Abstract
Slip-induced falls in gait often have devastating consequences. The purposes of this study were 1) to select the determinants that can best discriminate the outcomes (recoveries or falls) of an unannounced slip induced in gait (and to find their corresponding threshold, i.e., the limits of recovery, which can clearly separate these two outcomes), and 2) to verify these results in a subset of repeated-slip trials. Based on the data collected from 69 young subjects during a slip induced in gait, nine different ways of combining the center of mass (COM) stability, the hip height, and its vertical velocity were investigated with the aid of logistic regression. The results revealed that the COM stability ( s) and limb support (represented by the quotient of hip vertical velocity to hip height, S hip) recorded at the instant immediately prior to the recovery step touchdown were sufficiently sensitive to account for all (100%) variance in falls, and specific enough to account for nearly all (98.3%) variability in recoveries. This boundary ( S hip=−0.22 s−0.25), which quantifies the risk of falls in the stability–limb support quotient ( s– S hip) domain, was fully verified using second-slip and third-slip trials ( n=76) with classification of falls at 100% and recoveries at 98.6%. The severity of an actual fall is likely to be greater further below the boundary, while the likelihood of a fall diminishes above it. Finally, the slope of the boundary also indicates the tradeoff between the stability and limb support, whereby high stability can compensate for the insufficiency in limb support, or vice versa.
Highlights
Falls are a major cause of injury in older adults (Kannus et al, 2005)
The limits of stability, which differentiate backward balance loss and no balance loss in the center of mass (COM) state space, have been recently established (Pai and Iqbal, 1999; Pai and Patton, 1997; Yang et al, 2007; Yang et al, 2008). This stability measure could accurately predict that a backward balance loss must occur when COM motion state locates below the limits of stability (Bhatt and Pai, 2008b; Pai, 2003)
Hip height, hip vertical velocity, and limb support quotient continued to deteriorate during single-stance phase to their lowest point at L-TDpre (p < 0.001 for all variables) (Table 2, Fig. 3)
Summary
Falls are a major cause of injury in older adults (Kannus et al, 2005). Falls initiated by slip account for about 25% of all falls (Luukinen et al, 2000). As illustrated in our previous research, both center of mass (COM) stability and limb support against gravity play critical role in determining a fall during slip in gait. It is still unclear, whether these two factors or their combinations can quantify the boundary (i.e. the limits of recovery) that can clearly separate the falls from recoveries. The limits of stability (thick line in Fig. 1), which differentiate backward balance loss and no balance loss in the COM state space (i.e., its position and velocity), have been recently established (Pai and Iqbal, 1999; Pai and Patton, 1997; Yang et al, 2007; Yang et al, 2008). The limits of recovery against risk of falls are yet to be established
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