Abstract
The study purpose was to investigate whether older adults could improve their stability against a backward loss of balance (BLOB) after receiving repeated treadmill slips during walking and to see how such adaptive changes would be affected by practice dosage (combination of slip intensity and the number of slips at each intensity). Twenty-five healthy community-dwelling older adults received forty treadmill slips given over eleven blocks at five intensities (P1-P1-P2-P3-P4-P5-P4-P5-P5-P3-P1, larger number indicating higher intensity). Center of mass (COM) stability was calculated as the shortest distance of the instantaneous COM position and velocity relative to the base of support (BOS) from a theoretical threshold for BLOB (larger stability value indicated a better stability against BLOB). Stability, step length, and trunk angle were measured before and after slip onset to reflect proactive and reactive control, respectively. The first slips at each intensity block (i.e., P1, P3, P4, and P5) were compared with the first slips in the last blocks at those intensities to examine main effects of training dosage (intensity and repetition). Improvements in proactive and reactive stability were more pronounced for receiving more slips at larger intensities than fewer slips at smaller intensities. Older adults only demonstrated partial positive scaling effects to proactively, not reactively, establish a more stable initial COM state. The improved proactive stability was associated with an anterior shift of COM position relative to the BOS, resulting from a shorter pre-slip step length. The improved reactive stability was associated with an anterior shift of COM position, resulting from a larger compensatory step length and a faster COM velocity relative to the BOS. Our findings indicated that treadmill-gait slip perturbations elicited similar proactive and reactive control to that from over-ground slip perturbations, but greater slip intensity and repetition might yield more immediate adaptive improvements.