Full length articleThe influence of the aquatic environment on the center of pressure, impulses and upper and lower trunk accelerations during gait initiation
Introduction
Gait initiation is a common functional task defined as the transition from stationary standing to steady-state walking [1]. Gait initiation is frequently divided in two phases, the anticipatory postural adjustment (APA) phase and the execution of the first step [2], [3], [4]. The APA phase is known to generate the forward momentum [3] and to control the mediolateral (ML) stability [2] for the execution of the first step (EXE).
Effects of various sensorimotor and environmental conditions, such as with different speeds [2], different step lengths [5], stepping over obstacles [6], and changing initial stance position [7], on gait initiation have been investigated. However, to date, gait initiation has been underexplored in the aquatic environment, even though gait training in the aquatic environment is a common and effective approach for rehabilitation in people with various gait and balance impairments [8]. The physical properties of water appear to benefit people with low-functioning performance by providing body offloading due to buoyancy and moderate resistance due to water viscosity [9], [10].
In our previous study [11], we demonstrated that the aquatic environment affects the center of pressure (COP) trajectory, the impulse exerted by the body, and kinematics of lower limbs during gait initiation. However, as only the lower body segment receives resistance due to water viscosity and the upper body segment is left to move freely, the relation of movements between upper and lower body segments in water can be different from that on the land. Thus, in the current study, we investigated the relation of movements between upper and lower body segments in addition to the COP trajectory and impulses.
Further, in the current study, we propose to divide the EXE phase into two phases to differentiate two events on the execution phase of the COP, i.e., in one the COP predominantly moves in ML direction (EXE1) and in the other the COP predominantly moves in anteroposterior (AP) direction (EXE2). In this way, we were able to identify the trajectory that corresponds to the weight-transferring to the stance limb (the ML COP trajectory, EXE1) and one corresponding to stepping forward (the AP COP trajectory, EXE2). The aquatic medium may affect these COP trajectories differently in AP and ML directions due to the effects of water resistance and buoyancy when body moves during weight transfer and stepping forward in water. Under the support of buoyancy, the ML COP trajectory in EXE1 would increase due to a longer weight transfer and the AP trajectory during EXE2 would increase because individuals could take a longer first step or lean more forward. In other words, we hypothesize that the EXE1 trajectory would be predominantly longer in ML direction and EXE2 trajectory would be predominantly longer in AP direction. Furthermore, the water resistance would increase the duration of the trajectories as the COP velocity during EXE2 would decrease.
Therefore, the aim of the present study was to investigate the kinematics and kinetics of posture during gait initiation in water, focusing on trunk acceleration and with newly proposed divisions of COP. We first hypothesized that the aquatic medium would increase the length of COP trajectory during APA and execution phases, and would decrease velocity while individuals are stepping forward. Second, we hypothesized that a greater mean AP force would be required during the first step, in order to overcome water resistance. Third, we hypothesized that the ratio between upper and lower trunk accelerations in AP direction would increase (acceleration of upper trunk > acceleration of lower trunk) in water during execution phase compared to on land walking, due to greater resistance that the lower trunk and legs experience in water.
Section snippets
Participants and location
Ten able-bodied volunteers (5 females, age 19–35 years, weight 46–81 kg, height 164–178 cm, and body mass index 17–26), without contraindication to immersion in thermal water, participated in this study. Participants reviewed and signed a written informed consent. Ethical approval was obtained.
Tests in water and on dry land were conducted in the hydrotherapy pool area. The water temperature was set around 35 °C. Tests on dry land were performed at the pool side in order to avoid any environmental
Results
The percentage of BW offloading in water varied from 38.6% to 62.8% and was negatively correlated with individuals’ height (r = −0.712, P = 0.021) and positively correlated with the normalized AP impulse (r = 0.901, P < 0.001) and normalized AP mean force (r = 0.685, P = 0.029) in water. There was no difference on the COP mean position in relation to the ankle line (d = 0.08, P = 0.799) and to the lateral borders of the feet (d = 0.01, P = 0.974) (Table 1).
Table 1 depicts parameters of COP trajectories during APA,
Discussion
The present study proposed a new, detailed design of the COP trajectories defining APA based on the COP displacement in ML direction. In a previous study, the COP in ML direction has shown to be more consistent and reliable to detect APA than the acceleration signals [14]. Therefore the COP in ML direction was used to divide the gait initiation cycle in APA and EXE phases across all signals analyzed in the study. Further, we subdivided EXE phase in EXE 1 and EXE2 as we expected that the aquatic
Conclusions
Aquatic environment leads to an increased length of COP trajectories and slower COP execution during the first step compared to on land condition. The larger AP mean force aligned with a change in the trunk acceleration pattern seemed to be a compensatory strategy to overcome the drag force during gait initiation in water. Walking in water offers considerable BW offloading and movement resistance, which challenges postural control during anticipatory and execution phases of gait initiation.
Conflict of interest
None.
Acknowledgments
Mrs. Marinho-Buzelli acknowledges the support of Canadian Institutes of Health and Research (CIHR) through the Vanier Canada Graduate Scholarship on the development of this study [Grant # 95662]. She also acknowledges Dr. William McIlroy’s significant contribution on the design of the project as PhD committee member. Dr. Rouhani acknowledges the support of the Swiss National Science Foundation Grants [PBELP3-137539 and P300P2-147865] and Spinal Cord Injury Ontario Postdoctoral Fellowship. The
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