Gait motor control development: What is behind children kinematic variability?

Introduction: The onset of independent gait in children starts generally around age 1: the first 6 months of independent walking represent a process of integration of postural constraints into the dynamic necessities of gait movement, while after this period, a tuning phase begins, characterized by a more precise adjustment of the gait parameters [1]. As children grow, their gait pattern begins to approximate that of an adult: by age 3, most of the adult kinematic patterns are present, but gait maturation continues until age 6 [2]. Literature shows that variability of gait kinematic is higher in children than in adults [3]: thus, segmental kinematics has not been proved to be representative of gait development. The hypothesis of the present work is that the trajectory of the centre of mass (CoM) is a leading factor during gait maturation, allowing 4- and 6-year old children (4YC and 6YC) to reach steady state points in gait development, despite the shown variability: if this is true, the variability of CoM kinematic should be lower than that of other kinematic variables. Methods: Seven 4YC [4 ± 0 years, 101 ± 3 cm, 17±2 kg], seven 6YC [6 ± 0 years, 121 ± 2 cm, 22 ± 1 kg] and 7 young adults [23 ± 1years, 157 ± 8 cm, 61 ± 7 kg] participated in the study. Participants wore swimwear or close-fitting shorts. They were asked to walk barefoot along the gait analysis laboratory at self-selected speed: each participant performed 15 walks, divided in groups of 5. Every 5 walks, a 7 minute pause was scheduled as interval. During pauses, participants were asked to perform the following coordination exercises: standing on a foot for more than 8 seconds; tiptoes walking; heel to toe walking. Coordination exercises were used both for distracting participants from the walking tests and for evaluating children's level of development. 3D kinematic (SmartD, Bts, Italy) was collected using Plug-in-Gait protocol. One triaxial inertial sensor (Opal, Apdm, US) was positioned at CoM level. For each walking test, joint angles, CoM trajectory and CoM acceleration were obtained. Intra-test and inter-test variability (std) was calculated for each variable (respectively among the 5 walks of each test and among the repetitions of the 5-walk-test). Results: Joint angles results showed higher variability in children than in adults for both the intra-test and the inter-test conditions: e.g. children ankle flexion/extension std were 4 or 5 times greater than adults’ for 4YC and twice for 6YC. CoM trajectory in the inter-test condition showed similar std for the 3 groups; in the intra-test, 4YC and 6YC std was higher than adults’ on the mediolateral axis (4 times), while was similar for the other axes. In both intra-test and inter-test conditions, children CoM acceleration showed std twice the adults’ for the anteroposterior and the vertical axis, while higher on the mediolateral one. Discussion: Obtained results support the hypothesis that CoM trajectory is a leading factor during gait maturation and could be used as descriptive parameter: both 4YC and 6YC showed std of CoM kinematics along the sagittal plane close to the adults’ one, while std of joint angles was always higher. Moreover, present results suggest that control of gait is focused on the progression of the CoM and not on its lateral stabilization: children variability on the mediolateral axis was always higher than adults’ one.