The Influence of Cognitive Impairment on the Fitness/Cognition Relationship in Multiple Sclerosis

Background: Cognitive processing speed impairment (PSI) is common and debilitating in persons with multiple sclerosis (MS). Exercise training has been proposed as a behavioral approach for possibly managing MS-related processing speed dysfunction, largely based on cross-sectional studies of the relationship of cardiorespiratory fitness and cognitive processing speed. However, there is minimal evidence supporting exercise training as a treatment for MS-related PSI, as the vast majority of the existing studies have examined exercise and cardiorespiratory fitness effects on cognition in samples of non-cognitively impaired persons with MS.

Objectives: The current cross-sectional investigation examined whether cardiorespiratory fitness was differentially associated with processing speed in persons with MS with and without PSI.

Methods: 64 persons with MS undertook the Symbol Digit Modalities Test (SDMT) for establishing groups with and without MS-related PSI, a modified flanker task for measuring processing speed performance, and an incremental exercise test to exhaustion on a cycle ergometer for measuring cardiorespiratory fitness in a single testing session. Bivariate correlations were performed on cardiorespiratory fitness and processing speed outcomes in PSI group subsamples.

Results: Cardiorespiratory fitness was not significantly associated with processing speed (ρ=−.20, p=.08) in the sample without PSI. However, in the sample with PSI, cardiorespiratory fitness was significantly associated with processing speed (ρ=−.62, p=.01), such that lower cardiorespiratory fitness was strongly associated with slower processing speed.

Conclusions: This study provides preliminary evidence of a significant association between cardiorespiratory fitness and processing speed in persons with MS-related PSI. Such an investigation provides the first direct support for aerobic exercise training as a possible behavioral approach for managing/treating MS-related cognitive impairment, beyond merely improving cognitive performance.