Mechanics of a myriapod metachronal gait: Legged locomotion in the laterally undulating Arizona centipede, Scolopendra heros

This study of the Arizona centipede, Scolopendra heros, is the first on the locomotor biomechanics of a laterally-undulating, metachronally-gaited animal. One untested hypothesis on the evolution of the Chilopoda is that the axial musculature of scolopendromorph centipedes should actively resist lateral undulation. The electromyographic (EMG) study in this dissertation demonstrates that this long held hypothesis is incorrect and scolopendromorph centipedes actively undulate their bodies. In scolopendromorph centipedes wave of leg movements pass posteriorly along the body. All legs on one side of the body touch the ground on the concave side of the bent body at nearly the same location called a "focus." Measurements of the ground reaction forces show that the legs in the middle of the centipede produce the largest vertical and lateral ground reaction forces. The anterior legs decelerate and the posterior legs accelerate the centipede in the fore-aft direction. All legs on one side of a centipede passing through a focus produce ground reaction forces similar to those of the virtual legs of symmetrically-gaited animals and that can be modeled as being produced by a three-dimensional spring. Sir James Gray hypothesized that elongate, many-legged animals would not show the spring-mass dynamics of many symmetrically-gaited animals, but would roll more like wheels. The spring-mass properties of the ground reaction forces produced at foci allowed the summing of model focus ground reaction forces to predict whole-body ground reaction forces. This model predicted that only under very limited kinematic circumstances, that may never occur, would centipedes roll like wheels. Under most kinematic situations modeled, whole-body spring-mass dynamics were predicted. Furthermore, the model predicted that a whole-focus stride (WFS), or the time required for all legs to pass through a focus and the next focus to be placed on the ground, which is longer than a conventional stride, is required for the ground reaction force and mechanical energies of locomotion to complete full cycles. Measurements of the whole-body ground reaction forces confirmed that centipedes behave as whole-body spring-mass systems and not like rolling wheels. Furthermore a WFS was required for the ground reaction force and energy patterns to complete full cycles. The WFS is the virtual leg stride of centipedes. Many-legged, laterally-undulating, metachronally-gaited centipedes behave as spring-mass systems just as symmetrically-gaited animals with fewer legs do.