DOES PARKINSONIAN ACTION TREMOR CONTRIBUTE TO MUSCLE WEAKNESS IN PARKINSONS-DISEASE

The aim of this study was to see whether action tremor contributes to the weakness which can be measured in some muscles in patients with Pa rkinson's disease, by preventing fully fused contraction of motor unit s. Strength and action tremor were recorded during maximal wrist exten sion in patients when they were on and off antiparkinsonian medication , and in age- and sex-matched healthy subjects. Peak torque and mean r ectified EMG levels were reduced by 25% and 30% (n = 7), respectively, when patients were off medication (compared with when they were on me dication). In parkinsonian patients off treatment, action tremor was v isible in torque and EMG records, and had a frequency of similar to 10 Hz. The absolute amplitude of this tremor was considerably smaller in patients on medication and in control subjects. In patients, medicati on reduced action tremor in torque and EMG by 37% and 57%, respectivel y so that tremor amplitude approached that in normals. Similar changes were seen when action tremor was expressed as % peak torque or % mean rectified EMG. In parkinsonian patients off medication, a 10-Hz synch ronizing influence dominates muscle activity at the wrist. The result is an incompletely fused muscle contraction which is an important fact or contributing to the weakness present in the off-medication state. A ntiparkinsonian medication releases motor units from the 10-Hz synchro nizing influence, enabling higher discharge rates, fused contraction a nd improved force generation.