The Dynamics of Locomotor Neuromuscular Fatigue during Ramp-Incremental Cycling to Intolerance

Abstract

Introduction: Traditional neuromuscular fatigue assessments are not task-specific and are unable to characterize neuromuscular performance decline during dynamic whole-body exercise. This study used interleaved maximal isokinetic cycling efforts to characterize the dynamics of the decline in neuromuscular performance during ramp-incremental (RI) cycle ergometry exercise to intolerance.

Methods: Eleven young healthy participants (10 male/1 female) performed two RI cycle ergometry exercise tests to intolerance: 1) RI exercise with peak isokinetic power (Piso) at 80 rpm measured at baseline and immediately at intolerance from a maximal ~6 s effort, and 2) RI exercise where additional Piso measurements were interleaved every 90 s to characterize the decline in neuromuscular performance during the RI test. Muscle excitation was measured using EMG during all Piso assessments, and pulmonary gas exchange was measured throughout.

Results: Baseline Piso was 832 ± 140 W and RI exercise reduced Piso to 349 ± 96 W at intolerance ( P = 0.001), which was not different from flywheel power at intolerance (303 ± 96 W; P = 0.292). There was no reduction in Piso between baseline cycling and gas exchange threshold (GET; baseline Piso vs mean Piso below GET: 828 ± 146 vs 815 ± 149 W; P = 1.00). Piso fell progressively above GET until intolerance (Piso every 90 s above GET: 759 ± 139, 684 ± 141, 535 ± 144, 374 ± 117 W; each P < 0.05 vs baseline and mean Piso below GET). Peak muscle excitation (EMG) was also reduced only above GET (73% ± 14% of baseline, at intolerance; P < 0.05). However, the reduction in peak Piso preceded the reduction in peak muscle excitation.

Conclusions: The dynamics of the decline in neuromuscular performance (reduction in Piso and EMG) during RI exercise are consistent with known intensity-dependent metabolic and traditional pre-post neuromuscular fatigue responses to discrete bouts of constant-power exercise.

FIGURE 1

A, Schematic representation of the ramp-incremental exercise test with interleaved assessments of Piso. ■: measurement of baseline Piso. The solid black line describes flywheel power throughout the ramp-incremental test increasing at a rate of 20–25 W·min⁻¹. ●: peak ramp-incremental flywheel power. □: Piso measured at intolerance. Δ: Piso measured every 90 s during the ramp-incremental test. The four measures used to characterize locomotor neuromuscular performance, as described in the text, are also shown. B, A representative example the interleaved Piso measurement performed, as indicated by ▲ in panel A. Right and left powers every 2° of angular rotation from the ramp phase transitioning into the Piso phase with the corresponding cadence are shown. The gray box indicates the three pedal strokes from which Piso was calculated. The calculated Piso from these pedal strokes that is comparable with the flywheel power for this phase is shown by the thick black solid line.

FIGURE 2

The mean power responses that characterize locomotor neuromuscular performance induced by ramp-incremental exercise at the limit of tolerance. A, Ramp-incremental exercise. B, Ramp-incremental exercise with interleaved Piso measurements. ■: Baseline Piso; Δ: Piso measured every 90 s during the ramp-incremental test; ●: peak ramp-incremental flywheel power; □: Piso at intolerance.

FIGURE 3

A, Representative participant’s crank power during the ramp-incremental protocol with interleaved measurement of Piso every 90 s. Also shown is the mean crank power and 95% prediction limits of the power fluctuation while exercising in the cadence-independent hyperbolic mode, with the isokinetic phases excluded. B, Individual isokinetic power (Piso) responses during ramp-incremental exercise, following measurement of Baseline Piso. C, Group mean Piso during ramp-incremental exercise, using GET to demarcate moderate-intensity (

FIGURE 4

A, Individual (○) and group mean (■) muscle excitation responses during isokinetic power measurements (n = 9 due to EMG failure in 2 subjects). *Lower than baseline, #lower than below GET, $lower than post-GET 1, ^lower than post-GET 2. B, The relationship between the decrease in muscle activation (% baseline EMG) and the decrease in Piso (% Baseline Piso) during ramp-incremental exercise with interleaved measurements of Piso. ■: Baseline Piso; Piso below (●) and above (○) GET during ramp-interleaved exercise; □: Piso at intolerance.