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A study was conducted to investigate the discharge pattern of single motor units during submaximal prolonged activity at the tension of the recruitment threshold, and the relationship between the discharge pattern and the conduction velocity of the motor unit action potential, which has been used as an index of muscle unit characteristics (Andreassen & Arendt-Nielsen, 1987). The results were as follows : 1) In all motor units observed (32 units), the spike interval during prolonged activity increased in the first several minutes. However, there was some difference in the motor unit discharge pattern accoding to the degree of the initial increment in the spike interval and the discharge pattern after the initial elongation period. Therefore we divided the motor unit discharge pattern into four typical styles, i, e., 1 : units derecruited (7 units), 2 : units first derecruited and later rerecruited (9 units), 3 : units that fired continuously with gradual initial slowing (8 units), 4 : units that fired continuously with only slight initial slowing (8 units). 2) Recruitment of the motor units appeared after 5 min of the •gload•h according to their recruitment thresholds. 3) In most of the motor units observed, spike intervals became shorter 15 min after the onset of the •gload•h, and the recruitment thresholds decreased immediately after the •gload•h in comparison with the value before the •gload•h. It was suggested that most units were gradually excited by this prolonged load. 4) Conduction velocity of the muscle fibers was in the range between 2.59 and 4.99 m•Es-1. 5) When the conduction velocity of single motor units was divided into four groups according to the discharge pattern, there was no difference in the conduction velocity among the four groups. During submaximal prolonged activity, motor units showed individual discharge patterns, and their excitability was generally increased. It was concluded that the increased excitability was due to some •gcompensatory•h mechanism for maintaining the target tension, which probably differed from the neural control mechanism during •gmaximal•h prolonged activity. (Jpn. J. Phys. Fitness Sports Med. 1990, 39 :298306) key words : Single motor unit, Submaximal prolonged activity, Discharge pattern, Recruitment pattern, Conduction velocity 横 浜国 立 大 学 教 育学 部 運 動 生 理 学 研 究 室Laboratory of Applied Physiology,Faculty of Education, 神 奈川 県 横 浜 市 保 土 ケ谷 区 常 盤 台156 Yokohama National University,Hodogaya-ku,Yokohama, Kanagawa,240 単一運動単位の持続的筋力発揮時の放電様式とその筋興奮伝導速度 299

DISCHARGE PATTERN OF SINGLE MOTOR UNITS DURING PROLONGED ACTIVITY AT THE TENSION OF THE RECRUITMENT THRESHOLD AND ITS CONDUCTION VELOCITY