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Article Title

ARE MOTOR UNIT FIRING PROPERTIES CONTROLLED WITHIN DISTINCT REGIONS OF A MUSCLE?

Abstract

Micheal J. Luera1, Jesus Hernandez-Sarabia1, Carlos A. Estrada2, Jason M. DeFreitas1 1Oklahoma State University, Stillwater, OK2 Aurora University, Aurora, IL

Previous findings have suggested that the distribution of motor units within a muscle may display region-specific muscle activation. Consequently, if regionalization of motor units indeed exists, then force generating capacities would be highly task and joint dependent. PURPOSE: To examine for regional motor unit control from proximal and distal locations of biarticular [rectus femoris (RF)] and monoarticular [vastus lateralis (VL)] muscles during low-force knee extensions. METHODS: Following 2 maximal voluntary contractions (MVC), eighteen resistance-trained men (n = 9, age = 23 ± 3 yrs) and women (n = 9, 22 ± 2 yrs) performed a 10 sec isometric ramp contraction up to 30% MVC. On two separate occasions, surface electromyographic (EMG) signals were collected from proximal and distal locations of either the VL or RF. These were used to record EMG amplitude and were also decomposed into the constituent motor unit action potentials. The slope and intercept values were calculated across the motor units for relationships between mean firing rate, recruitment threshold, and action potential size for each subject. Paired samples t-tests were used to compare regression coefficients and EMG amplitude between proximal and distal locations of the RF and the VL separately. RESULTS: There were no differences in EMG amplitude between locations in the RF (p = 0.21, d = 0.43), however, amplitude in the distal location of the VL was greater than the proximal (p < 0.05; d = 0.64). There were no significant differences in slope or intercept coefficients for any of the motor unit relationships (see Table 1) (p = 0.08 –0.99, d = 0.00 –0.47) CONCLUSION: Although there was a regional difference in the activation across the VL, there were no region-specific differences in the motor unit firing properties. The differences in amplitude were likely due to other factors that affect EMG signals, such as the underlying morphology (muscle size, subcutaneous fat thickness, etc.).

Table 1 reported on page 2.

Luera Table 1.docx (147 kB)
Table 1

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