Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been shown to be an effective treatment for Parkinson's disease (PD). The intraoperative positioning of DBS electrodes and postoperative adjustment of the stimulation parameters, however, require continuous, precise evaluation. Moreover, ambulatory measurements of the symptoms would also help to evaluate changes in the progression of PD in these patients. To this aim, we objectified rigidity measurements via surface EMG recordings of the Mm. biceps (bic) and triceps brachii (tric) in patients treated with chronic stimulation of the STN. We show that cessation and initiation of DBS have effects on the EMG profile during standardized extension and flexion movements in the elbow joint. These data correlate significantly with clinical ratings. Thus, EMG recordings of the Mm. bic and tric during this standardized extension-flexion movement can be used to objectively measure rigidity and to monitor its course over time. In view of its low technical requirements, this technique lends itself to use during DBS implantation surgery and in the clinical environment.