Abstract
Human locomotion is a rhythmic task in which patterns of muscle activity are modulated by state-dependent feedback to accommodate perturbations. Two popular theories have been proposed for the underlying embodiment of phase in the human pattern generator: a time-dependent internal representation or a time-invariant feedback representation (i.e., reflex mechanisms). In either case the neuromuscular system must update or represent the phase of locomotor patterns based on the system state, which can include measurements of hundreds of variables. However, a much simpler representation of phase has emerged in recent designs for legged robots, which control joint patterns as functions of a single monotonic mechanical variable, termed a phase variable. We propose that human joint patterns may similarly depend on a physical phase variable, specifically the heel-to-toe movement of the Center of Pressure under the foot. We found that when the ankle is unexpectedly rotated to a position it would have encountered later in the step, the Center of Pressure also shifts forward to the corresponding later position, and the remaining portion of the gait pattern ensues. This phase shift suggests that the progression of the stance ankle is controlled by a biomechanical phase variable, motivating future investigations of phase variables in human locomotor control.
Highlights
Imagine walking along a riverbed and stepping on a rock that slips beneath your heel
Certain sources of state-dependent feedback appear to 1) gate discrete control events, such as the initiation of swing based on the hip angle [3,4] and unloading of leg muscles [5,6], and 2) modulate the timing of continuous behaviors [2], e.g., ankle-foot muscle and cutaneous afferents are used for active balance control [7]. These behaviors could be the result of state-dependent feedback modulating an internal timing variable that produces feedforward patterns of muscle activation [8], but it is possible for reflex mechanisms and mechanical self-stabilization to generate locomotor patterns without a time-dependent feedforward contribution [9,10,11]
[34], which was limited to Center of Pressure (COP) and ankle measurements, we focused our hypotheses on the response of the ankle joint
Summary
Imagine walking along a riverbed and stepping on a rock that slips beneath your heel. Your ankle normally dorsiflexes later in the walking gait (i.e., it would have encountered the dorsiflexed position anyway), so the response of minimal intervention [1] would be to continue from the later position and follow the remaining portion of the normal pattern This shift in phase–or location in the gait cycle–highlights the difference in controlling locomotor patterns with respect to the current state rather than the current time. Certain sources of state-dependent feedback appear to 1) gate discrete control events, such as the initiation of swing based on the hip angle [3,4] and unloading of leg muscles [5,6], and 2) modulate the timing of continuous behaviors [2], e.g., ankle-foot muscle and cutaneous afferents are used for active balance control [7]. The key difference is whether the underlying representation of phase in the human locomotor pattern is timedependent or time-invariant
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