SINGLE LEG SWAY QUANTIFICATION IN ABLE-BODIED ADULTS

Abstract

Researchers continue to use laboratory reference frames for center of pressure (COP) postural sway excursion quantification. PURPOSE: We suggest that the base of support principal axis as defined by the foot dimensions, talocrural and subtalar axes, and the medial longitudinal arch axis can affect the change in location and acceleration of the center of mass, and thus can effect the principal sway direction during single leg stance. Based on these anatomical characteristics we hypothesize that the principal sway direction will be oriented from posterior and medial to anterior and lateral with respect to the anatomical foot reference frame. METHODS: Thirty able-bodied young adults (7 males and 23 females) were recruited from the general community (mean age 27 ± 6 years). The subjects were asked to maintain a still position standing on the right limb, while maintaining the left limb flexed off the ground, for a 30 s period. Two successful trials were collected and analyzed for each subject, and sufficient time was provided between trials to avoid fatigue. Three dimensional forces and torques acquired at 600 Hz with an AMTI force plate were used to calculate the location of the COP within the base of support. Simultaneous kinematic data were collected at 60 Hz using a Vicon 370 system to establish an anatomical foot reference frame as defined by 3 position markers placed on the calcaneus, the head of the second metatarsal bone and the lateral malleolus. The sway direction was identified by the principal axis of a 95% best-fit ellipse to the COP scatter plot. RESULTS: The mean sway direction was determined to be 31.5 ± 17.4° from the anteroposterior axis of the anatomical foot reference frame in the hypothesized direction. The test-retest reliability (ICC) of the principal sway direction was 0.71. Given the established link between the postural sway characteristics and the balance ability of older adults, the frequency of injuries during single leg support in dynamic activities, and the fact that almost 80% of the stride duration during gait is in single leg stance, the methodology used for stability quantification becomes extremely important. CONCLUSION: Our data suggest that postural sway parameters for stability purposes need to be expressed in the principal direction of sway relative to anatomical foot reference frame instead of simply behavioral or laboratory frames. Supported by Women in Science Program, Indiana University