Kinematics and dynamics of burst transitions

Subjects (N = 14) were instructed to walk at comfortable walking speed and to start sprinting on an external (visual) stimulus. This is a burst transition. To accelerate maximally, different strategies can be used. The choice for a strategy was hypothesized to be (a) dependent of the body's dynamical status, which is in its turn dependent on the signal timing within the gait cycle; and (b) influenced by the performance and efficacy of the different strategies. Three-dimensional kinematics and ground reaction forces were used to discriminate between strategies and to calculate work (W(total)). Distance laser data yielded performance measures and the work related to the forward acceleration (W(objective)). Efficacy was calculated as the ratio of W(objective) to W(total). Subjects mainly used 2 strategies among others depending on the timing of the stimulus: (a) subjects placed their body center of mass (BCOM) in front of their center of pressure (COP) by tilting the trunk forward and flexing the knee, resulting in a sudden forward acceleration but a relatively fair efficacy; (b) subjects placed their COP behind their BCOM by placing the foot of the swing leg backward. This led to a high performance with high efficacy and was therefore the most ecologically relevant.