Causal Mediation Analysis of Factors Influencing Physical Activity and Community Access Among People With Mild-to-Moderate Parkinson Disease

Objective: To examine how known causal factors (exercise self-efficacy, balance, walking capacity) affect outcomes (moderate-intensity physical activity, community access) in people with Parkinson disease (PD): through a direct pathway, indirectly through potential mediators (nonmotor impairments), or through combined direct and mediated paths. Design: Causal mediation analyses using baseline and three-month data from pooled treatment groups in a randomized controlled trial. Setting: Data were collected at 2 university clinical research centers. Participants: One hundred thirty-eight people with PD. Interventions: Home and community-based walking and strength exercise program. Main Outcome Measure(s): Exposures were exercise self-efficacy, walking capacity, and balance. Potential mediators included pain, fatigue, mood (anxiety, depression, affect, apathy), stigma, and cognition (executive function and cognitive flexibility). The outcomes were physical activity and community access behavior at follow-up 3 months later. Separate models were developed for each causal factor-mediator-outcome combination. To minimize bias, all models were adjusted for known confounders (comorbidities, freezing of gait, severity of motor impairment, and/or age) and baseline values of the outcomes. Results: Self-efficacy of walking duration had a causal relationship with moderate-intensity physical activity through direct and combined paths (P<.001). Walking capacity had a causal relationship with community access through direct (P=.03-.04) and combined (P=.02-.03) paths. Balance did not affect community access (P>.05). There were no significant mediation effects through indirect pathways for either outcome. Conclusions: The effect of known causal factors on physical activity and community access was not mediated by nonmotor impairments. Walking self-efficacy and walking capacity remain the primary intervention targets for improving physical activity and community access, respectively, in people with PD.