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This Article Full Text Full Text (PDF) All Versions of this Article: 573/1/161    most recent jphysiol.2005.103044v1 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 Google Scholar Google Scholar Articles by Klein, C. S. Articles by Thomas, C. K. Search for Related Content PubMed PubMed Citation Articles by Klein, C. S. Articles by Thomas, C. K. Related Collections Skeletal Muscle and Exercise

¹ The Miami Project to Cure Paralysis, Department of Neurological Surgery
³ Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136, USA
² Department of Community Medicine and Rehabilitation, Umeå University, 901 87 Umeå, Sweden

Human muscles paralysed chronically by spinal cord injury (SCI) fatigue excessively. Whether these reductions in force reflect a decrease in the fatigue resistance of the motor units is unknown. Our aim was to determine the fatigability of thenar motor units paralysed chronically (10 ± 2 years) by cervical SCI. Surface electromyographic activity (EMG) and force were recorded from 17 paralysed motor units (n = 7 subjects) in response to intraneural motor axon stimulation (13 pulses at 40 Hz, 1 s^–1 for 2 min). Unit force decreased progressively, reaching 8–60% of initial after 2 min, whereas both the amplitude and area of the first EMG potentials in the trains increased significantly (both P < 0.05). Thus, transmission of neural signals to the sarcolemma was effective and the reduction in force must reflect impaired processes in the muscle fibres. The median fatigue index for paralysed units (0.31), the ratio of the force at 2 min compared to the initial force, was significantly lower than that for units from control subjects (0.85, P < 0.05), but the distribution of fatigue indices for each population had a similar shape (ranges: 0.08–0.60 and 0.41–0.95, respectively). Hence, chronic paralysis did not limit the range of fatigability typically found for thenar units, only its magnitude. These findings suggest that all paralysed units underwent similar reductions in fatigue resistance. After fatigue, paralysed unit forces were reduced at all frequencies (1–100 Hz, P < 0.05). Twitch contraction and half-relaxation times were increased, as was the frequency needed to produce half maximal force (P < 0.05). Thus, stimulation protocols used to produce functional movements in paralysed muscles need to accommodate the significant and rapid fatigue of the motor units.

(Received 5 December 2005; accepted after revision 26 February 2006; first published online 2 March 2006)
Corresponding author C. K. Thomas: The Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, 1095 NW 14 Terrace, R48, Miami, FL 33136-2104, USA. Email: cthomas{at}miami.edu