There have only been a few studies that have reported the effects of adding exercise to psychological treatments of depression. Previous reviews and meta-analyses have also raised concerns over the methodological quality of existing studies examining the effects of exercise on depression. Specifically, these reviews and meta-analyses noted overestimation of treatment effects, and lack of long term follow-up data as limitations to the existing literature [20, 38, 39]. Adherence rates for exercise-based interventions are comparable to those found in pharmacotherapy and other psychotherapeutic treatments, but still remain low. Another limitation of adjunctive exercise is that there appears to be many possible biomarkers of exercise that likely interact with one another, which makes interventions that target exercise challenging. It is also challenging to engage depressed individuals in exercise interventions, particularly those with anhedonia, given the lack of interest and motivation inherent in this condition [44, 45]. Thus, future research of adjunct exercise interventions should include randomized clinical trials with strong methodological designs and control groups, biological outcome measures, and examination of moderating variables (to identify those who may benefit most from these interventions).
