Journal of Applied Physiology AJP: Endocrinology and Metabolism
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J Appl Physiol (November 12, 2004). doi:10.1152/japplphysiol.01032.2004
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Submitted on September 17, 2004
Accepted on November 10, 2004

Conduction Velocity of Low-Threshold Motor Units during Ischemic Contractions Performed with Surface EMG Feedback

Dario Farina1*, Marco Gazzoni2, and Federico Camelia2

1 Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Aalborg, Denmark
2 Laboratorio di Ingegneria del Sistema Neuromusculare(LISiN), Dipartmento di Elettronica, Politecnico di Torino, Torino, Italy

* To whom correspondence should be addressed. E-mail: df{at}hst.aau.dk.

The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at approximately 8 pulses per second (pps) for five 3-min long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean ± SE, 4.6 ± 2.1 % of the maximal voluntary contraction force), discharge rate (8.6 ± 0.4 pps), interpulse interval variability (34.8 ± 2.5 %), and peak-to-peak amplitude of the target motor unit action potentials (176.6 ± 18.2 µV) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 ± 0.2 m/s, 195.2 ± 10.5 Hz, and 5.4 ± 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 ± 1.8 % and 8.50 ± 2.18 % during the 3 min, respectively) and action potential duration increased (8.2 ± 4.6 % in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.




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