Age-related differences in the control of weight-shifting within the surface of support

BACKGROUND AND AIM: An important reason for falling in elderly is incorrect weight-shifting¹. In many daily life activities quick and accurate weight-shifting is needed to maintain balance, especially in situations when balance is suddenly disturbed and anticipation on the upcoming movement is difficult. Considering the deterioration in postural control in elderly², it is expected that they have more difficulties with executing these quick and accurate weight-transfers³. The present study aims to gain more insight in age-related differences in postural control strategies during a postural control task requiring weight-transfers of different amplitudes and in different directions within the surface of support METHODS: Nine healthy older adults (70.3±6.9 years) and twelve young adults (20.9±0.5 years) participated in the study. The participants performed a weight-shifting task by moving the whole body in different directions to move a cursor, representing real time COP position, towards targets of different sizes and at different distances projected on a screen. Movement time (MT) was the time between the appearance of the goal target and the moment a target switch was realized (i.e. the cursor stayed in the goal target for 0.5 second). The accuracy of the movement was quantified by Counts on Goal (CoG), that is the number of times the cursor hit the goal target before a target switch was realized and by Dwelling Time (DT), the time required to realize a target switch after the goal target was hit by the cursor for the first time. Fluency was expressed by the maximal deviation (MD) of the performed path with respect to the ideal path and the number of peaks (nP), or inflections in the performed path. RESULTS: Significant main effects of target size, target distance and age on all outcome measures were found (p<.01). With decreasing target size, increasing target distance and increasing age, MT significantly increased and fluency and accuracy significantly decreased (nP, MD, CoG and DT increased). Elderly used a slower, less accurate and less fluent weight-shifting strategy compared to younger adults with increasing task difficulty (e.g. decreasing target size and increasing target distance) as indicated by significant interaction effects of size*age and distance*age (p<.05). CONCLUSION: The results of this study provided insight in how elderly control their weight-shifting when the movement cannot be anticipatorily planned. Elderly exhibited slower and more variable movements, especially with increasing task difficulty. This weight-shifting strategy seems characterizing for an increased fall risk in elderly, since the results indicate that elderly might have more difficulties with executing an adequate (quick and accurate) adaptation to a perturbation in daily life. ¹SN Robinovitch et al. Lancet. (2013), 381(9860), 47-54. ²FB Horak. Age Ageing. (2006), 35(2), 7-11. ³V Jongman et al. Stud Health Technol Inform. (2012), 181, 93-97.