Amplitude electromyography (EMG) is often used as an estimator of muscular load. Such measurements can, however, be biased by other factors, for example muscular fatigue. The aim of this study was to examine the influence of fatigue on amplitude parameters of the EMG. The test subjects raised the arm to 90(9) of abduction in the plane of the scapula. The hand was loaded with 0, 1 and 2 kg during 5, 3 and 2 min respectively. EMG was recorded from the trapezius muscle, and spectral and amplitude parameters were calculated. There was a significant rise of the EMG amplitude as a sign of fatigue at all load levels: 7% min(-1) at 0 kg, 15% min(-1) at 1 kg, and 19% min(-1) at 2 kg. At 0 kg hand load there was no change of the spectral parameters but at higher load levels there was a significant decline of mean power frequency: 3% min(-1) at 1 kg and 11% at 2 kg. The amplitude rise due to muscle fatigue may seriously jeopardize calibration measurements unless the duration of the load is kept limited.