Two types of slip-induced falls among community dwelling older adults

Abstract

Little is known about the landing behavior of the trailing (recovery) foot and ensuing types of falls following a forward slip in walking. The purposes of this study were to (1) determine if community-dwelling older adults experienced bilateral slips at the same rate as had been previously observed for young adults during over-ground walking; (2) determine if fall rate in older adults was dependent on slip type (unilateral vs. bilateral); and (3) identify differences in spatiotemporal variables of the trailing leg step between unilateral and bilateral slips. One-hundred-seventy-four participants experienced an unannounced, unrehearsed slip while walking on a 7-m walkway. Each trial was monitored with a motion capture system and bilateral ground reaction force plates. Although the experimental design, developed with original data from a young adult population, favored bilateral slips, more older adults (35%) than anticipated (10% previously observed in young, p<0.001) displayed a unilateral slip. The probability of fall was equal in the two types of slips. Eighty-two people recovered from the slip, while the remaining 92 (53%) fell. These 92 were classified into two exclusive categories based on the heel distance at the time of fall arrest using cluster analysis: those which resembled a fall into a "splits" position (n=47) or a feet-forward fall (n=45). All (100%) unilateral slips led to splits falls, as expected. Yet, not all bilateral slips (only 83%) resulted in feet-forward falls. A longer forward recovery step with a prolonged step time led to both feet slipping, nearly together, hence a feet-forward fall.

Fig. 1

Typical profile of vertical ground reaction forces (GRF) for subjects who aborted their recovery (left) step after the slip onset, which is approximately 30ms after right (slipping) foot touchdown (RTD) on the platform. The recovery foot did not completely take off from the ground to accomplish the recovery the step. Correspondingly, the vertical GRF under the left foot after slip onset first decreased (unloading phase) and then increased (re-loading phase) before it became zero. The instant of the minimum vertical GRF (point A) under the left foot was identified as recovery foot liftoff (LLO) and touchdown (LTD).

Fig. 2

Stick-figure animation sequence of a representative (a) feet-forward faller and (b) splits faller in responding to a slip induced in gait. The solid and dashed lines, respectively, indicate the right and left sides. The triangle is the starting position of the right heel at right foot touchdown. The five frames on each panel respectively correspond with the instants of the right foot touchdown (RTD), left liftoff (LLO), left touchdown (LTD), the middle frame between LTD and fall, and the instant of fall. Also shown is the definition of the heel-to-heel distance at fall instant.

Fig. 3

(a) The distribution of the left heel landing location relative to the right heel (0) for all fallers at left touchdown. (b) The distribution of the relative heel landing location at fall arrest for all fallers. Each symbol represents a subject. Cluster analysis successfully classified these distances into two clusters, as feet-forward (squares) and splits (circles) falls. The thin horizontal line indicates the threshold value of the recovery step length categorizing the fall type. On (a) and (b), open circles indicate the splits falls with unilateral slip while shaded circle represents splits fallers with bilateral slip. (c) The comparison of the heel distance at fall between the feet-forward falls and splits falls. The heel distance is normalized to body height (bh).

Fig. 4

Group means (column height) and standard deviations (bar) of the elapsed time between events in seconds for feet-forward and splits falls. The events include right foot touchdown (RTD), left foot liftoff (LLO), left foot touchdown (LTD), and the instant of fall.

Fig. 5

The comparison of the heel-to-heel distance at left (recovery) foot touchdown (LTD), and the recovery stride length from left liftoff (LLO) to LTD between the feet-forward fall and splits fall groups. All lengths are normalized to body height (bh). Also shown are the definitions of heel-to-heel distance at LTD, and recovery stride length of the recovery limb (shaded) from LLO to LTD. The feet are indicated by solid and dashed lines respectively at LLO and LTD.

Fig. 6

Comparison of (a) the heel-to-heel distance, (b) the trunk angle, and (c) the hip height at different events between feet-forward and splits fall groups. The events include right foot touchdown (RTD), left foot liftoff (LLO), left foot touchdown (LTD), and the instant of fall. Both heel-to-heel distance and hip height are normalized to body height (bh). Positive trunk angle represents that the trunk leans backward against the vertical line.