Assessing effects of neurodegenerative disorders, such as Parkinson's disease (PD), on muscle activation for maintaining balance is a research topic of interest to clarify the pathophysiology of postural instability. This study aims to a deeper insight into pathophysiological mechanisms associated with balance disorders in PD patients, under and not under dopaminergic treatment, through muscle synergy analysis. Ten patients with PD and ten age-matched healthy subjects were exposed to external perturbations around the vertical axis (yaw perturbations). PD patients were evaluated twice, both under their usual dopaminergic therapy (ON) and after a medication washout period of 12 hours (OFF). Two different external perturbations were provided using the RotoBit^1D. Muscle activities of 12 bilateral muscles of the upper body were gathered through surface electromyography. To extract muscles synergies, the Non-Negative Matrix Factorization was performed and the Variability Account For was used to assess the minimum Number of Synergies (NoS). Mean, standard error and Coefficient of Variability (CoV) of the total Variability Account For (tVAF) for synergies from 1 to the maximum NoS found were considered. No statistical differences were found considering the NoS that ranged from 2 to 7 among all participants. Statistical differences were found in the tVAF between PD patients and healthy subjects. No statistical differences were found between PD patients OFF and ON state of therapy for all NoS. Finally, the minimum NoS of PD strongly correlated with specific clinical evaluation scores, such as cognitive function and bradykinesia. These findings unveiled poor balance control strategies in PD patients, depending on cognitive and motor impairment and not responding to dopaminergic treatment.