Over the past decade (or more), there has been tremendous growth in the number of randomized controlled trials and meta-analyses revealing the effectiveness of physical therapy and exercise interventions for persons with Parkinson disease (PD).1 Given this accumulating body of evidence, exercise is now considered an important element in the treatment of PD. Although this marks important progress in the field, there is still much more to be learned to optimize the outcomes of the patients we serve. For example, which persons with PD benefit most from a particular intervention? Is one type of exercise better than another? Does dose matter? Does the timing of the intervention matter? What are the underlying mechanisms that may account for the benefits of exercise?

There are many aspects of physical therapy and exercise interventions that are beneficial for persons with PD; however, not all persons with PD respond similarly to all interventions. Many of the physical therapy and exercise studies to date include persons in Hoehn & Yahr stages 2 and 3. This represents a very heterogeneous group of persons with PD with a wide range of PD symptoms, physical mobility challenges, cognitive, and nonmotor impairments-all of which can influence responsiveness to a particular intervention. In RCTs, the benefits reported represent changes in the mean-not all study participants improve. This "one size fits all" approach is lacking in precision. To optimize outcomes, we must take a deeper dive. For example, identifying patient characteristics, disease phenotypes, or levels of disease severity that may be more responsive to a particular type of intervention would help to prescribe the optimal treatment to those who could benefit most. In this special issue, researchers address this issue to "drill down" to identify the "responders" to a variety of interventions or more specific deficits that may cause disability. For example, Skinner et al2 look beyond muscle strength and examine muscle force steadiness in lower extremity muscles critical for balance and gait. Strouwen et al3 investigate the characteristics of persons with PD who benefited most from dual-task training. Lofgren et al4 shed light on those individuals who were most responsive to a highly challenging gait and balance training program. Olivier et al5 characterize postural skill acquisition in people with PD to identify factors that predict postural motor learning. Taken together, these studies provide therapists targets for intervention and insight into what characteristics predict greater response to therapeutic interventions.

The optimal dose of exercise for persons with PD has not been determined. To understand this complex but important feature of treatment, it is important to rigorously assess the impacts of various exercise doses via comparative effectiveness studies. Furthermore, identification of biomarkers, which underlie functional changes, can provide surrogate outcomes on which to assess dosing studies. Some studies in PD have begun to examine the differences between higher-intensity and moderate-/low-intensity exercise within a particular modality of exercise (eg, aerobic training).6 In this special issue, Landers et al7 take a more pragmatic approach by examining the differences between a high- and low-intensity multimodal exercise program (aerobic, strengthening, and balance training). These investigators also examine potential disease-modifying effects of their high- versus low-intensity exercise program with the goal of providing critical insights into the underlying mechanisms that may account for the changes observed. Others have examined the timing of interventions. For example, are interventions more effective when applied earlier or later in the course of the disease? Lirani-Silva et al8 address this issue and examine how disease progression may influence the effects of auditory cueing on gait impairment.

Interestingly, Rawson et al9 showed that treadmill walking, tango dancing and stretching had differential effects across some outcomes but not others when examined off their PD medications.9 This suggests that, in some cases, identifying the most optimal treatment for an individual depends on the outcome of interest. When examined closely, different treatments may share common core elements that lead to similar results across outcomes. The relative similarity of some treatments (of similar dose) may allow clinicians to refocus their patient-specific lens toward a different type of individuality: patient preference. Identifying programs that are exciting and engaging at an individual level may increase adherence and dose, thus improving the patient experience and increasing the overall benefit from the intervention.

The contributions to the special issue on Parkinson disease address critical questions in the field to help advance neurologic physical therapy research leading to more targeted treatment of persons with Parkinson disease. With increased clarity on who benefits most from which types of treatments, physical therapists can tailor their interventions to optimize patient outcomes. This more refined approach is an essential step toward improving the lives of the patients we serve.

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