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Evidence for a time-invariant phase variable in human ankle control.

Gregg RD, Rouse EJ, Hargrove LJ, Sensinger JW - PLoS ONE (2014)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Departments of Mechanical Engineering and Bioengineering, University of Texas at Dallas, Richardson, Texas, United States of America.

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.

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Diagram of robotic force plate dorsiflexing the stance foot.The platform’s center of rotation coincides with the ankle joint. The center of pressure (COP) is a candidate phase variable that can be measured by the force plate of the perturbation mechanism.
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pone-0089163-g001: Diagram of robotic force plate dorsiflexing the stance foot.The platform’s center of rotation coincides with the ankle joint. The center of pressure (COP) is a candidate phase variable that can be measured by the force plate of the perturbation mechanism.

Mentions: If human locomotion was similarly controlled with a phase variable, your neuromuscular system would sense a shift in the variable caused by a slope change and your ankle would respond according to the new phase location. We postulate that human locomotion is in fact controlled in this time-invariant manner, employing feedback related to a single state variable as a continuous representation of phase. Given substantial evidence that load-related receptors are involved in the initiation of phase-specific behaviors during human locomotion [5],[28]–[30], we hypothesize that human joint patterns depend on the heel-to-toe movement of the Center of Pressure (COP)–the point on the plantar sole of the foot (Figure 1) where the resultant reaction force is imparted against the ground, i.e., where all moments sum to zero [31]–[33].


Evidence for a time-invariant phase variable in human ankle control.

Gregg RD, Rouse EJ, Hargrove LJ, Sensinger JW - PLoS ONE (2014)

Diagram of robotic force plate dorsiflexing the stance foot.The platform’s center of rotation coincides with the ankle joint. The center of pressure (COP) is a candidate phase variable that can be measured by the force plate of the perturbation mechanism.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3928429&req=5

pone-0089163-g001: Diagram of robotic force plate dorsiflexing the stance foot.The platform’s center of rotation coincides with the ankle joint. The center of pressure (COP) is a candidate phase variable that can be measured by the force plate of the perturbation mechanism.
Mentions: If human locomotion was similarly controlled with a phase variable, your neuromuscular system would sense a shift in the variable caused by a slope change and your ankle would respond according to the new phase location. We postulate that human locomotion is in fact controlled in this time-invariant manner, employing feedback related to a single state variable as a continuous representation of phase. Given substantial evidence that load-related receptors are involved in the initiation of phase-specific behaviors during human locomotion [5],[28]–[30], we hypothesize that human joint patterns depend on the heel-to-toe movement of the Center of Pressure (COP)–the point on the plantar sole of the foot (Figure 1) where the resultant reaction force is imparted against the ground, i.e., where all moments sum to zero [31]–[33].

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Departments of Mechanical Engineering and Bioengineering, University of Texas at Dallas, Richardson, Texas, United States of America.

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.

Show MeSH
Related in: MedlinePlus