Bipedal Walking Research Papers - Academia.edu (original) (raw)

To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upcoming ground surface without interfering with the exploitation of passive mechanical forces. We propose that walkers use visual information to initialize the mechanical state of the body before the beginning of each step so the resulting ballistic trajectory of the walker's center-of-mass will facilitate stepping on target footholds. Using a precision stepping task and synchronizing target visibility to the gait cycle, we empirically validated two predictions derived from this strategy: (1) Walkers must have information about upcoming footholds during the second half of the preceding step, and (2) foot placement is guided by information about the position of the target foothold relative to the preceding base of support. We conclude that active and passive modes of control work synergis-tically to allow walkers to negotiate complex terrain with efficiency, stability, and precision. human locomotion | visual control | foot placement | biomechanics | inverted pendulum H umans and other animals are remarkable in their ability to take advantage of what is freely available in the environment to the benefit of efficiency, stability, and coordination in movement. This opportunism can take on at least two forms, both of which are evident in human locomotion over complex terrain: (i) harnessing external forces to minimize the need for self-generated (i.e., muscular) forces (1), and (ii) taking advantage of passive stability to simplify the control of a complex movement (e.g., ref. 2). In the ensuing section, we explain how walkers exploit external forces and passive stability while walking over flat, obstacle-free terrain.* We then generalize this account to walking over irregular surfaces by explaining how walkers can adapt gait to terrain variations while still reaping the benefits of the available mechanical forces and inherent stability. This account leads to hypotheses about how and when walkers use visual information about the upcoming terrain and where that information is found. We derive several predictions from these hypotheses and then put them to the test in three experiments. Passive Control in Human Walking The basic movement pattern of the human gait cycle arises primarily from the phasic activation of flexor and extensor muscle groups by spinal-level central pattern generators, regulated by sensory signals from lower limb proprioceptors and cutaneous feedback from the plantar surface of the foot. This low-level neuromuscular circuitry serves to maintain the rhythmic physical oscillations that define locomotor behavior (see ref. 3 for review). This section will provide an overview of the basic biomechanics of the bipedal gait cycle to show how these inherent physical dynamics contribute to the passive stability and energetic efficiency of human locomotion. During the single support phase of the bipedal gait cycle, when only one foot is in contact with the ground, a walker shares the physical dynamics of an inverted pendulum. The body's center of mass (COM) acts as the bob of the pendulum and is supported above a single point of rotation in the planted foot (4, 5). At the onset of the single support phase—that is, at " toe off, " when the nonsupporting leg breaks contact with ground—momentum carries the COM up to a maximum height at midstance and then down again until the swinging foot contacts the ground, which is called " heel strike " (Fig. 1A). As the COM increases its height during the first half of the single support phase, some of the kinetic energy of the COM is transferred into potential energy, which is then transferred back into kinetic energy as the COM drops down to its original height in the latter half of the step. In the ideal case, the exchange between kinetic and potential energy is perfectly lossless and symmetric , and human walking closely approximates an idealized inverted pendulum acting conservatively. As such, humans can harness gravity and inertia and exploit the inverted pendulum dynamics of their bodies to traverse the distance traveled during the single support phase at a minimal cost in terms of work and muscle force (6, 7). Of course, in reality walking does incur costs. A primary determinant of the metabolic cost of walking is restoration of the energy lost when the swinging foot contacts the ground (refs. 8 and 9, but see ref. 10). Ground contact by the swinging foot marks the end of the single-support phase and the beginning of the double-support phase, during which the COM must be redirected from a downward trajectory to the upward trajectory it will need for the coming step (11). Upon ground contact, the force applied by the leading leg has a horizontal component opposite to the direction of motion of the COM; that is, the leading leg performs negative work on the COM. This collision is Significance The physical dynamics of the body are central to the generation and maintenance of the human gait cycle. The ability to exploit the force of gravity and bodily inertia increases the energetic efficiency of locomotion by minimizing the need for internally generated muscular forces and simplifies control by obviating the need to actively guide each body segment. Here we explore how these principles generalize to situations in which foot placement is constrained, as when walking over a rocky trail. Walkers can exploit external forces to efficiently traverse extended stretches of complex terrain provided that visual information about the up-coming ground surface is available during a particular (critical) phase of the gait cycle between midstance of the preceding step and toe-off. *The forthcoming description of the human gait cycle is an adaptation of the " simplest walking model, " which is an attempt to develop the most parsimonious description of bipedal walking that still captures key characteristics of human locomotion (13, 71, 72). Although more complete musculoskeletal models can be invaluable for determining the contributions of individual muscles to locomotor behavior (63), such models are compu-tationally expensive and conceptually difficult to analyze. By abstracting the highly complex action of human locomotion down to the simplest possible biomechanical model, it is possible to explore how the control of human locomotion is organized around the underlying physical dynamics of bipedal gait (16, 73–75).