1. The electrical activity of up to eight concurrently active motor units has been recorded from the human deltoid and first dorsal interosseous (f.d.i.) muscles. The detected myoelectric signals have been decomposed into their constituent motor-unit action potential trains using a recently developed technique.2. Concurrently active motor unit behaviour has been examined during triangular force-varying isometric contractions reaching 40 and 80% of maximal voluntary contraction (m.v.c.). Experiments were performed on four normal subjects and three groups of highly trained performers (long-distance swimmers, powerlifters and pianists).3. Results revealed a highly ordered recruitment and decruitment scheme, based on motoneurone excitability, in both muscles and in all subject groups.4. Differences were observed between the initial (recruitment) and final (decruitment) firing rates in each muscle. These parameters were invariant with respect to the force rates studied, although some differences were observed among subject groups.5. In general, firing rates of f.d.i. motor units increased steadily with increasing force (up to 80% m.v.c.). The firing rates of deltoid motor units rose sharply just after recruitment and then increased only slightly thereafter.6. Recruitment was found to be the major mechanism for generating extra force between 40 and 80% m.v.c. in the deltoid, while rate coding played the major role in the f.d.i.7. The potential of rate coding for increasing force levels up to m.v.c. is discussed.