Rigid Ankle Foot Orthosis Deteriorates Mediolateral Balance Control and Vertical Braking during Gait Initiation

Abstract

Rigid ankle-foot orthoses (AFO) are commonly used for impeding foot drop during the swing phase of gait. They also reduce pain and improve gait kinematics in patients with weakness or loss of integrity of ankle-foot complex structures due to various pathological conditions. However, this comes at the price of constraining ankle joint mobility, which might affect propulsive force generation and balance control. The present study examined the effects of wearing an AFO on biomechanical variables and electromyographic activity of tibialis anterior (TA) and soleus muscles during gait initiation (GI). Nineteen healthy adults participated in the study. They initiated gait at a self-paced speed with no ankle constraint as well as wearing an AFO on the stance leg, or bilaterally. Constraining the stance leg ankle decreased TA activity ipsilaterally during the anticipatory postural adjustment (APA) of GI, and ipsilateral soleus activity during step execution. In the sagittal plane, the decrease in the stance leg TA activity reduced the backward displacement of the center of pressure (CoP) resulting in a reduction of the forward velocity of the center of mass (CoM) measured at foot contact (FC). In the frontal plane, wearing the AFO reduced the displacement of the CoP in the direction of the swing leg during the APA phase. The mediolateral velocity of the CoM increased during single-stance prompting a larger step width to recover balance. During step execution, the CoM vertical downward velocity is normally reduced in order to lessen the impact of the swing leg with the floor and facilitates the rise of the CoM that occurs during the subsequent double-support phase. The reduction in stance leg soleus activity caused by constraining the ankle weakened the vertical braking of the CoM during step execution. This caused the absolute instantaneous vertical velocity of the CoM at FC to be greater in the constrained conditions with respect to the control condition. From a rehabilitation perspective, passively- or actively-powered assistive AFOs could correct for the reduction in muscle activity and enhance balance control during GI of patients.

Keywords: ankle rigidity; ankle-foot orthosis; balance control; gait initiation; vertical braking.

Figure 1

“Standard” short ankle foot orthosis. The figure portrays a frontal and side view of the rigid orthosis that was used in this study. The orthosis can block dorsi- and plantarflexion of the ankle in addition to reducing the eversion and inversion of the foot.

Figure 2

Anteroposterior (AP) and mediolateral (ML) center of mass (CoM)-center of pressure (CoP) distance and the bilateral tibialis anterior (TA) activity. Panel (A) shows, from top to bottom the timelines of the ML and AP CoP (solid lines) and CoM (dashed lines) trajectories in addition of the raw traces of swing and stance TA activity during gait initiation (GI). The traces were obtained from a single trial of a representative subject in the Ctrl (left), O-St (middle) and O-Bi (right) conditions. The vertical dash lines represent the instant of t0, first foot off (FO1), foot contact (FC) second foot off (FO2). Panels (B–E) show the grand means (N = 19) and standard deviations of the maximum AP and ML distance between the CoM and CoP, the swing and stance TA activity. The histograms show that wearing the ankle-foot orthoses (AFO) decreases the activity of TA and ML and AP CoP and CoM excursions. *Indicates significant difference (p < 0.05).

Figure 3

Vertical ground reaction force and CoM velocity and the stance leg Sol activity. Panel (A) shows, from top to bottom) the timelines of the vertical ground reaction forces and vertical CoM velocity in addition of the raw traces of stance Sol activity during GI. The traces were obtained from a single trial of a representative subject in the Ctrl (left), O-St (middle) and O-Bi (right) conditions. The vertical dash lines represent the instant of t0, first foot off (FO1), FC second foot off (FO2). Panels (B–E) show the grand means (N = 19) and standard deviations of the minimum vertical velocity of CoM, the vertical velocity of CoM at FC, the vertical braking of CoM vertical fall and the stance Sol activity during step execution. It can be noted that wearing the AFO decreases the activity of Sol, which affects the braking action on the CoM vertical fall. *Indicates significant difference (p < 0.05).