2 research outputs found
Evidence for a Time-Invariant Phase Variable in Human Ankle Control
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.United States Army Medical Research Acquisition Activity (USAMRAA grant W81XWH-09-2-0020)National Institute of Neurological Disorders and Stroke (U.S.) (NIH award number F31NS074687)Burroughs Wellcome Fund (Career Award at the Scientific Interface
Josef v. Karabacek's letter to Ignaz Goldziher
The cyclic and often linear torque-angle
relationship of locomotion presents the opportunity to innovate
on the design of traditional series-elastic actuators (SEAs). In
this paper, a novel modification to the SEA architecture was
proposed by adding a clutch in parallel with the motor within
the SEA—denoted as a CSEA. This addition permits bimodal
dynamics where the system is characterized by an SEA when
the clutch is disengaged and a passive spring when the clutch is
engaged. The purpose of the parallel clutch was to provide the
ability to store energy in a tuned series spring, while requiring
only reactionary torque from the clutch. Thus, when the clutch
is engaged, a tuned elastic relationship can be achieved with
minimal electrical energy consumption. The state-based model
of the CSEA is introduced and the implementation of the
CSEA mechanism in a powered knee prosthesis is detailed. The
series elasticity was optimized to fit the spring-like torqueangle
relationship of early stance phase knee flexion and
extension during level ground walking. In simulation, the
CSEA knee required 70% less electrical energy than a
traditional SEA. Future work will focus on the mechanical
implementation of the CSEA knee and an empirical
demonstration of reduced electrical energy consumption
during walking.United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship Award 1122374