Arm Movements and Slip Severity

Abstract

Slip-initiated falls often cause occupational injuries and deaths, especially in older workers. Previous slips and falls research has to a large extent focused on lower extremity reactions, yet arm responses are often part of postural reactions to such perturbations. It is unclear if arm responses play a role in balance recovery, are modulated by the severity of the postural perturbation and/or are a reflex-type response, e.g. reaching for external body support. In this study, the relationship between slip severity and shoulder biomechanics was examined. 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 to slip the leading/left foot). Bilateral sagittal plane kinematics and kinetics were derived. Slip severity was quantified using a measure of slip hazardousness based on the peak slip velocity (PSV) measured at the heel of the slipping foot. Specifically, if PSV ≥ 1 m/s, then the slip was classified as hazardous. Although arm responses were bilateral, only the biomechanics of the shoulder ipsilateral to the slipping foot, specifically moment generation rate, were affected by slip hazardousness. Specifically, a hazardous slip was associated with an extensor moment at the shoulder ipsilateral to the slipping foot, whereas a non-hazardous slip was associated with a flexor moment. Shoulder responses were triggered later than the hip and knee response based on moment onset data. Finally, overall, older adults appeared to generate a greater extensor moment at the shoulder compared to the response seen in the younger group of participants. In conclusion, evidence presented in this study implies that (1) arm responses play a role in balance recovery but also may be protective in nature when experiencing a severe slip, (2) a legs-to-arms response sequence appears to drive the reaction to a slip, and (3) older adults may use their arms as a protective strategy to a greater extent than their younger counterparts.