Can treadmill-slip perturbation training reduce immediate risk of over-ground-slip induced fall among community-dwelling older adults?

Abstract

The purpose of this study was to determine any potential falls-resistance benefits that might arise from treadmill-slip-perturbation training. One hundred sixty-six healthy community-dwelling older adults were randomly assigned to either the treadmill-slip-training group (Tt) or the treadmill-control group (Tc). Tt received 40 slip-like perturbations during treadmill walking. Tc received unperturbed treadmill walking for 30 min. Following their treadmill session, both groups were exposed to a novel slip during over-ground walking. Their responses to this novel slip were also compared to previously collected data from participants who received either over-ground-slip training (Ot) with 24 slips or over-ground walking (Oc) with no training before experiencing their novel over-ground slip. Fall rates and both proactive (pre-slip) and reactive (post-slip) stability were assessed and compared for the novel over-ground slip in groups Tt, Tc, and Oc, as well as for the 24th slip in Ot. Results showed Tt had fewer falls than Tc (9.6% versus 43.8%, p < 0.001) but more falls than Ot (9.6% versus 0%, p < 0.001). Tt also had greater proactive and reactive stability than Tc (Tt > Tc, p < 0.01), however, Tt's stabilities were lower than those of Ot (p < 0.01). There was no difference in fall-rate or reactive stability between Tc and Oc, though treadmill walking did improve the proactive stability control of the latter. While the treadmill-slip-training protocol could immediately reduce the numbers of falls from a novel laboratory-reproduced slip, such improvements were far less than that from the motor adaptation to the over-ground-slip-training protocol.

Keywords: Balance; Motor generalization; Slip perturbation; Stability; Treadmill.

Figure 1.

Flowchart of the number of participants. One hundred and sixty-six community-dwelling older adults were initially screened. One hundred and forty-six of them qualified and were randomized into two groups: the treadmill-slip perturbation training group (Tt, N=73) or the treadmill-control group (Tc, N=73).

Figure 2.

(a) The computer-programmed treadmill (ActiveStep, Simbex, Lebanon, NH) used for treadmill-slip perturbation training and treadmill walking. (b) The 7-meter walkway with imbedded low-friction movable platforms for inducing over-ground slips and over-ground walking. Beneath the surface, the low-friction bearings of both platform were mounted on top of the low-friction aluminum tracks, resting on four force plates (AMTI, Newton, MA) to record the ground reaction force which were used to control the release of the platforms to provide the over-ground slip. The movable platforms were firmly locked during regular (unperturbed) walking and unlocked electronic-mechanically without the subjects’ awareness only in the slip trial at the instant of their right heel strike on the right platform. Once released, the right platform could slide forward freely up to 0.9 m which was comparable to the maximum slip distance in the treadmill-slips. The left movable platform was constructed the same and operated in a similar manner. Participants were protected by a safety harness connected through a loadcell (Transcell Technology Inc., Buffalo Grove, IL) to a low-friction trolley-and-beam system mounted on the ceiling above the treadmill and the over-ground walkway during all trials. Kinematic data from all of the trials was recorded by an eight-camera motion capture system (MAC., Santa Rosa, CA and Qualysis., Gothenburg, Sweden). Motion data was synchronized with the force plates and the loadcell data.

Figure 3.

Schematics of study design. Two treadmill groups, the treadmill-slip perturbation training group (Tt) and the treadmill-control group (Tc) underwent five to ten trials of regular (unperturbed) walking on the 7-meter over-ground walkway. Group Tt then received 40 “slip-like” disturbances on the ActiveStep treadmill system as training which approximated 30 minutes in the duration. Group Tc received 30 minutes walking on the treadmill without perturbation at their selected speed. After the treadmill session, both treadmill groups immediately went back to the same walkway for five to ten unperturbed walking trials and then received their novel, over-ground slip as the post-training test. Two over-ground groups: over-ground-slip-training group (Ot) and over-ground-control group (Oc) took approximately ten regular (unperturbed) baseline walking trials on the same 7-meter walkway. Group Ot then received a total of 24 over-ground slips, in which, the final (24^th) slip represented the post training trial. Group Oc received one novel, over-ground slip after the baseline walking trials. The results were compared (square in dashed line) for the novel over-ground slips for the Tt, Tc, and Oc groups, as well as for the 24^th slip of the Ot group. Group Tt was compared with group Tc to test hypothesis one (H1). Group Tt was compared with group Ot to test hypothesis two (H2). Group Tc was compared with group Oc to test hypothesis three (H3).

Figure 4.

(a) The total 40 treadmill slips in the treadmill-slip perturbation training protocol were provided over 11 blocks of walking and their perturbation intensity underwent adjustments in three phases. The first phase was a stepwise increase in perturbation intensity (#1, Ascending phase). The second phase was designed to enhance the training effects with repetitive blocks of intensity (#2, Enhancement phase). The third phase was for cooling down (#3, Cooling down phase). P₁-P₅ indicated training profiles with varied perturbation intensity from lowest to highest as the number ascended. (b) The profile of one block of treadmill slips. Each slip on the treadmill began with 1.3-2 seconds ramping-up duration followed by three to five steps of steady state walking on a backward-moving belt at one of the preset speeds (−0.6 m/s, −0.8 m/s, −1.0 m/s or −1.2 m/s, negative sign indicated that the moving direction of the treadmill belt was opposite to the direction of participants’ COM). The preset speeds were selected by the participants as their most comfortable walking speed on the treadmill. Without any warning, the belt speed would abruptly reverse the direction (accelerating forward) to produce a single “slip-like” perturbation. The perturbation intensity of each training profile was determined by two factors, the acceleration of the belt (at two levels: 5 or 6 m/s²) and the duration of its application (ranged from 0.2-0.55 s). After each acceleration, the belt deaccelerated at 0.8 m/s² to the previous steady-state walking speed. The next “slip” occurred following another three to five steps of steady-state walking. There was a total of two or four slips in each block.

Figure 5.

Slip recovery outcomes and their percentages of falls (FALL, unfilled bar), backward loss of balance (BLOB, hatched lines), and full recovery (FULL, light grey) within the four groups. Significant differences were indicated with a solid line (*** = p < 0.001). Shown in the sub-plot at the right corner is the change in the incidence of falls (S1 through S8) in over-ground-slip-training group (Ot: over-ground-slip-training group).

Figure 6.

Dynamic stability within four groups. Proactive control (at the instant of slipping limb touch down) (a-c): (a) Proactive stability; (b) COM position relative to the base of support (BOS) (relative P_COM); (c) COM velocity relative to the BOS (relative V_COM). Reactive control (at the instant of trailing limb lift-off) (d-f): (d) reactive stability; (e) relative P_COM; and (f) relative V_COM. Both the relative P_COM and relative V_COM were relative to the rear edge of the base of support (BOS) and normalized by foot length (1_BOS) and $\sqrt{g\times bh}$ respectively (gravitational acceleration is represented by g and body height is represented by bh). *p<0.05, **p><0.01, ***p><0.001.