Fig 1.
Technical drawing and placement of PIMU sensors for land and underwater walking gait motion analysis.
(Left) Dimensions (mm) and locations of the main components. (Right) Placement and axial orientations of three PIMU on a test subject. The measurement axes of each sensor are highlighted in red.
Table 1.
Technical characteristics of the wearable, waterproof Pressure and Inertial Measurement Unit (PIMU).
Table 2.
Temporal gait and knee joint parameters estimated from the optoelectronic land trials and PIMU system for land and water trials.
The summary statistics for comparison include the median, first (Q1), third (Q3) quartiles and interquartile ranges (IQR) of the distributions, as well as the knee joint coefficients of variation (CV and CVz).
Table 3.
Validation parameters used to compare the optoelectronic and the PIMU system for land-based trials.
Root Mean Squared Error (RMSE), Spearman correlation coefficient (ρ), Bland-Altman plot coefficients: mean of the differences (bias), the standard deviation of the differences (σ), lower and upper boundaries of the confidence interval (CI = bias ± 1.96σ).
Fig 2.
Comparison of optoelectronic and PIMU systems estimation of the knee angle for land-based trials.
(Left) Mean and standard deviation of the knee joint flexion-extension angle, normalized over the gait cycle. Dashed vertical lines identify the toe-off. (Right) Bland-Altman plot of a single test subject with 10 repetitions (gray dots). A single gait cycle is highlighted (black points), and the bias (solid black line) and upper and lower 95% confidence interval boundaries (dashed gray lines) indicate that there were few substantial deviations between optoelectronic and PIMU knee angle estimates.
Fig 3.
Comparison of the temporal gait parameters between land and underwater trials.
(Left) Foot-mounted accelerometer magnitude time series during a single gait cycle on land and in water. The stride time of the underwater gait is nearly 2.5x longer in duration than that of the stride time on land. (Center) Boxplots of the temporal gait parameters. (Right) Boxplots of the gait phases. Boxes represent the interquartile range (IQR) over the 25th to 75th percentiles, the centerline corresponds to the median value of the distribution, error bars extend by a factor of 1.5 from the IQR and outliers are marked as circles.
Fig 4.
Normalized knee flexion-extension angles and hydrodynamic pressure over the gait cycle.
(Left) Mean and standard deviation envelope (shaded regions) of the knee joint angle calculated from land (green) and water (blue) trials with PIMUs. (Right) Hydrodynamic pressure envelopes (shaded regions) obtained from the underwater trials from the PIMU mounted on the thigh (red), shank (blue) and foot (black). Dashed vertical lines in both panels indicate the toe-off.
Table 4.
Hydrodynamic pressure statistics of underwater gaits.
Sensors were placed on the thigh, shank and foot and observations were summarized as the median, first (Q1), third (Q3) quartiles and interquartile ranges (IQR) of the Range of Pressure (ROP), maximal and minimal pressure, as well as the coefficient of variation (CV and CVz).