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 in associated activity versus 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 minutes following 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 EMG) remained depressed for only 5-10 minutes, whereas the MVCs stayed depressed for over 20 minutes. These decreases were accompanied by a reduction in MEP, MVC EMG-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 versus maximal contractions might contribute to these differences in post-fatigue behavior. However, we cannot exclude differences in processes that are specific to either voluntary or to associated contractions.