Modular control of varied locomotor tasks in children with incomplete spinal cord injuries

Abstract

A module is a functional unit of the nervous system that specifies functionally relevant patterns of muscle activation. In adults, four to five modules account for muscle activation during walking. Neurological injury alters modular control and is associated with walking impairments. The effect of neurological injury on modular control in children is unknown and may differ from adults due to their immature and developing nervous systems. We examined modular control of locomotor tasks in children with incomplete spinal cord injuries (ISCIs) and control children. Five controls (8.6 ± 2.7 yr of age) and five children with ISCIs (8.6 ± 3.7 yr of age performed treadmill walking, overground walking, pedaling, supine lower extremity flexion/extension, stair climbing, and crawling. Electromyograms (EMGs) were recorded in bilateral leg muscles. Nonnegative matrix factorization was applied, and the minimum number of modules required to achieve 90% of the "variance accounted for" (VAF) was calculated. On average, 3.5 modules explained muscle activation in the controls, whereas 2.4 modules were required in the children with ISCIs. To determine if control is similar across tasks, the module weightings identified from treadmill walking were used to reconstruct the EMGs from each of the other tasks. This resulted in VAF values exceeding 86% for each child and each locomotor task. Our results suggest that 1) modularity is constrained in children with ISCIs and 2) for each child, similar neural control mechanisms are used across locomotor tasks. These findings suggest that interventions that activate the neuromuscular system to enhance walking also may influence the control of other locomotor tasks.

Keywords: locomotion; module; spinal cord injury; synergies; walking.

Fig. 1.

Children with incomplete spinal cord injuries (ISCI) performing locomotor tasks: treadmill walking, overground walking, supine flexion/extension, tricycle pedaling, crawling, and stair climbing.

Fig. 2.

Muscle module weightings and timing profiles for 4 modules identified in control children during overground walking. Left: muscle weightings for the 10 lower extremities are shown for the tibialis anterior (TA), medial gastrocnemius (MG), vastus medialis (VM), rectus femoris (RF), medial hamstrings (MH), and gluteus medius (GM) muscles. Bars represent the relative activation of each muscle in the module. Each bar corresponds to data from 1 lower extremity; the data are sorted from greatest to least for each muscle. The weighting for the muscle with the greatest activation is set to 1. Right: activation timing profiles indicate the relative level of activation of a module across the gait cycle (0–100%). The timing profiles from 1 representative lower extremity are shown. Each module was active at a distinct time point consistent with key biomechanical demands for overground walking (Clark et al. 2010; Neptune et al. 2009).

Fig. 3.

Muscle module weightings and timing profiles for the 3 modules identified in 1 control child during treadmill walking. Left: muscle weightings for both lower extremities are shown for TA, MG, VM, RF, MH, and GM muscles. Bars represent the relative activation of each muscle in the module. The weighting for the muscle with the greatest activation is set to 1. Right: activation timing profiles indicate the relative level of activation of each module across the gait cycle (0–100%).

Fig. 4.

Muscle module weightings and timing profiles identified during treadmill walking in children with ISCIs. A, left: muscle module weights for both lower extremities in 2 children with ISCIs. Right: activation timing profiles represent the relative activation of the 2 modules for the right and left legs across the gait cycle (0–100%). B, left: muscle module weights for both lower extremities of a child with ISCI who used 4 modules during treadmill walking. Right: activation timing profiles for both lower extremities of this child are displayed. Muscle weights for the TA, MG, VM, RF, MH, and GM muscles are represented.