In a secondary analysis of the SPARX Phase 2 randomized clinical trial (128 de novo Parkinson's disease patients), sedentary time significantly modified cardiorespiratory fitness gains from moderate-intensity treadmill exercise over 6 months. In a sensitivity analysis restricted to complete cases, each 10-percentage-point increase in average daily sedentary time was associated with a 2.51 mL/kg/min reduction in VO2max improvement compared with usual care. No equivalent interaction was detected in the high-intensity exercise group, suggesting that higher exercise intensity may be resilient to the dampening effects of prolonged sitting.
This finding matters because VO2max is one of the strongest predictors of all-cause mortality and functional longevity in adults, and Parkinson's patients already face accelerated fitness decline. The data hint at a practical prescription principle: if patients with PD spend most of the day sedentary, moderate-intensity exercise alone may not be enough to meaningfully lift cardiorespiratory fitness — pushing toward high-intensity protocols could be the more effective strategy. That said, several cautions apply. This is a secondary, hypothesis-generating analysis from a trial not powered to detect interaction effects, and the key result appeared only in a sensitivity analysis of complete cases rather than the primary model. The 128-person sample is small for subgroup work, and residual confounding cannot be excluded. Being a preprint not yet peer-reviewed, these findings should be treated as preliminary and directional. Clinicians should await a confirmatory, adequately powered trial before adjusting exercise prescriptions for PD patients based on sedentary behavior.