Reversal of interleg coupling in backward locomotion implies a prime role of the direction of locomotion.

Conventional descriptions of interleg coupling relate to anatomical definitions such as fore- or hindlegs, right or left legs (i.e. the body is the frame of reference). This convention is obvious for forward walking, where forelegs (in anatomical terms) are also the leading legs (in terms of direction). In backward locomotion, however, the leading legs in terms of direction are the hindlegs in terms of anatomy. What effects do the anatomy and direction of movement have on the sequence of stepping? Our observations on the locomotion of mole rats in a transparent acrylic tunnel revealed that, as in nature, mole rats moved both forwards and backwards. They typically employed a diagonal sequence of steps in forward walking, whereas in backward walking they typically employed a lateral sequence. However, when stepping was described with movement direction as the frame of reference, both forward and backward walking were made up of the same sequence of steps. The same invariant trend was recorded during backward galloping, but to a lesser extent than during walking. We suggest that the backward sequence is simply a reversal of the forward sequence: a hindleg during backward locomotion acts like a foreleg in forward locomotion, while a foreleg acts like a hindleg in forward locomotion. Interleg coupling therefore remains invariant in relation to the direction of locomotion.