Differences between the Time Course of Changes in Neuromuscular Responses and Pretest versus Posttest Measurements for the Examination of Fatigue

Cory M. Smith, Terry J. Housh, Ethan C. Hill, Kristen C. Cochrane, Nathaniel D.M. Jenkins, Richard J. Schmidt, Glen O. Johnson

Department of Nutrition and Health Sciences, Human Performance Laboratory, University of Nebraska-Lincoln, Lincoln, NE 68505, USA. Department of Kinesiology and Health Promotion, CPP Human Performance Research Laboratory, California State Polytechnic University, Pomona, CA 91768, USA. Department of Health and Human Performance, Applied Musculoskeletal and Human Physiology Research Laboratory, Oklahoma State University, Stillwater, OK 74078, USA.

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Purpose: The purposes of the current study were to examine the differences during the time course of changes in neuromuscular responses and pretest versus posttest measurements and to differentiate the information provided by these two methods (time course vs. pretest and posttest) regarding the motor unit activation strategies used to control force production during the process of fatigue.

Methods: Twelve men performed concentric-only dynamic constant external resistance leg extensions to failure at 70% 1 repetition maximum (1-RM) and 1-RM measurements were taken before and after the fatiguing workbout.

Results: The results indicated decreases in pretest versus posttest 1-RM strength and electromyographic (EMG) frequency, but no changes in EMG amplitude, mechanomyographic (MMG) amplitude, or MMG frequency. The time course of changes in neuromuscular responses during the 70% 1-RM protocol indicated 4 unique phases (1 to 20, 20-60, 60-80, and 80-100% of the repetitions to failure) that generally exhibited increases in EMG amplitude and MMG amplitude, but decreases in EMG frequency and MMG frequency.

Discussion: These findings indicated that maximal pretest versus posttest measurements and the time course of changes in neuromuscular responses provide different information regarding the process of fatigue which may explain the reduction in maximal and submaximal force production. Journal of Nature and Science (JNSCI), 3(10):e456, 2017

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