Generalization of gait adaptation for fall prevention: from moveable platform to slippery floor

Abstract

A person's ability to transfer the acquired improvements in the control of center of mass (COM) state stability to slips induced in everyday conditions can have profound theoretical and practical implications for fall prevention. This study investigated the extent to which such generalization could take place. A training group (n=8) initially experienced 24 right-side slips in blocked-and-random order (from the 1st unannounced, novel slip, S-1 to the last, S-24) resulting from release of a low-friction moveable platform in walking. They then experienced a single unannounced slip while walking on an oil-lubricated vinyl floor surface (V-T). A control group (n=8) received only one unannounced slip on the same slippery floor (V-C). Results demonstrated that the incidence of balance loss and fall on V-T was comparable to that on S-24. In both trials, fall and balance-loss incidence was significantly reduced in comparison with that on S-1 or on V-C, resulting from significant improvements in the COM state stability. The observed generalization indicates that the control of COM stability can be optimally acquired to accommodate alterations in environmental constraints, and it may be broadly coded and easily modifiable within the CNS. Because of such mechanisms, it is possible that the locomotor-balance skills acquired with the aid of low-friction moveable platforms can translate into resisting falls encountered in daily living.

FIG. 1.

Left: kinematic and kinetic characteristics of a typical response to the st, unannounced, novel slip induced by the release of the moveable platform (solid line, S-1, for the subjects in the training group) and that resulting from slippery vinyl floor (dashed line, V-C, for subjects in the control group). Middle: the training effects of the same person (left) were transferred from the last (24th, S-24) slip to the 1st vinyl slip (V-T). Right: the mediolateral constraint from the linear bearing of the moveable platform was removed in the “free” slip on vinyl floor in the same trials as the right panel. The S-1 and V-C trials resulted in loss of balance, whereas the response in S-24 and V-T trials (middle) led to successful recovery. Shown for the left and middle panels from top to bottom are the representative time histories of the following variables: A, anterior-posterior displacement of the slipping heel (x_heel); B, its horizontal velocity (X̄_heel), the ground reaction force normalized to the body weight (bw in kg.m.s^-2) in the (C) anteriorposterior (F_x) and (D) vertical directions (F_z), and (E) the ratio F_x/F_z or F_y/F_z, which cannot exceed surface friction coefficient. This ratio can increase, however, if the surface coefficient of friction changes due to the distribution of oil-based lubricant applied on the vinyl surface as seen during later part of the trial. Shown in the right panel are the same variables but in the mediolateral direction. The vertical thick gray line indicates the time of slipping (right) limb touchdown (R-TD), which approximates slip onset. The thin-solid and thin-dash vertical lines, respectively, indicate the time of contralateral liftoff for the S-1 (or S-24) and V-C (or V-T), respectively. Note positive values in A and B for the left and middle panels indicate forward heel displacement, whereas for the right panel, it indicates heel displacement to the left. Positive ground reaction force is in anterior (F_x), medial (to the left; F_y), or upward (F_z) direction.

FIG. 2.

The mean incidence of fall (A) and loss of balance (B) for the training group (•) on the 1st (S-1) and the last (S-24) slip induced by the release of the low-friction moveable platform and on the 1st slip on the slippery vinyl floor. Also shown is the fall and balance loss incidence for control group (□) on the 1st vinyl slip. Significant comparisons at P < 0.05 are indicated (*).

FIG. 3.

The means ± SD of preslip (A) and postslip stability (B) for the training group (•) on the 1st (S-1) and the last (S-24) slip induced by the release of the low-friction moveable platform and the transfer slip on the slippery vinyl floor. Also shown is the stability for control group (□) on the first vinyl slip. Significant comparisons at P < 0.05 are indicated (*).

FIG. 4.

The means ± SD of stability at touchdown (a) of slipping/leading limb and liftoff of contralateral limb for the training group on the following trials: regular, unperturbed walking trial preceding the 1st platform slip (Reg), the first platform slip (S-1), the nonslip trial (i.e., trial 36) preceding the last training slip (NS), the last (i.e., 37) training slip (S-24), the regular, unperturbed walking vinyl baseline trial (Reg-VB), the regular, unperturbed walking vinyl trial preceding the vinyl slip (Reg-V) and the transfer vinyl slip (V-T). Note for the slip trials, the stability at touchdown and liftoff respectively equals preslip and postslip stability. For the vinyl baseline trials, subjects were explicitly informed that a slip would not occur. Significant comparisons at P < 0.05 are indicated (*). To maximize clarity, some significant differences are not indicated (e.g., Reg vs. Reg-VB and Reg vs. Reg-V are significant with (P < 0.05).

FIG. 5.

The means and (SD) of (a) anterior-posterior, slipping heel displacement (X_heel) and (b) its velocity (X̄_heel) at postslip, contralataral limb liftoff. The training group's results (filled circles) were reported on the first (S-1), the last (S-24) moveable platform slips, and the transfer vinyl slip, together with those of the control group's vinyl slip (open square). Significant comparisons at P < 0.05 are indicated with an *.