Parkinson's disease patients compensate for balance control asymmetry

In Parkinson's disease (PD) subtle balance abnormalities can already be detected in early-stage patients. One feature of impaired balance control in PD is asymmetry: one leg produces more corrective joint torque than the other. We hypothesize that in mild to moderately affected PD patients, the least impaired leg compensates for the more impaired leg. Twenty PD patients and eleven healthy matched control subjects participated. Clinical asymmetry was determined by the difference between the left and right body side scores on the Unified Parkinson's Disease Rating Scale. Balance was perturbed with two independent continuous multisine perturbations in the forward-backward direction. Subsequently, we applied closed-loop system identification, which determined the spectral estimate of the stabilizing mechanisms, for each leg. Balance control behavior was similar in PD patients and control subjects at the ankle, but at the hip stiffness was increased. Control subjects exhibited symmetric balance control, but in PD patients the balance contribution of the leg of the clinically least affected body side was higher whereas the leg of the clinically most affected body side contributed less. The ratio between the legs helped to preserve a normal motor output at the ankle. Our results suggest that PD patients compensate for balance control asymmetries by increasing the relative contribution of the leg of their least affected body side. This compensation appears to be successful at the ankle but is accompanied by an increased stiffness at the hip. We discuss the possible implications of these findings for postural stability and fall risk in PD patients.