Bergstrom, H. C., Housh, T. J., Cochrane, K. C., Jenkins, N. D., Buckner, S. L., Baker, B., Schmidt, R. J., Johnson, G. O., & Cramer, J. T. (2014). Neuromuscular responses during continuous exercise at, above, and below critical power. Medicine & Science in Sports & Exercise, 46(5), Supplement abstract number 2472.

“Critical power has been used to demarcate fatiguing from non-fatiguing exercise intensities. Fatigue is characterized by specific motor unit activation strategies including changes in motor unit recruitment, firing rate, and action potential conduction velocity that are reflected in the time and frequency domains of the electromyographic and mechanomyographic signals.”

This study examined the neuromuscular (electromyographic and mechanomyographic) responses during continuous exercise at, above, and below critical power. Ss (N = 11; mean ± SD age: 21 ± 2 years) performed an incremental cycle ergometer test to exhaustion. During a separate visit, critical power was determined from a 3-minute all-out test. The electromyographic and mechanomyographic amplitude and mean power frequency responses as well as time to exhaustion were examined during randomly ordered, continuous rides at critical power minus 10%, critical power, and critical power plus 10%. Linear regression analyses were performed to determine if the slope coefficients were significantly different from zero (p ?0.05) for the neuromuscular responses versus time relationships for the exhaustive rides at critical power minus 10%, critical power, and critical power plus 10%.

The mean time to exhaustion at critical power minus 10% was 23.93 ± 9.42 minutes. At critical power, Ss maintained exercise for only 12.76 ± 7.54 minutes. At critical power plus 10%, exhaustion occurred at 7.41 ± 4.18 minutes. There were significant, positive slope coefficients for electromyographic amplitude, electromyographic mean power frequency, and mechanomyographic amplitude, and non-significant slope coefficients for mechanomyographic mean power frequency at critical power minus 10% and critical power. At critical power plus 10%, electromyographic amplitude and mechanomyographic amplitude increased, while electromyographic mean power frequency and mechanomyographic mean power frequency decreased.

Implication. At critical power minus 10% and critical power, the increases in electromyographic and mechanomyographic amplitude as well as electromyographic mean power frequency likely reflected the fatigue-induced recruitment of fast-twitch muscle fibers with faster conduction velocities. Above critical power, the fatigue-induced recruitment of additional motor units (i.e., an increase in electromyographic and mechanomyographic amplitude) was associated with decreases in action potential critical velocity and firing rate (i.e., decreases in electromyographic and mechanomyographic mean power frequency). In addition, exercise at critical power minus 10% and critical power were not sustainable for greater than 30 minutes for most Ss. Critical power did not demarcate fatiguing from non-fatiguing exercise intensities.

Return to Table of Contents for this issue.