Gait performance of walking in or out of the water: Objective and interpretative observation using variability and stability indices

Introduction: Hydrotherapy is used as a treatment in arthrosis conditions, backache, coronary disease, etc. [1]. Movements performed in water have different characteristics when compared to the same movements out of water, and these differences change depending on the water temperature and depth. Subjects’ characteristics, such as proprioception, sensory feedback, vestibular apparatus responses are influenced by the presence of water, which has proprieties, like friction, viscosity, reaction forces, different from those of air [2]. Walking in water could be used in aquatic therapy. In literature several methods were proposed to quantify the stability during walking out of water: the related synthetic indices can be grouped as stability indices and variability indices [3]. The aim of this work is to understand how these indices (describing elements such as complexity, stability, variability, smoothness in walking and postural control) change among tests conducted in or out of the water, in order to provide objective indications on the motor control response in relation to the different characteristics of the walking in the water. Methods: Eleven (6 male and 5 female) healthy subjects participated in the study (age 27.0 ± 3.4, height 174.2 ± 8.2, mass 70.2 ± 11.8). The participants were asked to walk barefoot at self-selected speed for 10 meters, repeating the task for three times, first out then in the water. Measures were collected using 3 tri-axial wireless inertial sensor (OPALS, Apdm, USA, sampling frequency 128 Hz), inserted in waterproofed boxes and fixed to the body segments (shanks, pelvis) [2]. 30 strides for participant were considered. The following parameters for quantifying gait stability and variability were applied: Harmonic Ratio (HR), Poincaré plots (SD1, SD2), maximum Floquet multipliers (FM), Multiscale Entropy (MSE), recurrence qualification analysis (RQA), Lyapunov exponents (sLE) [4]. Results: Most significant results are shown in Table 1. HR and sLE on the three axis and RQA on the vertical (V) axis showed higher values out of the water, while RQA on the mediolateral (ML) and anteriorposterior (AP) axis, MSE on the three axis, SD1 and SD2 showed higher values in the water. Table 1. Mean values and standard deviation of the most significant indices, in and out of the water. sLE HR SD2 V ML AP V ML AP In 0.5 ± 0.2 0.6 ± 0.3 0.7 ± 0.3 1.2 ± 0.2 1.6 ± 0.4 1.0 ± 0.2 0.2 ± 0.2 Out 1.1 ± 0.2 1.0 ± 0.2 1.1 ± 0.2 3.6 ± 0.7 3.3 ± 1.0 3.5 ± 0.5 0.5 ± 0.2 MSE τ = 6 Recurrence SD1 V ML AP V ML AP In 1.4 ± 0.1 1.3 ± 0.2 0.8 ± 0.2 13 ± 3 26 ± 3 28 ± 6 0.20 ± 0.10 Out 1.0 ± 0.1 1.1 ± 0.3 1.1 ± 0.1 22 ± 2 15 ± 3 25 ± 2 0.03 ± 0.01 Discussion: Preliminary variability and stability results suggested that walking in water is more stable (lower values sLE), less fluid (lower values HR) and more variable (higher values SD1 e SD2) compared to walking out of the water. The RQA in the water shows a greater postural control along the AP and ML axes, while along the V axis a greater postural control is observed out of water. MSE along the three axes is higher in water: this could suggest that walking in water is conducted in a less automatic way [4]. Future research will be focused on the interpretation of the physiological meaning of these indices, in relation to the different characteristics of walking in the water environment and to the effects that water has on walking performance.