We have described the changes in aerobic fitness, cognitive function, and functional measures in a small group of individuals with chronic stroke following participation in 12 weeks of aerobic and strengthening exercise in this pilot study. Improvements in a measure of working memory (Digit Span Backwards test) were noted following exercise. However, the magnitude of this change was small (mean = 0.56 digits) and may not be clinically significant. Other researchers found a 1.0 digit change in healthy older adults after 4 weeks of strengthening exercise.16 Although this measure has not been used extensively in people with stroke, previous researchers have described differences in the Digit Span Backwards test between older adults with and without mild cognitive impairment (range of 3.3-4.4 digits and 4.6-5.6 digits, respectively).33 The postintervention scores for our participants are within the range reported for adults with mild cognitive impairment.
A significant correlation between change in aerobic fitness and improved score on an executive function test (the Flanker-incongruent score) was found following the intervention. The incongruent condition was more challenging than the congruent condition, as indicated by the higher error rate. Executive function as measured with the Flanker test has been previously described with very low error rates in healthy adults, with correct scores in approximately 99% of congruent trials and in approximately 97% of incongruent trials.2,35 Our participants scored lower, with 92.4% correct responses in the congruent condition and 71.8% correct responses in the incongruent condition at baseline. We found that our participants had a 29% increase in reaction time with incongruent stimuli compared to responses to congruent stimuli. This level of interference is comparable to the 26% increase found in older adults with low levels of physical fitness.2 Our participants showed a 7.7% improvement in their ability to overcome interference following participation in exercise. Although the clinical significance of this change is unknown, it is less than the 11% improvement reported following a 6-month exercise intervention in older adults.2
A meta-analysis of the effects of exercise and cognition in older adults found strong support for the hypothesis that aerobic fitness training improves cognitive performance on a variety of tasks.5 The strongest benefit was noted for executive-control processes, as compared to other measures of cognition (speed, visuospatial, or controlled processing). Both of our cognitive outcome measures (Digit Span Backwards and Flanker tests) could be considered to be in the executive-control domain. Furthermore, the meta-analysis found that combined aerobic and strengthening exercise programs have a greater influence on cognitive measures than aerobic exercise alone.5 Although our subjects underwent both strength and aerobic training, our analysis focused on changes in aerobic fitness. It would be interesting to assess the relationships between changes in motor strength or fat-free mass (eg, with dual-energy x-ray absorptiometry measures) with measures of cognition in future studies.
The effect of exercise on cognition in people with stroke is only beginning to be explored. A randomized controlled trial in people with chronic stroke examined the effect on executive function, motor learning, and mobility of 8 weeks of aerobic-only exercise compared to a control intervention (home stretching program).24 Greater improvements in information-processing speed and motor learning were found in the aerobic intervention group. However, no differences were noted between the groups in tests of executive function (eg, Trail Making B-A, Stroop, and Wisconsin Card Sorting tests). Direct comparison between studies is limited by the different cognitive outcome measures and the overlap between different cognitive domains assessed by these measures. These early results are encouraging, and examination of the dose and type of exercise on cognition in people with stroke seems important for further study.
Most of our participants improved aerobic fitness following the intervention, and these changes showed a trend toward statistical significance. Other authors have found improved aerobic fitness and motor function following exercise in people with chronic stroke.24,53-56 The baseline
The FM motor score improved significantly in our subjects following the intervention. It has been suggested that a change of 6 to 7 points in the FM score indicates clinical significance.57 Our participants had a smaller change (mean 3.6 points) although their baseline score was more than 80, which indicates a relatively mild severity of motor deficits.58 Total SIS scores also improved significantly, even though the intervention did not include functional task practice. The SIS score change of 33.8 noted in our participants reflects change over 8 different domains, and interpretation of changes in this self-report measure must be interpreted with caution in the absence of a control group that received a comparable amount of attention, as it is possible that the subjects improved simply by virtue of participation in a study. Stroke Impact Scale total scores have been reported to distinguish between mild and moderate stroke (605.3 and 512.5, respectively, at 6 months following stroke),36 and the total SIS scores of our subjects fell within this range.
One interesting factor is that 4 of our 9 participants had a diagnosis of diabetes mellitus, and older adults with type 2 diabetes may have deficits in memory and learning (specifically list learning tasks).59 These deficits may be secondary to a chronic hyperglycemia-induced “central neuropathy,”60 cortical atrophy,61,62 or atrophy specifically in the hippocampus and amygdala.63,64 The possible confounding influence of the dual diagnosis of diabetes and stroke on executive function has not received attention in the literature but would be another important area of future study.
One of the primary limitations of this pilot project was the difficulty with recruitment and retention during this 12-week project. The intensity of this project (3 times each week for 12 weeks) and the requirement that participants come to an urban academic medical center for the intervention may have contributed to the difficulty with recruitment and retention. The small number of subjects increased the possibility of a type II error, when the sample size has insufficient power to detect a statistically significance difference.65
In summary, the findings of this pilot study indicate that participation in a 12-week aerobic and strengthening exercise program improved selected measures of executive function in people with stroke. These results contribute to the emerging literature on improvements in cognition following exercise in people with stroke. These benefits indicate the need for future study with a larger group to further explore these relationships.
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