Adaptive control of locomotion: How do stick insects master the transition from level to slope walking?

Günzel Y, Dallmann CJ, Schmitz J, Dürr V. Adaptive control of locomotion: How do stick insects master the transition from level to slope walking? Proceedings of the German Zoological Society. 2018: NB 1.Goal-directed, unrestrained motor behaviour in a variable and unpredictable environment requires animals to adjust their limb movements appropriately. In mammals, neuromuscular strategies with functionally distinct transition strides between level and slope walking have been found (Gottschall & Nichols, 2011, Phil.Trans. R.Soc.L.B, 366(1570), 1565-1579). So far, this has not been described for insects, where slope-dependent changes have been studied mainly in steady state situations with differing slope, during upside-down walking, or in climbing a stair of given height. With the present study, we evaluate the behavioural adjustments of unrestrained walking stick insects (Carausius morosus) in response to a step change in slope of the walkway (±45°). Larsen et al. (1995, J.Comp.Physiol.A, 176(2), 229-238) suggested that stance phases can be divided into an early support and a later propulsive part, in line with the protractor coxae muscles showing braking activity during early stance (Graham & Wendler, 1981, Physiol.Entomol., 6(2), 161-174). Here, we analyse the hind leg protraction/retraction system of the stick insect, using simultaneous recordings of whole-body kinematics and muscle activity in freely walking animals. Our results suggest an early load-dependent activation of both muscles with delayed retractor activity in case of only little load, supporting the idea of a biphasic power stroke. The situation, however, is quite different for leg kinematics, where overall differences among steady state situations were small, and the most prominent differences occurred only during the transitional steps between level walking and slope walking. Further studies may address the contribution of other muscles (e.g., depressor trochanteris) and legs