Summary

1. 1. A re-examination of fast stepping in free-walking cockroaches showed that there was considerable variation in the alternating tripod gait.

2. 2. Small load increments applied to the whole animal significantly reduced this variability and concentrated the range and speed of movements used.

3. 3. Animals suspended above oiled glass surfaces moved their legs in normal gait patterns indicating that mechanical coupling via the substrate was not a critical factor for normal high-speed coordination.

4. 4. Increasing the viscosity of the oil produced the same reduction in variability found with load increment so the change cannot be due to increased mechanical coupling.

5. 5. Increasing the viscosity of the oil under one leg alone resulted in concentration over the whole pattern so the result was not considered to be due to viscosity changes.

6. 6. The changes seen in artificial loading situations were mimicked in natural loading situations such as walking up a slight incline.

7. 7. The functional significance of the phenomenon for the stability of the rapidly moving animal is considered and the role of load afference in biasing central pattern generators to produce a continuous range of functional output is discussed.