["This study assessed: 1) the effects of 12 weeks of resistance training (RT) and resistance training with instability (RTI) on presynaptic inhibition (PSI) and disynaptic reciprocal inhibition (DRI) of patients with Parkinson's disease (PD); 2) the effectiveness of RT and RTI in moving PSI and DRI values of patients towards values of age-matched healthy controls (HC [Z-score analysis]); and 3) associations between PSI and DRI changes and clinical outcomes changes previously published.", 'Thirteen patients in RT group, 13 in RTI group, and 11 in a non-exercising control group completed the trial. While RT and RTI groups performed resistance exercises twice a week for 12 weeks, only the RTI group used unstable devices. Soleus H-reflex was used to evaluate resting PSI and DRI before and after experimental protocol. The HC (n=31) was assessed at pre-test only.', 'There were significant group×time interactions for PSI (P<0.0001) and DRI (P<0.0001). RTI was more effective than RT in increasing the levels of PSI (P=0.0154) and DRI (P<0.0001) at post-training and in moving PSI (Confidence interval [CI] 0.1-0.5) and DRI (CI 0.6-1.1) levels to those observed in HC. There was association between DRI and quality of life changes (r=-0.69, P=0.008) and a strong trend toward association between PSI and postural instability changes (r=0.60, P=0.051) after RTI.'] Is resistance training with instability more effective than resistance training in improving spinal inhibitory mechanisms in Parkinson 's disease?