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This Article Full Text Full Text (PDF) All Versions of this Article: 586/8/2183    most recent jphysiol.2008.150698v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Riley, Z. A. Articles by Enoka, R. M. Search for Related Content PubMed PubMed Citation Articles by Riley, Z. A. Articles by Enoka, R. M. Related Collections Skeletal Muscle and Exercise

¹ Department of Integrative Physiology, University of Colorado, Boulder, CO, USA

The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n = 22) discharged action potentials tonically after being recruited, whereas others (n = 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 ± 2.5%; intermittent, 10.7 ± 2.9%. Discharge rate for the tonic units decreased progressively (13.9 ± 2.7 to 11.7 ± 2.6 pulses s^–1; P = 0.04) during the 99 ± 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 ± 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 ± 3.3 pulses s^–1) than the tonic motor units (13.7 ± 3.3 pulses s^–1; P = 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 ± 12.3%) than the tonic motor units (21.2 ± 9.4%) at recruitment (P = 0.001) and remained elevated for discharge duration (34.6 ± 9.2% versus 19.1 ± 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 ± 1.9% and 10.5 ± 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force.

(Received 7 January 2008; accepted after revision 19 February 2008; first published online 21 February 2008)
Corresponding author Z. Riley: Department of Integrative Physiology, University of Colorado, Boulder, CO 80309-0354, USA. Email: zachary.riley{at}colorado.edu

This article has been cited by other articles:

				Z. A. Riley, S. Baudry, and R. M. Enoka Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction J Neurophysiol, November 1, 2008; 100(5): 2843 - 2851. [Abstract] [Full Text] [PDF]