Neuromechanical control of leg length and orientation in children and adults during single-leg hopping.

Adult-like fine control of cyclical motor patterns found in locomotion develops into adolescence. Single-leg hopping in place is one such motor pattern where children have demonstrated a reduced capacity to control horizontal motion and match metronome cues. These developmental differences might arise from immature inter-segment coordination strategies and variability regulation. Therefore, the purpose of this study was to use an uncontrolled manifold (UCM) analysis to evaluate the control of segment angle variance (i.e., local variables) to stabilize leg length and leg orientation (i.e., task variables) in the sagittal plane between young adults and children aged 5-11 years old while hopping at different frequencies. The UCM space and its orthogonal space were constructed and segment angle variance was partitioned into these two spaces. Increased variance in the UCM space represents the stabilization of a task variable, while increased variance in its orthogonal space indicates a greater deviation of a task variable from its mean value. Our results indicated that children have developed an adult-like inter-segment coordination strategy of stabilizing leg length at mid-stance and leg orientation during flight. However, children might have an underdeveloped capacity to modulate leg length at take-off from cycle-to-cycle. Moreover, when increasing hopping frequency, children showed limited capacity to selectively increase leg-length stabilization. When decreasing hopping frequency, children illustrated an increased stabilization of leg orientation over the entire stance phase. Mid-stance leg-length stabilization might emerge with the motor skill; however, other inter-segment coordination strategies might continue to develop beyond 11-years of age.