Critical Peripheral Fatigue Thresholds Among Different Force-Velocity Conditions: An Individual-Based Model Approach

Abstract

During high intensity exercise, metabosensitive muscle afferents are thought to inhibit the motor drive command to restrict the level of peripheral fatigue to an individual's critical threshold. No evidence exists of an individual relationship between peripheral fatigue and the decrease in voluntary activation reached after prolonged all-out exercise. Moreover, there is no explanation for the previously reported large decrease in voluntary activation despite low metabolic stress during high force contractions. Thirteen active men completed two maximal intensity isokinetic knee extension tests (160 contractions) under conditions of low force - high velocity and high force - low velocity. Neuromuscular testing including maximal torque, evoked torque and voluntary activation, was done every 20 contractions. The exponential modeling of these variables over time allowed us to predict the stable state (asymptote) and the rate of decrease (curvature constant). For both high and low force contractions the evoked torque and voluntary activation asymptotes were negatively correlated (R ² = 0.49 and R ² = 0.46, respectively). The evoked torque asymptotes of the high and low force conditions were positively correlated (R ² = 0.49). For the high force contractions, the evoked torque and voluntary activation curvature constant were negatively correlated (R ² = 0.43). These results support the idea that a restrained central motor drive keeps peripheral fatigue under this threshold. Furthermore, an individual would show similar fatigue sensibility regardless of the force generated. These data also suggest that the decrease in voluntary activation might not have been triggered by peripheral perturbations during the first high force contractions.

Keywords: evoked torque; exercise; group III/IV muscle afferents; performance; voluntary activation.

FIGURE 1

Typical torque traces obtained during the fatigue procedure (left panel) and a neuromuscular measurement (right panel). An isometric neuromuscular measurement was conducted after every twenty 1–s contractions performed either at 30°.s^-1 or 90°.s^-1. NM_x, neuromuscular measurements after x seconds of accumulated contraction time; T_stim, torque level at the point of the stimulation; T_max, maximal torque produced before the stimulation; ST, superimposed twitch, i.e., difference between the maximal torque attained after the stimulation and Ts_tim; Q_tw, resting potentiated twitch, i.e., maximal torque response following a high frequency doublet stimulation on the relaxed potentiated muscle.

FIGURE 2

Mean fatigue kinetics for isokinetic knee extensions performed with high force (HF; black) and low force (LF; white). Individual data are represented by the small markers. (A) Torque (Isometric Maximal Voluntary Contraction) – time relationship. (B) Evoked torque (Q_tw) – time relationship. (C) Voluntary activation (VA) – time relationship; IMVC, Q_tw and VA are expressed as a percentage of the maximal value. The solid/dotted line corresponds to exponential modeling of the relationship for HF and LF, respectively. Markers representing HF and LF conditions are slightly shifted for a visualization purpose.

FIGURE 3

Mean evoked torque (Q_tw; panel A) and voluntary activation (VA; panel B)expressed as a function of torque (IMVC) decrease for high force (HF; black) and low force (LF; white) condition. Each neuromuscular testing are represented by the small markers. The big markers represent the mean (pooled data) for VA/Q_tw decrease at 10, 20, and 30% IMVC decrease estimated from individual models and Equation 4. All data are expressed as a percentage of the maximal value obtained during the pre-fatigue neuromuscular testing. The solid/dotted line corresponds to mean exponential modeling of the relationship for HF and LF, respectively. ^∗Statistical difference between HF and LF.

FIGURE 4

Evoked torque asymptote correlation for high force (HF) and low force (LF) condition. Each dot represent an individual. The evoked torque asymptote for HF (A_Qtw-HF) and LF (A_Qtw-LF) are expressed as a percentage of the maximal value obtained during the pre-fatigue neuromuscular testing. The correlation was statistically significant, p < 0.001.

FIGURE 5

Correlation of evoked torque and voluntary activation for asymptotes and curvature constant of the exponential fatigue kinetics model. Each dot represent an individual. (A) Evoked torque (Q_tw) and voluntary activation (VA) asymptote correlations for knee extensions performed with high force (HF) and low force (LF). (B) Evoked torque (Q_tw) and voluntary activation (VA) curvature constant (τ) correlations for knee extensions performed with high force (HF) and low force (LF). All correlations are statistically significant, p < 0.05.