Post M, Bayrak S, Kernell D, Zijdewind I. Contralateral muscle activity and fatigue in the human first dorsal interosseous muscle. During effortful unilateral contractions, muscle activation is not limited to the target muscles but activity is also observed in contralateral muscles. The amount of this associated activity is depressed in a fatigued muscle, even after correction for fatigue-related changes in maximal force. In the present experiments, we aimed to compare fatigue-related changes

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... associated activity vs. parameters that are used as markers for changes in central nervous system (CNS) excitability. Subjects performed brief maximal voluntary contractions (MVCs) with the index finger in abduction direction before and after fatiguing protocols. We followed changes in MVCs, associated activity, motor-evoked potentials (MEP; transcranial magnetic stimulation), maximal compound muscle potentials (M waves), and superimposed twitches (double pulse) for 20 min after the fatiguing protocols. During the fatiguing protocols, associated activity increased in contralateral muscles, whereas afterwards the associated force was reduced in the fatigued muscle. This force reduction was significantly larger than the decline in MVC. However, associated activity (force and electromyography) remained depressed for only 5-10 min, whereas the MVCs stayed depressed for over 20 min. These decreases were accompanied by a reduction in MEP, MVC electromyography activity, and voluntary activation in the fatigued muscle. According to these latter markers, the decrease in CNS motor excitability lasted much longer than the depression in associated activity. Differential effects of fatigue on (associated) submaximal vs. maximal contractions might contribute to these differences in postfatigue behavior. However, we cannot exclude differences in processes that are specific to either voluntary or to associated contractions. associated activity; recovery; transcranial magnetic stimulation; electrical stimulation MOTOR FATIGUE IS CHARACTERIZED by a gradual decline in maximal force or by an increase in effort to maintain a constant submaximal contraction. To stress that the decline in force is due to fatigue-related changes in both the periphery (muscle and peripheral nervous system) and the central nervous system (CNS; Ref. 23), we use the term motor fatigue. Although several studies have followed time-related changes due to peripheral and central fatigue, only a few studies have looked at fatigue-related changes in contralateral muscles (3, 11, 30, 37, 40, 46, 53, 54) . During strong unilateral contractions, muscle activity is not restricted to the target muscle but activity is also observed in both ipsilateral and contralateral (nontarget) muscles (e.g., Address for reprint requests and other correspondence: I. Zijdewind, A.