Preparatory adjustments during gait initiation in 4–6-year-old children

Abstract

The preparatory adjustments related to gait initiation in a group of six children (4–6 years old) were studied in comparison to a group of six adults (50–61 years old). Muscle activity, ground reaction forces and body kinematics were recorded during the initiation of gait in subjects standing with one foot on each of two forceplates. Like adults, children had consistent anticipatory activations of the tibialis anterior (TA) muscle accompanied by center of pressure (COP) displacements, but the relative magnitude of both the backward COP displacement and concomitant initial TA activation burst were lower (P<0.03 and P<0.001 respectively) in children. The latter findings were associated with a lower rate of forward progression in children (P=0.01), and a lack of significant covariance between the backward shift and forthcoming velocity. In contrast, the preparatory adjustments in the medio-lateral (M/L) direction were prominent in children. Larger M/L peak force rates in children (P=0.01) were associated with an earlier (P=0.007) weight transfer to the stance limb; moreover, children initiated gait from a wider (P=0.04) base of support and had a trajectory of the total COP that was lateral rather than posterior like in adults. The consistent preparatory adjustment responses indicate that the motor program for initiating gait is functional at this age. The prominence of the preparatory adjustments in the M/L direction together with a reduced magnitude of the responses in the antero-posterior direction suggest that the anticipatory behavior for initiating gait develops first in the frontal plane and that more walking experience and better postural stability are required to fully achieve the adult-like control of the gait initiation process.

Introduction

Gait initiation, which is defined as the transition from quiet stance to the cyclic movements of walking, involves a centrally mediated motor program [1], [2], [3] that releases the preparatory adjustments needed to propulse the body forward and toward the stance limb. These preparatory adjustments are characterized by bilateral anticipatory electromyographic (EMG) sequences (soleus inhibition and tibialis anterior activation) resulting in a displacement of the center of foot pressure (COP) backward and toward the initial swing limb [3], [4], [5], [6], [7]. These preparatory adjustments promote the initiation of gait by accelerating the center of mass (COM) forward and toward the stance limb [5], [8], [9]. Because these adjustments occur prior to voluntary movement of the lower limbs, they are qualified as anticipatory [3], [10]. Muscle activations that precede the onset of gait initiation will also be qualified as anticipatory.

Very little is known about the anticipatory behavior of children when they initiate gait. Results from one study [11] suggest that anticipatory responses are still poorly developed after 6 months of independent walking. It was found, that unlike adults, the steady state velocity (forward progression velocity) of inexperienced walkers is reached after two to four steps [11]. The backward shift of the COP was also very rarely observed in the children [11] and when present the amplitude of the backward shift and the gait velocity did not covariate as in adults. It was concluded that in children with little experience of autonomous walking, poor stability and insufficient muscle strength (biomechanical factor) to accelerate the body forward, or difficulty in consistently activating the motor program (neural factor) could explain the lack of preparatory adjustments during gait initiation. More recently, however, in a study involving children aged between 2.5 and 8 years, it was found that the occurrence of anticipatory COP displacements increased progressively with age [12]. The covariance between the magnitude of the backward shift and the forthcoming velocity only became significant at 6 years of age, suggesting that accurate tuning of the feedforward control takes place later with improvement of postural control. To our knowledge, however, no study has examined the muscle activations during gait initiation in children. The latter information could help determine if the characteristics of the preparatory adjustments in children are related to inconsistent activation of the motor program, inappropriate muscle activation patterns or to a level of muscle activations that is not powerful enough in younger children to overcome the body mass and inertia as suggested by others [11].

To gain some insight on the organization of the anticipatory behavior during gait initiation, a group of children with more than 3 years experience of independent walking was investigated in this study. Muscle activation patterns and concomitant displacements of the COP were recorded during gait initiation in a group of 4–6-year-old children. It was expected that if the gait initiation motor program was functional and well integrated in the locomotor program, anticipatory muscle activations should be consistently elicited during the initiation of gait. If, despite consistent anticipatory muscle activations, the mechanical responses or the time to reach steady state velocity were different compared to adults, it would then suggest the motor command (motor unit recruitment) or muscle strength (biomechanical) to be the factors limiting the expression of adult-like preparatory adjustments.

The main purpose of this study was thus to determine if children aged between 4 and 6 years who had experienced independent walking for more than 3 years, demonstrated consistent anticipatory motor behavior when initiating gait and if the gait initiation process had adult-like characteristics. To do so the data collected in children were compared to a group of adults who were tested with the same methodology.