In normal subjects, fast arm movements are accomplished by a diphasic or triphasic EMG activation of the agonist and antagonist muscles. The duration of the first agonist burst (Ag1) has been said to be constant for movements of different size, whilst its amplitude is variable. Previous studies focused on relatively small movements (10-40 degrees). We have studied the behaviour of the Ag1 duration over the full physiological range of wrist and elbow flexion movements in normal subjects. The results showed that the principle of invariance of Ag1 duration was true when small movements of about 15-30 degrees were studied, but when larger movements were made burst length increased. A similar increase in Ag1 burst duration also was seen during movements performed against a load and in contractions made with fatigued muscles. Changes in duration of the Ag1 burst appear to be part of the normal mechanism for increasing the impulsive force provided in rapid contractions.