The development of running in children
Authors:   , Midtgaard Bach Margit 
Published: April 2023

In this thesis I study the development of running in young children by combining neuromuscular and biomechanical measures using novel techniques and state-of-the-art statistics. I introduce a frame of reference for the factors underlying the development of running in children. Chapter 1 introduces muscle synergies and clustering techniques which are integral to the results in this thesis.
Chapter 2 focuses on the biomechanics of walking and running on a treadmill of children aged 2-9 years old in a cross-sectional design. Clustering of kinetic and kinematic parameters across strides revealed no direct agreement between chronological age and maturity in young children walking and running when comparing their gait patterns to adults. When learning to run, young children made use of a “walk-run-strategy”. This strategy is characterized by the ability to run with a combination of double support and flight phase causing in-phase oscillations of potential and kinetic energies of the center-of-mass.
Chapter 3 focuses on the neuromuscular control of locomotion in children aged 2-9 years old in a cross-sectional design. I assessed muscle activity and in particular the number and structure of muscle synergies. Children in this study used a walk-run-strategy when learning to run. Older children incorporated exploratory muscle synergies when “optimizing” their gait patterns whereas the children younger than 3.5 years made use of a “simpler” motor control pattern trending toward larger bursts of activation. The increase in the number of muscle synergies for individual participants could be related to motor learning and exploration.
To further detail the development of locomotion, I investigated the running in a longitudinal design assessing two children from their first independent steps until about 32 months after onset of independent walking. In Chapter 4 I combined both biomechanics and neuromuscular control of locomotion. Using clustering like in Chapter 2 and comparing the running development of the two toddlers to a group of adults, I determined that the development of running can proceed along different trajectories including the co-existence of immature and mature running within the same session in a child.
Collecting large amounts of data is easy to achieve on a treadmill and most of the experimental data presented in this thesis were therefore collected on a treadmill. However, a treadmill does not necessarily represent natural locomotion. So, in Chapter 5 I explored avenues to record outside of the lab. I employed recursive neural networks to predict vertical ground reaction force data from shank accelerometer data in adults with high accuracy which led to a high accuracy in the gait event detection.
In Chapter 6 I reflected on the findings and put these into a broader context. I showed that neither flight phase nor the in-phase oscillations of kinetic and potential energies of the center-of-mass were proper indicators of running in children from onset of independent walking to 9 years of age. I proposed some aspects of future research such as not focusing on the presence of a flight phase to determine running in children and to apply methods requiring advanced statistics to get proper insights into running in children.
The work in this thesis supports the notion that development of running in children is not a linear process. Generally, children limit the degrees of freedom available to them resulting in a reduced range of movement, both in terms of biomechanics and neuromuscular control. Based on my findings, I conclude that it is not possible to distinguish biomechanical from neuromuscular constraints. They interact and are equally important in understanding the motor control of running in children. The development of running is a subtle process and cannot be characterized with just a few parameters.