Dynamic stability during level walking and obstacle crossing in children aged 2-5 years estimated by marker-less motion capture

Abstract

In the present study, dynamic stability during level walking and obstacle crossing in typically developing children aged 2-5 years (n = 13) and healthy young adults (n = 19) was investigated. The participants were asked to walk along unobstructed and obstructed walkways. The height of the obstacle was set at 10% of the leg length. Gait motion was captured by three RGB cameras. 2D body landmarks were estimated using OpenPose, a marker-less motion capture algorithm, and converted to 3D using direct linear transformation (DLT). Dynamic stability was evaluated using the margin of stability (MoS) in the forward and lateral directions. All the participants successfully crossed the obstacles. Younger children crossed the obstacle more carefully to avoid falls, as evidenced by obviously decreased gait speed just before the obstacle in 2-year-olds and the increased in maximum toe height with younger age. There was no significant difference in the MoS at the instant of heel contact between children and adults during level walking and obstacle crossing in the forward direction, although children increased the step length of the lead leg to a greater extent than the adults to ensure base of support (BoS)-center of mass (CoM) distance. In the lateral direction, children exhibited a greater MoS than adults during level walking [children: 9.5%, adults: 6.5%, median, W = 39.000, p < .001, rank-biserial correlation = -0.684]; however, some children exhibited a smaller MoS during obstacle crossing [lead leg: -5.9% to 3.6% (min-max) for 4 children, 4.7%-6.4% [95% confidence interval (CI)] for adults, p < 0.05; trail leg: 0.1%-4.4% (min-max) for 4 children, 4.7%-6.4% (95% CI) for adults, p < 0.05]]. These results indicate that in early childhood, locomotor adjustment needed to avoid contact with obstacles can be observed, whereas lateral dynamic stability is frangible.

Keywords: adaptive locomotion; balance; development; margin of stability (MOS); obstacle advoidance; preschoolers.

Figure 1

Experimental setup and overview of estimating body feature points by OpenPose. (A) Obstacle-crossing tasks were recorded by three RGB video cameras placed in front [perspective in (C), middle image] and diagonally in front [perspective in (C), left and right images] of an obstacle. The data from a total of 4 steps, including two steps before crossing the obstacle (called the lead approach step and trail approach step) and two steps during crossing the obstacle (called the lead cross step and trail cross step), were analyzed. (B) Camera positions for the children were closer to the obstacle than those for the adults. (C) OpenPose was applied to the three recording videos to estimate the 2D coordinates of the 25 body landmarks; these coordinates were transformed into 3D coordinates using a direct linear transformation (DLT).

Figure 2

Definition of the margin of stability (MoS). The extrapolated center of mass (XCoM) was derived from the position and velocity of the center of mass (CoM). MoS in the anteroposterior (AP) direction was defined as the heel position minus the XCoM at the time of heel contact. The MoS in the mediolateral (ML) direction was defined as the minimum distance from the small toe or heel position minus the XCoM. A positive MoS indicates dynamic stability. A negative MoS indicates that the walker would theoretically need an extra step to ensure that the CoM remained within the base of support (BoS).

Figure 3

Normalized step length, step width, and step speed during level walking. The step length and step width were normalized to leg length (L), and the step speed was normalized to the root square of gravity (g) × L. Circles indicate the mean within-participant value. The gray area shows the 95% confidence interval (CI) of the mean of adults. * Indicates p < 0.05.

Figure 4

Examples of sagittal stick pictures normalized to leg length during obstacle crossing. Most children aged 2–5 years were able to clear an obstacle set to a height of 10% of leg length as well as adults [(A), top: adult, 20 years old; bottom: child, 5.7 years old]. Children varied in obstacle-crossing behavior, with one taking several steps on the spot before crossing an obstacle [(B): child, 2.4 years old] and one bringing the trail leg into contact with the obstacle [(C): child, 2.9 years old].

Figure 5

Normalized margin of stability (MoS) in the anteroposterior (AP) and mediolateral (ML) directions during level walking and crossing an obstacle (with the lead and trail leg, separately). The MoS was normalized to leg length (L). Circles indicate the mean value of within participants. The gray area shows the 95% confidence interval (CI) of the mean of adults. * Indicates p < 0.05.

Figure 6

Normalized base of support (BoS)-center of mass (CoM) distance and CoM velocity in the anteroposterior (AP) and mediolateral (ML) directions during level walking and obstacle crossing (with the lead and trail leg, separately). The BoS-CoM distance was normalized to leg length (L). The CoM velocity was normalized to the root square of gravity (g) × L. Circles indicate the mean value of each participant.

Figure 7

Normalized vertical foot clearance, maximum toe height, and swing time during obstacle crossing. Circles indicate the mean value of within participants. The gray area shows the 95% confidence interval (CI) of the mean of adults. * Indicates p < 0.05.

Figure 8

Change ratios of the step length, step width, and step speed from level walking to obstacle crossing. A ratio value greater than 1 indicates that the gait parameter during obstacle crossing was greater than that during level walking. Circles indicate the mean value of within participants. The gray area shows the 95% confidence interval (CI) of the mean of adults. The dotted line indicates a change ratio equal to 1. * Indicates p < 0.05.