1. The patterns of electrical activity of selected muscles of the rear legs were studied during locomotion in unrestrained, freely moving cockroaches.

2. During walking, muscle potentials in both extensor and flexor muscles were always grouped into bursts separated by periods of silence, with one burst occurring per step.

3. During very slow walking, the occurrence of flexor and extensor bursts was synchronized with, respectively, the flexion and extension movements of the leg. At increasingly greater speeds of locomotion, the movements of the leg lagged increasingly behind the muscle activity, so that during very fast walking the extensor or flexor burst often began while the preceding leg movement was not quite half completed.

4. ‘Slow’ extensor and ‘fast’ flexor bursts were studied in detail. The frequency of impulses within each extensor burst was usually steady, but sometimes declined in the middle or increased at the end of a burst. Frequency within ‘fast’ flexor bursts was often more varied, but there was no systematic change in frequency with any other parameter which was examined.

5. For ‘slow’ extensor bursts, the duration of each burst, the number of impulses per burst and the mean interval between impulses in a burst each increased significantly as the duration of the associated step increased, i.e., there was a positive correlation between each pair of parameters. The rate at which the number of impulses per burst and mean interval per burst changed as a function of step duration was often significantly higher for steps shorter than about 300 msec than for longer ones.

6. For ‘fast’ flexor bursts, the duration of each burst, the number of impulses per burst and the mean interval per burst were each extremely variable; a weak correlation with step duration could only be demonstrated for the latter. However, both mean interval and the number of impulses per burst were positively correlated with the duration of the burst from which they were taken.

7. The data qualitatively support current models of step generation in insects, but also suggest that a useful distinction could be made between very slow walking (below 3-4 steps per sec), termed ambling, and faster walking.

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