SHORT REPORT ABSTRACT: The surface EMG area often exhibits progressive enlarge- ment during a submaximal fatiguing contraction, but the underlying reasons still remain uncertain. Fatigue-induced changes in the surface-detected mo- tor unit action potentials (S-MUAPs) of 10 human thenar motor units (MUs) with widely differing physiological properties were examined. After 2 min of repetitive 40-Hz stimulation, the size of the S-MUAPs of all MUs increased, the magnitude of which was negatively correlated with their tetanic tension changes. These findings suggest that during muscle fatigue, in addition to reflecting recruitment of new MUs and increases in firing rates of the active MUs, the surface EMG may also be markedly influenced by changes in the S-MUAPs, especially in fast fatigable muscles. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1786–1789, 1998 DISSOCIATION OF THE ELECTRICAL AND CONTRACTILE PROPERTIES IN SINGLE HUMAN MOTOR UNITS DURING FATIGUE K. MING CHAN, MD, FRCP(C), 1 LEONARD P. ANDRES, MSc, 2 YELENA POLYKOVSKAYA, MD, 1 and WILLIAM F. BROWN, MD, FRCP(C) 1 1 Department of Neurology, New England Medical Center, Boston, Massachusetts, USA 2 Nutrition, Exercise Physiology & Sarcopenia Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts, USA Accepted 5 May 1998 Surface EMG is commonly used in the study of muscle fatigue. The progressive enlargement in its rectified integrated area during submaximal fatigu- ing contraction is thought to primarily reflect an in- crease in the number of recruited motor units (MUs) and in their firing rates. 7,10,14 A third possi- bility is that the size of the motor unit action poten- tial (MUAP) might change during fatiguing contrac- tion. Such possibility indeed has been shown in animal studies, 1,8,18 but has not been systematically investigated in humans. Therefore, the purposes of this study were to examine: (1) the effect of fatigue on the size of the MUAPs; and (2) the relationship of the latter changes to the contractile characteristics of these MUs. METHODS We recently described a noninvasive method for studying the contractile and electrical properties of human thenar MUs. 2,3 To briefly summarize, single motor axons with the lowest stimulation threshold were excited by finely graded electrical impulses de- livered through a surface bipolar stimulator placed at multiple points over the accessible portions of the median nerve at the wrist and upper arm. Their as- sociated MUAPs were detected by using self-adhesive surface recording electrodes (1.5 × 1 cm) with the active electrode placed over the thenar muscles and the reference electrode over the first metacarpopha- langeal joint. The tension output of each motor unit was detected by a Grass FT-10 force transducer placed against the first interphalangeal joint aligned in the plane of its prime vector. Using this method, the twitch properties, tetanic tensions, and electrical properties of 10 MUs from 5 young healthy subjects (1–3 MUs per subject) have been thoroughly char- acterized and reported. 3 The fatigue response of each MU was determined by delivering 13 stimuli at 40 Hz every second for a total of 2 min. 1 Changes in the size of the surface-detected MUAP (S-MUAP) Abbreviations: EMG, electromyography; FI, fatigue index; MU, motor unit; MUAP, motor unit action potential; S-MUAP, surface-detected motor unit action potential Key words: human; muscle fatigue; motor unit action potential; potentia- tion; Na/K pump Correspondence to: Dr. K. Ming Chan, Division of Neuroscience, 513 Heritage Medical Research Center, University of Alberta, Edmonton, Al- berta, T6G 2S2 Canada CCC 0148-639X/98/121786-04 © 1998 John Wiley & Sons, Inc. 1786 Short Reports MUSCLE & NERVE December 1998