Co-Principal Investigators: Steven Gard, PhD, and Dudley Childress, PhD
Student Investigator: Steven Miff, PhD
Co-Investigators: Margrit Meier, PhD; Andrew Hansen, PhD; and Rebecca Stine, MS
Funded by: National Institute on Disability and Rehabilitation Research (NIDRR) of the U.S. Department of Education under Grant No. H133E030030.
"Quiet" standing and walking (e.g., at a constant mean velocity) are considered "steady-state" activities, meaning that on average the body is neither accelerating nor decelerating. Various aspects of steady-state walking have been analyzed extensively in an attempt to better understand both healthy and pathological gait. However, human locomotion is not entirely a steady-state progression but rather consists of three components: starting, walking, and stopping. The dynamic processes of starting and stopping are much more complex since the human body needs to accelerate and decelerate, respectively, often in a limited amount of time. As a result, the skills necessary to maintain stability, weight transfer, foot clearance, etc., become more critical during these transition phases than during the steady-state conditions. Such requirements become even more significant in people with a lower-limb amputation, where one or multiple joints are missing. Although a large number of scientific studies have examined the various characteristics of steady-state locomotion, only a limited number have analyzed how a person initiates and terminates gait. The focus of this research is to understand how acceleration and deceleration are achieved during gait initiation and termination, in both able-bodied subjects and in people with lower-limb loss. We hypothesize that the process of gait initiation can be characterized through two distinct patterns: a "passive" initial forward fall, followed by an "active" process that leads to steady-state walking.
Gait initiation, stead-state walking and gait termination were analyzed in ten able-bodied subjects and ten people with unilateral transtibial limb loss for slow, normal, and fast walking speeds. A rocker-based inverted pendulum model was developed and used to simulate gait initiation. The relationships predicted by the model compared favorably with the measured data.
We used kinematic and anthropomorphic data to determine the body center of mass (BCOM) position of each subject. The BCOM acceleration was derived by double-differentiating the position data. An averaged BCOM acceleration was calculated by a filtering of the instantaneous acceleration data at a cutoff frequency set by the cadence for elimination of the step-to-step variation. We used this averaged acceleration to calculate the time the volunteers needed to initiate and terminate gait.
We also examined the effective rocker (roll-over shape) behavior of the knee-ankle-foot (KAF) system during gait initiation, steady-state walking (i.e. constant speed gait), and gait termination. The KAF roll-over shapes were characterized by transforming COP data of the 10 able-bodied subjects from a laboratory-based coordinate system into a leg-based coordinate system. The resulting roll-over shapes (effective rockers) were characterized using a circular arc model.
The results indicate that rapid gait termination is in many aspects a "mirror image" of gait initiation. Both able-bodied ambulators and the subjects with unilateral transtibial limb loss initiate and terminate gait in approximately two steps, regardless of the steady-state walking speed (see figure below). Higher acceleration and deceleration magnitudes account for increasing walking speeds. For gait initiation, the invariant time interval can be explained by the phases of this process. The initial conditions set by the ankle and by trunk lean dictate the magnitude and speed of the passive-like forward fall during the first step. During the second step, additional forward momentum is generated by a push from the trailing leg. Compensations in persons with transtibial amputation appear to occur at the hip.
The KAF roll-over shapes exhibit an overall ''flexed'' orientation during the first step of gait initiation and an ''extended'' orientation during the last step of gait termination. The results suggest that gait initiation begins with a realignment of the effective rocker created by the KAF system (i.e. a planar rotation of the KAF roll-over shape). This ''flexed'' re-alignment moves the COP backwards, encouraging the body to roll forward. During rapid termination, the KAF system creates an ''extended'' effective rocker. This rocker re-alignment moves the COP further in front of the ankle, encouraging the body to stop moving forward.
Hansen AH, Miff SC, Childress DS, Gard SA, Meier MR. Net external energy of the biologic and prosthetic ankle during gait initiation. Gait Posture. Jan 2010; 31(1):13-17.
Miff, SC, Hansen, AH, Childress, DS, Gard, SA, and Meier, MR (2008). Roll-over shapes of the able-bodied knee-ankle-foot system during gait initiation, steady-state walking, and gait termination. Gait and Posture, 27:316-322.
Miff, SC, Childress, DS, Gard, SA, Meier, MR, and Hansen, AH (2005). Temporal symmetries during gait initiation and termination in nondisabled ambulators and in people with unilateral transtibial limb loss. Journal of Rehabilitation Research and Development, 42(2):175-182.
Miff, S (2004). "Gait Initiation and Termination in Non-Disabled Ambulators and in People with Unilateral Lower Limb Loss," PhD Dissertation, Biomedical Engineering, Northwestern University.
Miff, S, Childress, DS, Hansen, A, Gard, SA, and Meier, M. (2004). The Behavior of the Knee-Ankle-Foot System During Gait Initiation, Steady State Walking and Gait Termination. 9th Annual Meeting of the Gait and Clinical Movement Analysis Society (CGMAS), Lexington, KY, April 21-24.
Miff, S, Childress, DS, Hansen, A, Gard, SA, and Meier, M. (2004). Temporal-Spatial Symmetries During Gait Initiation and Termination. 9th Annual Meeting of the Gait and Clinical Movement Analysis Society (CGMAS), Lexington, KY, April 21-24.
Miff, SC, Childress, DS, Gard, SA, Meier, M, and Hansen, A. (2003). Initiation of Walking: Passive or Active? 8th Annual Meeting of the Gait and Clinical Movement Analysis Society (GCMAS), Wilmington, DE, May 7-10.