Dynamics of Locomotor Fatigue during Supra-critical Power Exercise

Abstract

Purpose We aimed to measure 1) the dynamics of locomotor fatigue during constant supra-critical power cycling and 2) the magnitude of any reserve in locomotor power at intolerance to constant and ramp-incremental cycling in recreationally active volunteers. Methods Fifteen participants (7 women and 8 men, 22 ± 3 yr, 3.34 ± 0.67 L·min−1 V˙O2peak) completed ramp-incremental and very-heavy constant power (205 ± 46 W) exercise to the limit of tolerance. Immediately after intolerance, the ergometer was switched into the isokinetic mode, and participants completed a short (~5 s) maximal isokinetic effort at 70 rpm. The time course of locomotor fatigue during constant supra-critical power exercise was characterized with these short maximal isokinetic sprints at 30, 60, 120, and 180 s and at the limit of tolerance. Each bout was terminated after the isokinetic sprint. Results Constant power exercise duration was 312 ± 37 s. Isokinetic power production values at 30, 60, 120, and 180 s and at the limit of tolerance (at 312 ± 37 s) was 609 ± 165, 503 ± 195, 443 ± 157, 449 ± 133, and 337 ± 94 W, respectively. Of the total decline in isokinetic power, ~36% occurred within the first minute of exercise, and significant (P < 0.05) reductions in isokinetic power occurred at all time points versus the baseline maximal isokinetic power (666 ± 158 W). In addition, a significant power reserve of 132 ± 74 W (64% of the task requirement) and 119 ± 80 W (47%) was present at the limit of constant power and ramp-incremental exercise, respectively. Conclusions Locomotor fatigue occurred rapidly during supracritical power exercise with pseudo-exponential kinetics. Instantaneous isokinetic power production at the limit of tolerance exceeded that of the task requirement, regardless of the constant or ramp work rate profile. Thus, the perceptual and physiologic limits were dissociated at the limit of tolerance in recreationally active volunteers.

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“…The substantial power reserve at V̇ o 2peak in patients with COPD we found in this study and previously suggests that peripheral fatigue did not develop sufficiently to directly limit exercise tolerance. Of note, a power reserve is also observed in some healthy subjects during constant power exercise ( 25 ). This effect is more prevalent during longer (∼20 min) than shorter (∼5–10 min) exercise tasks ( 35 ), which may reflect greater contribution of central fatigue during longer tasks in healthy participants.…”

“…The proposed mechanisms limiting whole body exercise are that exercise causes accumulation of peripheral fatigue-associated metabolites (such as intramuscular Pi, H + and interstitial K + ), and impaired intramuscular calcium handling and sensitivity, which together directly reduce muscle power production to a level below the demands of the exercise task (a direct muscle fatigue limitation to task performance) ( 21 – 23 ) and also that this peripheral metabolite accumulation indirectly limits power production through feedback via group III/IV muscle afferents that activate interneuron inhibition of motor efferent activity in the dorsal horn of spinal cord and/or higher in the neuromuscular chain (indirect central fatigue limitation to performance of exercise) ( 24 ). In health, when measured serially during constant power exercise, power reserve declines approximately exponentially, with ∼36% of the total performance fatigue accumulating within the first minute of exercise ( 25 ). These dynamics are roughly similar to intramuscular PCr breakdown and Pi accumulation ( 26 , 27 ), consistent with the concept that the early dynamics of neuromuscular performance fatigue are strongly associated with peripheral muscle metabolite accumulation ( 25 , 28 ).…”

“…In health, when measured serially during constant power exercise, power reserve declines approximately exponentially, with ∼36% of the total performance fatigue accumulating within the first minute of exercise ( 25 ). These dynamics are roughly similar to intramuscular PCr breakdown and Pi accumulation ( 26 , 27 ), consistent with the concept that the early dynamics of neuromuscular performance fatigue are strongly associated with peripheral muscle metabolite accumulation ( 25 , 28 ). The later development of central fatigue during time trial exercise suggests that sensory feedback contributes relatively more to limiting muscle activation during sustained exercise tasks than during acute ones ( 28 ).…”

Power Reserve at Intolerance in Ramp-Incremental Exercise Is Dependent on Incrementation Rate

“…In addition, during RI exercise in which a cuff is rapidly inflated around the leg at task failure, and a 10-s isokinetic sprint performed 10 s later, mean power during this sprint was within 10% of the value achieved at RI task failure (13). Although in some healthy participants P iso at intolerance can be as much as 50% greater than the peak RI power (14), in patients with chronic lung disease P iso at intolerance can greatly exceed the demands of the task (P iso ~ 160% greater than the peak RI power) (12): exposing a “power reserve” in patients with pulmonary limitations to cycle ergometry. Using single-leg electrically stimulated contractions of the quadriceps, peripheral muscle fatigue was less in those with a power reserve at the limit of RI cycle ergometry despite V˙O 2max being attained in all participants (15).…”