Parkinson disease (PD) could result in functional limitations through both motor and nonmotor symptoms. Although patients with advanced disease have well-documented and profound functional limitations, less is known about the determinants of function in early to mid-stage disease where interventions may be more likely to benefit and preserve function. The aim of this study was to identify motor, cognitive, and gait determinants of physical functional performance in patients with early to mid-stage PD. This was a secondary analysis of cross-sectional baseline data from a randomized clinical trial of exercise. The study was comprised of 121 patients with early to mid-stage PD. The functional performance outcomes included the following: the Continuous Scale Functional Performance Test (CS-PFP; primary outcome); the timed Up and Go test (TUG); and Section 2 (Activities of Daily Living) of the Unified Parkinson’s Disease Rating Scale (UPDRS). Explanatory variables included measures of disease severity, motor function, cognitive function, balance, and gait. Stepwise linear regression models were used to determine correlations between explanatory variables and outcome measures. In the regression models, the CS-PFP significantly correlated with walking endurance (Six-Minute Walk Test; r2= 0.12, P< .0001), turning ability (360° Turn Test; r2= 0.03, P= .002), attention (Brief Test of Attention; r2= 0.01, P= .03), overall cognitive status (Mini–Mental State Examination; r2= 0.01, P= .04), and bradykinesia (timed tapping; r2= 0.02, P= .02). The TUG significantly correlated with walking speed (5-Meter Walk Test; r2= 0.33, P< .0001), stride length (r2= 0.25, P< .0001), turning ability (360° turn, r2= 0.05, P= .0003), and attention (r2= 0.016, P= .03). Section 2 of the UPDRS was significantly correlated with endurance (r2= 0.09, P< .0001), turning ability (r2= 0.03, P= .001), and attention (r2= 0.01, P= .03).
Gait, motor, and cognitive function all contribute to objectively measured global functional ability in mild to moderate PD. Subjectively measured functional activity outcomes might underestimate the effect of motor as well as nonmotor symptoms.