Volitional processes in changing physical activity: A randomized controlled trial with individuals with knee osteoarthritis.

A health action process approach (HAPA)-based intervention was designed to support moderate-to-vigorous physical activity (MVPA) in individuals with osteoarthritis of the knee (OAK). In secondary analyses of the randomized controlled trial "preventing the impairment of primary osteoarthritis by high-impact long-term physical exercise regimen-psychological adherence program," we examined long-term effects of the intervention on HAPA determinants and MVPA, and explored the former as mediators of change.