In this study of human locomotion we investigate to what extent the normal frequency and amplitude of leg movements can be modified voluntarily at different constant velocities, and how these modifications are accomplished in terms of changes in duration and length of the support and swing phases of the stride cycle. Eight healthy male subjects performed walking and running on a motor-driven treadmill at speeds ranging from 1.0 to 3.0 m s-1 (walking) and 1.5 to 8.0 m s-1 (running), respectively. At each speed the subjects walked and ran with: normal stride frequency; the highest possible stride frequency, and the lowest possible stride frequency. Time for foot contact was measured with a special pressure transducer system under the sole of each shoe. At all speeds of walking and running it was possible to either increase or decrease the frequency of leg movements; that is, to decrease or increase stride cycle duration. The range of variation decreased with increasing speed. The mean overall stride frequency range was 0.41 (low frequency walk 1.0 m s-1)-3.57 Hz (high-frequency run 1.5 m s-1). Stride length ranged 0.40 (high frequency walk 1.0 m s-1)-5.00 m (low frequency run 6.0 m s-1). At normal frequency the overall ranges of stride frequency and length were 0.83-1.95 Hz and 1.16-4.10 m, respectively. The stride frequency increased with speed in low frequency walking and running (as in normal frequency) and decreased in high frequency, despite the effort to maintain extreme frequencies. Only in high frequency walking could the stride frequency be kept approximately constant.(ABSTRACT TRUNCATED AT 250 WORDS)