The invention relates to determining the state of a muscle between a normal non-tired state, a tired state and a passive involuntary tension state. A signal from the muscle is recorded at rest by using an electrode arrangement, where an earth body may prevent the electrodes from picking up signals beyond the extent of the earth body. The frequency content of the signal is determined by spectral analysis, e.g. by computing a moment of the spectrum. A normal frequency content indicates a non-tired muscle state, whereas a low and a high frequency content indicate a tired and a passive involuntary tension muscle state. A mapping is used to improve accuracy of state determination, e.g. with a reference database.