Isometric Torque Generation in a Parkinsonian tremulous elbow and the effect of medication

Individuals afflicted with Parkinson´s disease (PD) have impaired force or torque control. It has also been reported that force or torque production, either dynamic or isometric, may be debilitated by PD in certain muscles and joints. During isometric contractions, the reported impairments were mostly in the rate of force changes, in latency, and in Maximum Voluntary Contraction (MVC) levels. However, in tremor dominant PD patients, it is of interest to investigate the effect of medication on the overall intended torque generation, with the tremor (unwanted) component being averaged out. This study examined average (tremor removed) isometric torque generation in submaximal contractions of elbow flexion-extension. The patient in this case study had unusually strong, but dopa-responsive, rest and isometric tremors. Ridge regression models were used to relate the average generated torque to average EMG in four main elbow flexor and extensor muscles. If changes in MVC were the result of medication, a reasonable assumption would be a mere scaling in the force production model. However, the results suggest that, in both tremor-dominant and non-tremor-dominant hands, the model parameters were different for the on-medication state compared to the off-medication state. This finding may indicate that relative activation of each muscle involved in this motion is affected by anti-Parkinsonian medication.