing details of sex-related differences in trapezoid muscles. 3 Unlike MR renography, the measurement of GFR by clearance techniques would require much longer sampling periods.…”

“…A striking example from human integrative muscle physiology revealed that a reduction in activation level reduces residual force depression in tibialis anterior muscle. 6 For the assessment of muscle architecture changes including fascicle length and pennation angle, B-mode US was employed using a temporal resolution of 16 ms (framerate: 60 Hz). 6 Trevino et al used US to probe muscle cross-sectional area and muscle tissue composition (contractile vs non-contractile tissue), thereby revealing details of sex-related differences in trapezoid muscles.…”

“…6 For the assessment of muscle architecture changes including fascicle length and pennation angle, B-mode US was employed using a temporal resolution of 16 ms (framerate: 60 Hz). 6 Trevino et al used US to probe muscle cross-sectional area and muscle tissue composition (contractile vs non-contractile tissue), thereby revealing details of sex-related differences in trapezoid muscles. 7 On the microscopic scale optical imaging highlights recently published in Acta Physiologica include the application of transmission electron microscopy for the assessment of mitochondrial densities and profiles in humans.…”