We found that cockroaches and crabs generate one Joule of mechanical energy/kg to move their body one meter just as do birds, mammals and humans do. Equivalent gaits (e.g. trots and gallops) may be present in 4-, 6- and 8-legged runners. Ghost crabs change gait from a trot to a gallop at the same stride frequency and speeds as a mouse.
Two-, four-, six- and eight-legged animals can produce similar force patterns during locomotion. They all can bounce as they run using 2 alternating sets of legs as springs. One human leg works like 2 dog legs, 3 cockroach legs and 4 crab legs. All show a pattern similar to a bouncing ball or a pogo stick.
Crabs and cockroaches do not move like most robots do. Most legged robots try to be wheel-like. They try to achieve a smooth ride with little oscillation of the body up and down, little acceleration and deceleration. No legs push against one another, so as to be most efficient. Also they try to be like a stool – 3 legs on the ground to always keep their balance and be statically stable.
Crabs and cockroaches have the advantage of many legs, a wide base of support and a low center of gravity to be very stable, as do many of the legged robots do. They can be stool-like. However, even when they walk, their bodies move up and down and function like upside-down pendulums to conserve energy. Crabs and cockroaches can also be fast, maneuverable and run. We have redefined a walk and a run. We use to think that a run meant you had to leave the ground, have an aerial phase. But no more. A run is where you bounce like a ball or your legs work like a pogo stick. Crabs and cockroaches can move dynamically and bounce like a ball. They are not restricted to moving one leg at a time until they are sure they have a good foot-hold. They can have kinetic energy, energy of motion, bridge the gap between poor or non-existent foot-holds. Crabs and cockroaches can be dynamically stable (not stable at every instant, but over one cycle of leg movements).
Moving up and down, accelerating and decelerating the body is desirable for an animal. Animals use there legs like pendulums and springs. Legs pushing against one another can minimize the forces at the joints. Animals move dynamically. Why not use these ideas to give robots both stability and speed and maneuverability?