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Slip-initiated falls often cause injury and death, especially in older adults. Previous research involving perturbed walking has to a large extent focused on lower extremity reactions, yet arm responses are often part of postural reactions to such perturbations. This research is focused on arm responses to an unexpected slip. In Aim 1, the relationship between slip severity and shoulder biomechanics was examined. In Aim 2, we determined if the vestibular system is involved in the triggering of arm responses. The correlation between shoulder moment magnitude and the extent of the body center of mass (COM) perturbation was examined in Aim 3.
Subjects (17 younger and 12 older adults) were exposed to two conditions: (1) baseline dry (subjects knew the floor was dry), and (2) unexpected slip (a diluted glycerol solution was spread on the floor beneath the stance/left foot). Shoulder Euler angles and moments in flexion/extension and abduction/adduction were derived. The spherical elevation angle was used to further describe shoulder kinematics. Slip severity was quantified using measures reported in the literature.
Although arm responses were bilateral, only left (side of slipping foot) shoulder biomechanics, specifically moment generation rates, spherical elevation angle, and abduction angle were positively correlated with slip severity. Left shoulder responses were triggered later than left hip and knee responses. Delayed shoulder moment onsets, slower abduction moment generation rate, and reduced range of motion were found in older adults compared to their younger counterparts. Aim 2 results indicated a weak but statistically significant positive relationship between the timing of the slip-initiated downward head acceleration and the onset of the left shoulder flexion/extension moment (true only when slip severity was controlled in the analysis). In Aim 3, increased left shoulder flexion generation rate correlated with decreased COM perturbation.
In conclusion, evidence presented in this study implies (1) arm responses play a role in balance recovery, (2) a legs-to-arms response sequence appears to drive the reaction to a slip, although the potential implication of the vestibular system cannot be ruled out, (3) age-related effects on arm responses may aggravate the risk of slips and falls in older adults.