Impaired reactive stepping adjustments in older adults

Abstract

Background: The ability to redirect the path of the foot during walking is critical for responding to perturbations and maintaining upright stability. The purpose of the current study was to compare mechanisms of reactive stepping adjustments in young versus older adults when responding to an unexpected perturbation during voluntary step initiation.

Methods: We tested 13 healthy community-dwelling older adults and an equal number of young control participants performing stepping movements onto a visual target on the floor. In some trials, perturbations were introduced by unexpectedly shifting the target, at various time points, from its usual location to a new location 20 cm to the right. We measured ground reaction forces under the supporting leg and three-dimensional kinematics of the stepping leg in baseline and target shift trials.

Results: During target shift trials, that is, when reactive adjustments were required, older adults demonstrated the following: delayed responses in modifying the lateral propulsive forces under the supporting foot, reduced rates of lateral force production, delayed responses in modifying the stepping foot trajectory, and prolonged movement execution times.

Conclusions: The current study quantitatively distinguishes between healthy older and young adults in generating reactive stepping adjustments to an unpredictable shift of a visual target. The decreased capability for rapidly planning and executing an effective voluntary step modification could reveal one potential cause for the increased risk of falls in the older population.

Figure 1.

(A) Overhead view of the experimental setup for baseline and target shift trials. The supporting leg was on a force plate (gray rectangle). After an auditory “ready” cue, the appearance of the visual target acted as the “go” cue. In baseline trials, the target (filled circle) was positioned directly in front of the stepping foot. In target shift trials, the target unpredictably and instantaneously shifted from its initial location to a new location 20 cm to the right. (B) Lateral and vertical ground reaction forces under the supporting foot and movement trajectories of the stepping foot in the medial/lateral direction from a typical young control participant during a baseline (left) and target shift (right) trial. Vertical lines delineate the boundaries of the response time (RTP), weight transfer (WTP), and step execution (SEP) phases. Response time phase = time from initial target illumination to onset of the first decrease in the vertical force under the supporting leg to below 5% of the mean vertical force during quiet standing. Weight transfer phase = time from the end of response time phase to onset of heel-off on the stepping leg. The weight transfer time encompasses the very well characterized and stereotyped lateral weight shift, first toward the stepping side, then the supporting side, that rapidly unloads the stepping leg before step initiation (–22). Stepping execution phase = time from the end of weight transfer phase to subsequent initial contact of the stepping leg onto the target. Dashed vertical line indicates the time of the target shift (in this case, 550 ms after initial target illumination).

Figure 2.

Stepping foot endpoint accuracy. Average step accuracy across all conditions and groups. Accuracy is depicted as the medial/lateral error in the foot endpoint position. Negative numbers indicate errors in the leftward (undershoot) direction. Error bars, ±1 SEM.

Figure 3.

Movement phase durations. Average duration of the (A) response time phase, (B) weight transfer phase, and (C) stepping execution phase across all conditions and groups. Asterisks show specific significant post hoc differences. Error bars, ±1 SEM.

Figure 4.

Stepping foot trajectory data. Individual foot trajectories in the medial/lateral direction for each target shift condition from a typical (A) young and (B) older participant. Time is shown as a percentage of the total step trial. Onset times of the target shifts in each of the three target shift conditions are indicated by filled inverted triangles. (C) Average time to initiate a stepping foot trajectory modification following a target shift, shown for all target shift conditions and groups. Asterisks show specific significant post hoc differences. Error bars, ±1 SEM.

Figure 5.

Support foot force data. Individual lateral ground reaction forces under the support foot for each target shift condition from a typical (A) young and (B) older participant. Time is shown as a percentage of the total step trial. Onset times of the target shifts in each of the three target shift conditions are indicated by filled inverted triangles. Average (C) time to initiate a lateral force modification, (D) peak lateral force amplitude, and (E) rate of lateral force production under the supporting foot, shown for all target shift conditions and groups. Asterisks show specific significant post hoc differences. Error bars, ±1 SEM.