Effect of speed on mediolateral dynamic stability during stepping in older adults

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Keywords:

• Balance
• step initiation
• elderly
• margin of stability
• biomechanics

1. Introduction

Falls in the elderly are a major public health concern, because of devastating physical and psychological consequences among this population. A large proportion of falls in older adults occurs during daily activities such as walking or stepping. These falls often result from incorrect weight transfers (Robinovitch et al. 2013), which could be associated with difficulties experienced by the elderly in controlling mediolateral stability (Rogers et al. 2001).

Initiation of stepping, which involves the transition from a bipedal to a unipedal stance, constitutes a task providing a challenge for mediolateral dynamic stability. Indeed, the act of lifting the foot to execute the step induces a reduction of the dimension of the base of support, which results in a lateral fall of the whole-body toward the swing leg during the subsequent step execution. During volitional stepping, it is well known that this lateral instability is minimized by the anticipatory postural adjustments (APA) preceding the swing foot-off. These APA are manifested by a lateral shift of the centre of pressure (CoP) toward the swing foot, which acts to propel the centre of mass (CoM) toward the stance leg prior to swing foot, thereby attenuating the subsequent lateral CoM fall (Rogers et al. 2001).

The question of whether and how the mediolateral stability control during volitional gait initiation is influenced by the speed of progression has been recently addressed (Caderby et al. 2014). It has been reported that young individuals are able to finely tune the temporo-spatial features of APA and the mediolateral swing foot placement at heel-contact time so as to maintain mediolateral dynamic stability unchanged during gait initiation whatever the gait speed (Caderby et al. 2014). However, to date, it is unknown whether these findings are generalizable to the elderly population.

The aim of the present study was to examine the effect of speed on mediolateral dynamic stability during initiation of stepping in older adults. We hypothesized that older adults would exhibit impairment in the mediolateral dynamic stability when pace increases.

2. Methods

Eighteen old participants (age: 65.8 ± 2.5 years) volunteered for this study. All participants were free from any disorders that could interfere with gait and balance. All subjects gave their written consent after being fully informed of the test procedure.

Stepping was initiated from a first force-plate. A second force-plate was located immediately in front of this initial force-plate so that the first step landed onto it. Reflective markers were fixed on bony landmarks of the participant’s body according to a simplified kinematic model (Tisserand et al. 2016). A motion capture system equipped with 12 cameras was used to collect simultaneously the kinematic data at 200 Hz and force-plate data at 1000 Hz.

Initially, participants were instructed to stand upright as still as possible on the first force-plate. At the verbal signal from the experimenter, participants initiated stepping with their dominant leg and followed with the non-dominant leg to stop in a confortable upright posture on the second force-plate. Stepping was performed under two speed conditions: preferred pace (Normal condition) and as fast as possible (Fast condition). After two familiarization trials, five trials were completed in each speed condition. The order of the experimental conditions was randomized across the individuals.

Both the durations of the APA phase (i.e., the time between the onset of movement to the toe-off of the swing leg) and the step execution phase (i.e., the time between the swing toe-off and the swing heel-contact) were reported. The amplitude of APA was calculated as the peak of lateral CoP shift toward the swing foot during APA phase. Mediolateral CoM displacement at swing toe-off was computed. To quantify mediolateral dynamic stability, we calculated the margin of stability (MoS) at the time of heel-contact (Hof et al. 2005). MoS was calculated as the mediolateral distance between the boundary of the base of support (i.e., ankle marker of the swing leg) and the extrapolated CoM position (XCoM). Student t-tests were conducted on each of these variables to assess the effect of speed.

3. Results and discussion

In line with our previous study (Caderby et al. 2014), the present results showed that the amplitude of APA increased, while the duration of APA decreased with speed (Figure 1). This decrease in the APA duration, i.e., the time allocated to propel the CoM toward the stance foot, resulted in a significant decrease in the mediolateral CoM displacement toward the stance foot at the end of APA, i.e., at swing toe-off time (Figure 1). As a consequence, we noted a higher lateral fall of the whole-body toward the swing leg during the subsequent step execution with the increasing speed, although the duration of step execution decreased (Figure 1). This was attested by a mediolateral position of XCoM at swing heel-contact located further toward the swing leg when speed increased (Figure 1). As step width did not change, MoS decreased with the increasing speed (Figure 1). These latter results markedly contrast with the previous findings obtained in young adults (Caderby et al. 2014), revealing that these individuals were able to compensate for the higher lateral instability by enlarging the step width so as to maintain MoS unchanged when gait speed increased. Although the motor task performed in the present study is not exactly similar to that of our previous study (step initiation vs. gait initiation), the decrease in MoS (about 1 cm) observed in the present study may suggest that older adults exhibit an impaired ability to control mediolateral stability during stepping when pace increases. Further investigation is nevertheless required to confirm this assumption.

Figure 1. The main effect of speed on dependant variables. yCoM_TO: mediolateral CoM displacement at toe-off. dEXE: step execution duration. XCoM_HC: extrapolated CoM position at heel-contact. MoS: margin of stability. *, ***: Significant effect of speed with p < 0.05 and p < 0.001, respectively.

4. Conclusions

The present results stress that mediolateral dynamic stability during initiation of stepping is impaired with the increasing speed in older adults. These findings may serve as a basis for the future studies aiming to reduce the incidence of falls among the elderly.

Additional information

Funding

A Regional Research Grant from the Réunion Region and from the European Regional Development (FEDER) supported this work.