The overlap in PA and PF outcomes of these two studies presented a unique opportunity to examine the effect of the SG-endorsed PA guidelines on PF. The recommended 150 minutes of moderate PA per week or more is well integrated into clinical practice,23
yet there is a paucity of data pertaining to the direct functional effect of these guidelines on older adults. Maintenance of PF is considered a hallmark of successful aging.24
One limitation in assessing global PF as an outcome relative to PA is that it potentially requires a large sample size. Estimates of projected sample sizes needed to obtain clinically meaningful changes in a variety of outcomes following a PA intervention have been previously published.13
Estimates projected for global PF measures required the largest sample size (N = 261 per group in a two-armed trial). A sample size of this magnitude is frequently beyond the reach of most PA interventionists. By pooling two modest-sized intervention trials, which used identical measures of PA and PF, it was possible to obtain a large enough sample and enhance generalizability to address the research questions relative to the association between PA and PF and whether change in PA affects the functional trajectory.
This study confirms that PA is an important contributor to PF, given the strong cross-sectional and longitudinal association between PA and PF. This study is unique in assessing PA relative to the specific threshold set by the national recommendations. For the first time, to the authors' knowledge, clinicians will be better able to characterize functional differences between individuals meeting or failing to meet PA guidelines. The 16-point difference in PF observed in this study is substantial. Of subjects exceeding PA guidelines, the baseline PA score of 74 was at approximately the 70th percentile of PF for mean and women aged 75 and older.16
A reduction of 16 points in PF would be comparable with the combined comorbidity burden of arthritis and congestive heart failure.22
Therefore, these findings are clinically meaningful.
A cross-sectional view is limited in that one cannot determine whether individuals with low PF had conditions that inhibited their ability to be physically active. Therefore, the second research objective, using longitudinal trajectories of PF, allowed the independent effect of the PA threshold on functional status to be examined. As a group, individuals who changed PA level also experienced a change in functional status. This is of particular interest to inactive individuals with poor functional status. Individuals who, independent of functional status, were able to increase their PA level improved 5.1 points in PF from an average baseline score of 63.4. As was noted earlier, a change in PF of this magnitude is clinically meaningful and would be comparable with medical interventions such as treatment for asthma with fluticasone versus placebo and treatment of sleep apnea with continuous positive airway pressure.22
The changes noted in the current study are comparable, in trajectory but not magnitude, with changes observed in a study of older patients with severe chronic obstructive pulmonary disease (baseline PF = 30) whose PF score increased 9 points after a pulmonary rehabilitation program.25
The difference in magnitude of change makes sense, given that the interventions were of home-based PA in contrast to a medically supervised structured exercise program. Two other studies have examined PA relative to global PF (physical disability) and found a protective effect of PA on incident disability, but in these studies, the rates of PA were not specific to the threshold levels purported by the SG guidelines.26,27
These studies, in addition to the current one, are highly relevant, because they suggest that exercise initiated in later life is highly beneficial.
Conversely, a reduction in PA level is associated with a clinically meaningful reduction in PF. Unfortunately it was not possible to determine the causes of the decreases in PA levels (whether reductions were due to sudden illness, injury, caregiving needs, or lack of interest). Nonetheless, a reduction in PA level was associated with a clinically meaningful loss in PF at a level that is associated with a greater inability to work because of health problems, the onset of a disease such as congestive heart failure, or the combined functional burden of diabetes mellitus and hypertension.22
The functional trajectory of individuals who maintained PA, although not surprising, is equally important in confirming that PF is preserved with maintenance of PA. An earlier study examined functional trajectories in a group of older women who sustained PA, sustained inactivity, or were irregularly active over several years and found lower rates of reported difficulty with activities of daily living in the women who were consistently active.11
Other studies confirming this finding have used tests of physical performance such as gait speed and walking ability to determine PF.11,12
This article is a contribution to the field, because the literature pertaining to PA and global PF is so sparse. A review article of exercise intervention outcomes found that 97% of outcomes reported were fitness impairment outcomes (e.g., improved strength, aerobic capacity, flexibility), and although 87% of studies reported functional outcomes, most of these were based on physical performance tests such as walking or stair climbing. A global indicator of health-related PF was rarely assessed.3
Although this pooled analysis offers valuable findings, there are limitations that must be borne in mind in generalizing these data. One limitation is that the measures, although validated, are based on self-report. Although self-report of PF is considered to be a reliable indicator of functional status, self-report of PA is more prone to misclassification. Both studies had a single-item screening question designed to exclude individuals engaging in regular exercise. The screening items targeted structured exercise and not general physical activities. In contrast, the PA outcomes in this study were derived from the moredetailed CHAMPS questionnaire, which assesses all types of PA. Although 68% of the sample reported less than 150 min/week of PA, the average number of reported minutes of PA was 162. This suggests that the single-item screening question was not sensitive enough to capture nonexercise PA that typically count toward the assessment of total daily PA, especially with regard to gardening and household activities, or that some individuals may have overreported their PA. Thus these data should be interpreted conservatively. Also, consideration should be given to potential sex differences in that this sample consisted predominantly of men and the findings may be less applicable to women.
In conclusion, this study is an initial step in quantifying the relationship between evidence-based guidelines for PA and PF in older adults. The guidelines endorsed by the SG were based upon an exhaustive review of the evidence. The current study confirms the validity of the SG recommendation and adds insight to the utility of the recommendation for older adults. Newly released PA recommendations for older adults advocate strength training and flexibility exercises in addition to the recommendation to perform moderate PA for 30 minutes on 5 or more days of the week.23
Although adoption of these guidelines will strengthen the association between PA and PF, prospective trials assessing the effect of these guidelines on functional status are warranted.