Antiparkinson medications improve agonist activation but not antagonist inhibition during sequential reaching...

The execution of sequential arm movements is critical to activities of daily living such as eating and grooming. It is known that movement sequences are bradykinetic in people with Parkinson's disease (PD) and that antiparkinson medications improve the speed of movement sequences. However, it is unclear how muscle activity is modulated during sequential movements and what effect antiparkinson medications have on muscle modulation. We studied subjects with PD and age- and gender-matched control subjects making sequential reaching movements. Subjects with PD were tested before and after their morning dose of antiparkinson medications (levodopa and/or dopamine agonists). We examined the effect of antiparkinson medications on the modulation of muscle activity (i.e., the ability to activate and inhibit each muscle throughout the course of a sequence). Results showed that the group with PD, before medication, moved more slowly and modulated muscle activity poorly compared to the control group. Antiparkinson medications improved movement speed as expected, although sequential movements remained slower than normal even after medication. Medication improved the ability to activate agonist muscle activity but did not improve the ability to inhibit antagonist activity. Instead, antagonist activity was also increased, resulting in minimal improvements in muscle modulation during sequential reaching movements.