Fatigue is defined as the failure of muscle to produce the initial peak force. This may be due to failure at one or more sites between the motor cortex, the motoneuron and the internal machinery of the muscle fibre. Issues of the topic under consideration include sensation of motor command during fatigue, voluntary activation of motoneuron pools and motor unit behaviour and electromyographic activity during fatigue. At one end of this motor chain there may be failure to generate an adequate descending voluntary drive to the motoneuron pool causing central fatigue and at the other a failure of excitation-contraction coupling. The contractile apparatus within the cell may also be abnormal. Changes in the central motor commands, the discharge of motoneurons and the output of muscle fibres during muscle fatigue are described.