Gradually strengthening electric stimulation impulses are sent to the median nerve at the wrist. Thereby an action potential caused on some level of the stimulation signal proceeds along the sensory nerve to the spinal cord, from where the nerve impulse (Hoffmanns reflex) returns via the alpha motor neuron pool along a motor nerve to a muscle and causes the movement of the muscle. This movement is detected by an accelerometer. The intensity of the stimulation signal and the time from the stimulation signal to the motoric response are saved, and by means of these the level of fatigue is evaluated. A device comprises a cuff with two electrodes and a current generator for forming impulses for the electrodes. The cuff can be placed on the wrist such that at least one of the electrodes is on the volar side of the hand at the point of the median nerve. The measuring device further comprises an accelerometer, that is provided to produce a signal, when the accelerometer detects a movement. The measuring device further comprises a control arrangement, which is provided for guiding the current generator to produce impulses and to monitor the signal of the accelerometer. The control arrangement stops the production of impulses, when the signal created by the accelerometer is detected, and saves the intensity value of at least the latest impulse and the time &DgrT, which is the difference between the points of time of the signal produced by the accelerometer and said latest impulse. From these the value describing the level of fatigue can be calculated.
