Greater Short-Time Recovery of Peripheral Fatigue After Short- Compared With Long-Duration Time Trial

Abstract

The kinetics of recovery from neuromuscular fatigue resulting from exercise time trials (TTs) of different durations are not well-known. The aim of this study was to determine if TTs of three different durations would result in different short-term recovery in maximal voluntary contraction (MVC) and evoked peak forces. Twelve trained subjects performed repetitive concentric right knee extensions on an isokinetic dynamometer self-paced to last 3, 10, and 40 min (TTs). Neuromuscular function was assessed immediately (<2 s) and 1, 2, 4, and 8 min after completion of each TT using MVCs and electrical stimulation. Electrical stimulations consisted of single stimulus (SS), paired stimuli at 10 Hz (PS10), and paired stimuli at 100 Hz (PS100). Electrically evoked forces including the ratio of low- to high-frequency doublets were similar between trials at exercise cessation but subsequently increased more (P < 0.05) after the 3 min TT compared with either the 10 or 40 min TT when measured at 1 or 2 min of recovery. MVC force was not different between trials. The results demonstrate that recovery of peripheral fatigue including low-frequency fatigue depends on the duration and intensity of the preceding self-paced exercise. These differences in recovery probably indicate differences in the mechanisms of fatigue for these different TTs. Because recovery is faster after a 3 min TT than a 40 min TT, delayed assessment of fatigue will detect a difference in peripheral fatigue between trials that was not present at exercise cessation.

Keywords: electromyography; femoral nerve electrical stimulation; maximal voluntary contraction; motor unit recruitment; peripheral fatigue; recovery; self-paced exercise.

FIGURE 1

Schematic view of the protocol. (A) The neuromuscular function assessed prior to the TTs. MVCs were performed three times with a 1-min break between MVCs. SS, PS100, and PS10 were assessed after the first two MVCs, and PS100 (superimposed doublet) was applied during the third MVC. (B) The three time trials in randomized order, followed by assessment of neuromuscular function (MVC + electrical stimulation after 4–8 s and 1, 2, 4, and 8 min, MVC with superimposed doublet after 16–20 s). MVC, maximal voluntary contraction; SS, single stimulus; PS100, paired stimuli at 100 Hz; PS10, paired stimuli at 10 Hz; TT, time trial.

FIGURE 2

Photo of a subject secured to the dynamometer. Electromyography (EMG) of surface sensors and the reference sensor applied to the patella is visible.

FIGURE 3

Responses in% of pre-TT for 3, 10, and 40 min duration TTs pre-TT; at the end of TT; and during the first 1, 2, 4, and 8 min following exercise cessation. MVC (maximal voluntary contraction) force (A), peak force for SS (single stimulus) (B), peak force for PS10 (paired stimuli at 10 Hz) (C), peak force for PS100 (paired stimuli at 10 Hz) (D), and FPS10/FPS100 (force for paired stimuli at 10 Hz divided by that for 100 Hz) (E). Data are shown as means ± SD, n = 12. ^TSignificant difference from previous, main effect of time. *Significant difference from previous. ^β Significant difference between 3 min TT and 10-min TT. ^£Significant difference between 3 min TT and 40 min TT. ^&Significant difference between all TTs’ β, P < 0.05. The number of symbols indicate significance at *P < 0.05; **P < 0.01; ***P < 0.001; and ****P < 0.0001. Numerical data for these parameters is provided in Supplementary Table S1. TT, time trial.

FIGURE 4

Electromyography (EMG) responses during recovery, expressed as % of pre-TT for 3, 10, and 40 min duration TTs. MVC RMS normalized to M-wave amplitude (average vastus lateralis and vastus medialis (A). M-wave peak-to-peak amplitude (average vastus lateralis and vastus medialis) expressed as % of pre-TT (B). Data are shown as means ± SD, n = 11. MVC, maximal voluntary contraction; RMS, root mean square; M, M wave; PPA, peak-to-peak amplitude; VL, vastus lateralis; VM, vastus medialis. *Different from previous. ^#Different from 3-min TT. ^&All three TTs are different from each other at this time point. The number of symbols indicate significance at *P < 0.05 and **P < 0.01.