K. Doesburg, T. Leahy, S. Thompson, M. Clark, E. Eckmann, J. Portillo, C. Stahl, R. McCulloch

Gonzaga University, Spokane, WA

PURPOSE: To determine the effects of added weight on stability at the hip, knee, and ankle joints of both the trailing and leading leg while stepping down. METHODS: Twenty Gonzaga female subjects with a healthy BMI (BMI: 22.0 ±1.4 kg*m-2) completed 3 trials of the following conditions: 0, 10, 20, and 30% added body weight (BW). Subjects stepped down onto a force plate from a 17 cm raised platform, a standard step height, at a pace of 88 min-1. Force plate data was collected to determine the effects of added weight on postural stability. Force plate variables included: anterior-posterior forces, ground reaction force, and center of pressure (CoP) range and speed. CoP can be used to quantify postural sway by evaluating an individual’s ability to control their center of mass over their base of support, therefore was used as an indicator of stability. Two-dimensional motion analysis was used to find trunk, hip, knee, and ankle angles of both the trailing and leading leg. Statistical analysis included t-tests, regression analyses, and correlation matrices. RESULTS: Force plate data indicated that with increased weight, subjects had greater anterior-posterior sway and ground reaction forces (p < 0.05). CoP speed positively correlated with BW (p<0.05). Joint kinematic data showed that the max hip angle of the trailing leg increased by 3.6° from 0 to 30% added BW and by 2.0° from 20 to 30% added body weight (p< 0.05). Max ankle angle of the leading leg increased by 3.1° from 20 to 30 % (p< 0.05). A positive correlation between increased BW and max hip angle of the trailing leg was found. DISCUSSION: The current study revealed that increasing BW by 30% yielded an increase in CoP speed, suggesting greater instability. Kinematic data suggested that 30% added BW caused a proportionally greater loading effect than 20% BW on hip and ankle joints. The small but significant changes in some of the measured joint angles indicated that small changes in angles may have large implications for stability during stair descent. CONCLUSION: The findings imply that the maximum added load should be limited to no more than 20% of an individual’s healthy BW. Increasing BW from above 30% may induce larger changes in joint angle compensations and compromise postural stability.

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