The purpose of this study was to test the hypothesis that group III and IV muscle afferent activity would differ after concentric- and eccentric-type fatiguing tasks. Tibialis anterior afferent activities from adult rats were measured in three conditions: before and after a rest period (C), and after concentric (CC) or eccentric (EC) exercise. Specific activators were used to elicit increases in afferent discharge rates, i.e., electrically induced fatigue (EIF), or potassium chloride (KCl) and lactic acid (LA) injections. After the rest period (POST-condition), the control group displayed a pattern of response to stimuli similar to that obtained in baseline condition (PRE-condition). However, responses were significantly different in the exercise groups: afferent responses were blunted in the CC group and were almost suppressed in the EC group. These results demonstrate that the type of muscular contraction involved in the fatiguing task can affect group III and IV afferent fiber activity differently and, potentially, can differentially affect the regulation of central motor command. Muscle Nerve, 2009.