The Gap between Walking and Running

The Gait that closes the Gap

The mostly used gaits of human locomotion are walking and running. In walking the important characteristics are that always one or both legs have ground contact, and the center of mass is lifted during a single leg stance phase. In running exist true flight phases and the center off mass reaches a minimum during stance phase. In order to change from one gait to another, an abrupt switching is observed. Maybe, there exists no smooth transition over many steps. The bipedal spring-mass model can reproduce both gaits, but there were significant differences in velocity, respectively system energy observed. However, the observations had a limited view because the simulation results were reduced to self-stable gait solutions. A more general investigation on model predictions should focus on periodic gait solutions and could propose additional strategies for stabilizing the identified gait patterns.

In order to investigate periodic gait patterns for analyzing walking and running, a common platform for gait simulations is required. As mentioned, the bipedal spring-mass model is able to show both gaits and using the methodology of Poincaré maps, periodic solutions can be identified. For applying Poincaré maps, the definition of start and stop conditions is necessary, which should exist in both gaits similarly. A useful start and stop event was established in the Locomotion Lab Jena, namely the instant of Vertical Leg Orientation (VLO). The issue regarding VLO is explained in another article.

Does there still exist a gap between walking and running when focussing on periodic gait solutions?

Classification of the gaits. R: running, W2,W3: walking, GR: grounded running.

Previously, it was known that walking operates at low velocities only. When increasing the energy respectivley the energy, the body will lift off at some time. Due to that reason, the following investigation will be focussed on gaits at low velocities. Analyses on running revealed that it is easy to find running at low speeds. It is depends mainly on the setting of the leg’s angle of attack $\alpha_0$, allowing for running or not. With our novel map for gait solutions, the systems variables at the instant of VLO are shown. Here, we focus on symmetric gait patterns, which means that the first half of the stance phase is symmetrically identical to the second half. The system state is then reduced to a single, independent variable, i.e. the height of the center of mass $y_0$ at the event of VLO. In the figure, two system parameters, i.e. the leg’s angle of attack $\alpha_0$ and the system energy (thick lines) were varied.

The simulation results for typical walking (green area, W2) reveal a height of the center of mass at midstance $y_0$ always above the height at touch down $y_{TD}$. The difference between them can be very small. In contrast, the center of mass is always lowered in running (blue area, R) with $y_0 < y_{TD}$. There is another difference regarding the angle of attack $\alpha_0$ found simulations with the same system energy. In conclusion, the gap between walking and running clearly exists.

Is there a chance to reduce the gap between the typical gaits of walking and running?

Inside the model simulations exists one condition, not explained so far. It is implied that the center of mass moves downward at the event of leg’s touch down. It feels like curious when a leg with a fixed angle at touch down would approaching the ground from a lower level. From now we reject this thought and allowing the center of mass to move upward when the leg is touching the ground. What happens with the leg? In fact, the simulated leg is not defined when it does not have ground contact. We assume of course that the leg will lengthen or rotating before hitting the ground.

With this modified assumption, the center of mass is lifted during touch down, we can identify a novel gait (orange area), which connects both, walking and running. In this gait, we observe that the center off mass is clearly lowered during single leg stance phase as usual in running. However, there exists a distinct double support phase, typical for walking.

The gait called Grounded Running. Motion of the center of mass and the ground reaction forces with distinct double support phases of the legs.

With the spring-mass model, a novel gait is identified, that could be classified as a running gait where $y_0 < y_{TD}$ although there is always ground contact respectively no flight phase exists. Based on simulations, this is a new gait, but it is already observed in locomotion of birds and is called Grounded Running. The simulations have shown, that grounded running exists for low velocities only. In contrast, the same simulation study revealed that walking can be faster than assumed.


Featured Paper

J. Rummel, Y. Blum, A. Seyfarth.
From walking to running.
Autonome Mobile Systeme 2009, R. Dillmann, J. Beyerer, C. Stiller, J.M. Zöllner, T. Gindele (Eds.) Springer: 89-96, 2009.
DOI: 10.1007/978-3-642-10284-4_12