Centrally initiated postural adjustments in parkinsonian patients on and off levodopa

This study investigates the effects of parkinsonism and dopamine replacemen t therapy (levodopa) on centrally initiated postural activity preceding ris ing onto the toes. The electromyographic (EMG) and force magnitude, scaling , sequencing, and postural stabilization were compared when rising-to-toes under two conditions, slow/low versus fast/high, for parkinsonian patients and elderly control subjects. Parkinsonian subjects were tested after withh olding their levodopa medication for 12-16 h and again 1 h after taking the ir medication when parkinsonian signs were diminished. Parkinsonian subject s showed reduced magnitudes and delayed timing of the postural and voluntar y components of the rise-to-toes task, as if they had difficulty turning of f the postural, tibialis anterior (TIB) component and initiating the volunt ary, gastrocnemius (GAS) component. Dopamine improved the relative timing, as well as the magnitude of both postural and voluntary components of rise- to-toes. Although the magnitude of dorsiflexion torque was smaller for park insonian subjects ON and OFF than for healthy elderly controls, the parkins onian subjects showed intact scaling of the magnitude of postural activity. Parkinsonian subjects do not perform the rise-to-toes task like normal sub jects who are instructed to rise slowly; the relative timing of TIB and GAS activation was different even at comparable speeds of performance. Parkins onian subjects, both ON and OFF, exhibited greater risk of falling than eld erly control subjects when rising to toes. This increased risk of falling w as reflected in a smaller safety margin between the peak center of mass (Co M) and peak center of pressure (CoP) during the task. The magnitude of mean postural dorsiflexion torque in the rise-to-toes task was highly correlate d with a clinical rating scale of gait and balance, suggesting that force c ontrol is a critical factor influencing postural control in patients with P arkinson's disease.