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1. The fatigue in rat anterior tibial (a.t.) motor units was studied and related to microphotometric determinations of succinate dehydrogenase (SDH) activity of the motor unit muscle fibres. 2. Anterior tibial contains fast-twitch type II fibre units with an average contraction time of 11 msec and about 5% slow-twitch type I fibre units with an average contraction time of 20 msec. 3. In type II fibres stained for SDH, absorbance varied continuously from 0.046 to 0.569 and inversely to fibre size, except for the largest fibres. 4. Resistance to fatigue of fast motor units to 100 Hz intermittent stimulation varied continuously within a wide range in near linear relations to absorbance for SDH of unit fibres and inversely to tetanic tension, except for motor units with the largest fibres and the largest tetanic tension. 5. Neither resistance to fatigue nor SDH activity lent itself to any categorization of motor units or fibres into well demarcated functional or histochemical types, since both parameters varied continuously in the unit and fibre population of the muscle. 6. The direct relation between resistance to fatigue of fast-twitch motor units and SDH activity of unit fibres appeared valid for fatigue resistance of: (a) neuromuscular transmission, tested with 100 Hz intermittent stimulation which gave concomitant failure of electrical and mechanical response, (b) excitation--contraction coupling, demonstrated by post-stimulatory depression of twitch tension with preserved maximum tetanus tension and action potential, and (c) contractile mechanism; excitation--contraction coupling?, tested with low frequency stimulation which gave decline of twitch and maximum tetanus tension with preserved action potential. 7. It is suggested that the endurance of each link in the chain of events leading to contraction, including neuromuscular junction and the excitation--contraction coupling system, is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity.
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