Methodological Considerations

The ways in which regular PA and emotional functioning constructs were measured in the current study may have also influenced the pattern of results in unexpected ways. Regular PA can be measured using many different approaches. The current study sought to capture dis- tinctions between those who do and those who do not report engaging in longstanding regular PA, which is thought to engender enduring structural and functional brain changes in regions relevant to the cognitive processes under study (Dishman et al., 2006). However, at least one study has linked levels of regular PA that are as low as 20 minutes per week with mental health benefits (Hamer, Stamatakis, & Steptoe, 2009). Thus, it is possible that the criteria used to cre- ate the dichotomous PA variable in the current study did not yield a variable that was optimal for capturing group differences in emotional functioning and the use of this method of categoriza- tion obscured individual differences in the sample (MacCallum, Zhang, Preacher, & Rucker, 2002). For example, the PA- group comprised individuals who reported a wide variety of PA lev- els, ranging from no PA to regular recommended levels of PA for less than 6 months; this

within-group heterogeneity may have diluted our ability to detect important emotional health dif- ferences between the PA- group and the PA+ group.

Thus, whereas the dichotomization strategy was selected to correspond meaningfully to PA’s brain-related changes, it may not have been the best choice for distinguishing between in- dividuals likely to obtain high versus low emotional health benefits. An alternative dichotomiza- tion strategy might have been to compare emotional functioning between participants who en- dorsed being sedentary and those who reported longstanding engagement in recommended levels of PA. Use of such an extreme groups approach would still allow for the distinction of a PA level and chronicity that is in line with CDC recommendations and that is also thought to lead to changes in the brain. It would also, theoretically, increase the likelihood that group differ- ences in emotional functioning would be large, and thus confer greater power to detect effects (Preacher, Rucker, MacCallum, & Nicewander, 2005). This approach, however, also has disad- vantages, such as loss of information, magnification of results or increased risk of false positive results, potentially spurious grouping of individuals with considerable variability, and obscuring findings about individuals with scores that fall between the extremes (Altman & Royston, 2006; Preacher, MacCallun, Rucker, & Nicewander, 2005).

Although recommended levels of PA released by the CDC are often used as a logical starting point in research, the dose of regular PA required to confer emotional health benefits is unclear. Some studies indicate that any regular PA, regardless of intensity, is associated with fewer indicators of poor emotional functioning (e.g., Harvey, Hotopf, Øverland, & Mykletun, 2010), while others suggest that specific intensities or doses are necessary (e.g., Dunn, Trivedi, Kampert, Clark, & Chambliss, 2005). Future research is encouraged to measure regular PA in ways that allows for greater distinction among its many different types and levels; such work would be useful in providing a firmer foundation for recommendations regarding optimal PA for

mental health. Such studies might, for example, gather data on type, intensity, energy expendi- ture, frequency, duration, and chronicity of activity to better understand which aspects of PA may be more reliably associated with emotional health benefits.

It is also important to consider ways in which the choice of an emotional functioning measure may have influenced results in the current study. In particular, it is possible that regular PA could be associated with emotional functioning in ways that are not reflected in scores on the emotional functioning questionnaire used in the present research. Research linking regular PA/fitness and mental health commonly uses questionnaires designed to measure depressive or anxious symptoms (e.g., Conroy et al., 1982; Morgan et al., 1970,) mood, (e.g., Steptoe et al., 1989) or general wellbeing or mental health status (e.g., Bhui & Fletcher, 2000; Greist et al., 1978; Steptoe & Butler, 1996). The DASS, a measure that includes questions about depression, anxiety, and stress in the preceding week, shows good convergent validity with other measures of depression and anxiety (Lovibond & Lovibond, 1995; Nieuwenhuijsen, De Boer, Verbeek, Blonk, & Van Dijk, 2003). However, its focus on negative emotional symptoms in the past week, which may be easily influenced by situational factors during that time frame, may set it apart from measures that cover longer periods of time. It is plausible that emotional functioning during the relatively brief reference period of one week may not be consistent with more enduring as- pects of participants’ general emotional health status, which might be differentially associated with the effects of regular PA and/or cognitive control performance (Rutherford, MacLeod, & Campbell, 2010). Further, past-week self-report questionnaires may be more suitable for longi- tudinal studies that explore differences in emotional functioning across two or more time points (e.g., before and after engaging in regular PA). Cross-sectional studies, on the other hand, might be better served by the use of trait-based measures that inquire about how the respond- ent views their emotional functioning in general to assess emotional health.

5.3 Cognitive Control