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This Article Full Text Full Text (PDF) All Versions of this Article: 571/3/683    most recent jphysiol.2005.099705v1 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 Fuglevand, A. J. Articles by Keen, D. A. Search for Related Content PubMed PubMed Citation Articles by Fuglevand, A. J. Articles by Keen, D. A. Related Collections Skeletal Muscle and Exercise

¹ Department of Physiology, College of Medicine, University of Arizona, Tucson, AZ, USA

Spinal motor neurones can exhibit sustained depolarization in the absence of maintained synaptic or injected current. This phenomenon, referred to as a plateau potential, is due to the activation of monoamine-dependent persistent inward currents. Accordingly, activation of a plateau potential should result in a decrease in the excitatory synaptic drive required to activate a motor unit. This, in turn, has been suggested to cause a progressive decline in the muscle force at which motor units are recruited during repeated voluntary contractions. Such a progressive decrease in threshold force associated with preceding activation of a plateau potential is referred to as ‘warm up’. Furthermore, activation of a plateau potential is thought to manifest itself as a decrease in the derecruitment force compared to recruitment force. Multiple muscles, however, can contribute to the detected force and their relative contributions may vary over time, which could confound measures of recruitment and derecruitment force. Therefore, the purpose of this study was to compare the recruitment and derecruitment forces of single motor units in the human extensor digitorum and tibialis anterior during repetitive triangular-force contractions in which the contributions of other muscles had been minimized. In both muscles, we found that the recruitment thresholds of single motor units were unchanged during repeated contractions, and that the derecruitment force was consistently greater than the recruitment force. These results suggest either that plateau potentials were not engaged (or were rapidly extinguished) under these experimental conditions or that changes in recruitment and derecruitment force are not suitable criteria for detecting them.

(Received 6 October 2005; accepted after revision 18 January 2006; first published online 19 January 2006)
Corresponding author A. J. Fuglevand: Department of Physiology, College of Medicine, University of Arizona, P.O. Box 210093, Tucson, AZ 85721-0093, USA. Email: fuglevan{at}u.arizona.edu

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