TIME COURSE OF CHANGES IN NEUROMUSCULAR PARAMETERS DURING SUSTAINED ISOMETRIC MUSCLE ACTIONS CORY M. SMITH, 1 TERRY J. HOUSH, 1 TRENT J. HERDA, 2 JORGE M. ZUNIGA, 3 CLAYTON L. CAMIC, 4 HALEY C. BERGSTROM, 5 DOUG B. SMITH, 6 JOSEPH P. WEIR, 2 ETHAN C. HILL, 1 KRISTEN C. COCHRANE, 1 NATHANIEL D.M. JENKINS, 1 RICHARD J. SCHMIDT, 1 AND GLEN O. JOHNSON 1 1 Department of Nutrition and Health Science, University of Nebraska–Lincoln, Lincoln, Nebraska; 2 Department of Health, Sport, and Exercise Sciences, University of Kansas, Lawrence, Kansas; 3 Department of Exercise Science & Pre-Health Professions, Creighton University, Omaha, Nebraska; 4 Department of Exercise & Sport Science, University of Wisconsin–La Crosse, La Crosse, Wisconsin; 5 Department of Kinesiology & Health Promotion, University of Kentucky, Lexington, Kentucky; and 6 Department of Health and Human Performance, Oklahoma State University–Stillwater, Stillwater, Oklahoma ABSTRACT Smith, CM, Housh, TJ, Herda, TJ, Zuniga, JM, Camic, CL, Bergstrom, HC, Smith, DB, Weir, JP, Hill, EC, Cochrane, KC, Jenkins, NDM, Schmidt, RJ, and Johnson, GO. Time course of changes in neuromuscular parameters during sustained isometric muscle actions. J Strength Cond Res 30(10): 2697–2702, 2016—The objective of the present study was to identify the time course of changes in electromyographic (EMG) and mechanomyo- graphic (MMG) time and frequency domain parameters during a sustained isometric muscle action of the leg extensors at 50% maximal voluntary isometric contraction. The EMG and MMG sig- nals were measured from the vastus lateralis of 11 subjects to identify when motor unit activation strategies changed throughout the sustained isometric muscle action. The EMG amplitude (mus- cle activation) had a positive linear relationship (p = 0.018, r 2 = 0.77) that began to increase at the initiation of the muscle action and continued until task failure. Electromyographic frequency (motor unit action potential conduction velocity) and MMG fre- quency (global motor unit firing rate) had negative quadratic rela- tionships (p = 0.002, R 2 = 0.99; p = 0.015, R 2 = 0.94) that began to decrease at 30% of the time to exhaustion. The MMG amplitude (motor unit activation) had a cubic relationship (p = 0.001, R 2 = 0.94) that increased from 10 to 30% of the time to exhaustion, then decreased from 40 to 70% of the time to exhaustion, and then markedly increased from 70% to task failure. The time course of changes in the neuromuscular parameters suggested that motor unit activation strategies changed at approximately 30 and 70% of the time to exhaustion during the sustained isometric muscle action. These findings indicate that the time course of changes in neuromuscular responses provide insight into the strategies used to delay the effects of fatigue and are valuable tools for quantifying changes in the fatiguing process during training pro- grams or supplementation research. KEY WORDS fatigue, recruitment, neurophysiological, resistance training, quadriceps muscle, skeletal INTRODUCTION S imultaneous assessments of electromyographic (EMG) and mechanomyographic (MMG) time and frequency domain parameters have been used to examine the motor unit activation strategies during fatiguing isometric muscle actions. Recording the EMG and MMG signals during a fatiguing isometric muscle action allows for the identification of changes in muscle activation (EMG amplitude), motor unit action potential conduction velocity (EMG frequency), motor unit recruitment (MMG amplitude), and global motor unit firing rate (MMG frequency) (2,25,26). Typically, during a fatiguing submaximal isometric muscle action, there are increases in muscle activation and motor unit recruitment and decreases in motor unit action potential con- duction velocity and global motor unit firing rate (24,31,33). For example, Vaz et al. (34) reported decreases in EMG frequency (25%) and MMG frequency (50%) but increases in EMG ampli- tude (35%) and MMG amplitude (48%) from the vastus lateralis (VL) after a fatiguing isometric muscle action of the leg exten- sors that began at 70% maximal voluntary isometric contraction (MVIC) and ended when the subjects could not maintain 50% MVIC. Furthermore, previous EMG and MMG studies (19,24,26) have reported that the motor unit activation strategies that modulate torque production are specific to the mode of muscle action (i.e., isometric, concentric, or eccentric) (5,6,16) and protocol performed (continuous isometric, intermittent isometric, or dynamic) (4,22,26). In addition, neuromuscular pa- rameters are often measured at the beginning and end of a fatigu- Address correspondence to Cory M. Smith, CSmith@unl.edu. 30(10)/2697–2702 Journal of Strength and Conditioning Research Ó 2016 National Strength and Conditioning Association VOLUME 30 | NUMBER 10 | OCTOBER 2016 | 2697 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.