Physical inactivity, neurological disability, and cardiorespiratory fitness in multiple sclerosis.

The maintenance of cardiorespiratory fitness is important for ensuring independence in daily life among persons with chronic disabling conditions such as multiple sclerosis (MS; 1). Cardiorespiratory fitness or peak oxygen consumption (VO2peak) is defined as the highest rate of oxygen uptake attained during physical work (2) and is a product of the peak cardiac output and the peak difference in arteriovenous oxygen (3). There is evidence for a reduction in cardiorespiratory fitness among persons with MS (4) and this is likely associated with physical inactivity and neurological disability (1). Physical inactivity is one factor that may contribute to the reduction of cardiorespiratory fitness in persons with MS. Physical inactivity is prevalent in persons with MS (5, 6), but there have been few examinations of an association between lifestyle physical activity and cardiorespiratory fitness in this population. One study reported no significant association between self-reported physical activity and VO2peak in persons with MS (7). We are not aware of other studies that have examined objectively measured physical activity as a correlate of VO2peak in this population. Neurological disability is another factor that might be associated with reduced cardiorespiratory fitness. To date, there is inconsistent evidence of an association between neurological disability and cardiorespiratory fitness in the MS population. Some studies have reported an inverse association between cardiorespiratory fitness and neurological disability (8, 9), whereas other studies have reported no association between cardiorespiratory fitness and neurological disability (10).We further note that researchers have not examined the possibility that neurological disability is associated with cardiorespiratory fitness independently of physical activity. This is important as physical inactivity has been associated with neurological disability in persons with MS (11, 12) and it might be associated with cardiorespiratory fitness. This article includes two studies that examined the association between physical activity and cardiorespiratory fitness (Study 1) and the associations among physical activity, neurological disability, and cardiorespiratory fitness (Study 2) in persons with MS. Study 1 examined the association between physical activity and cardiorespiratory fitness given there is minimal and conflicting evidence regarding physical inactivity as a factor that may contribute to the reduction of cardiorespiratory fitness in persons with MS. Study 2 was a follow-up that examined the associations among physical activity, neurological disability, and cardiorespiratory fitness given that researchers have not examined the independent associations of physical inactivity and neurological disability with reductions in cardiorespiratory fitness in persons with MS. Importantly, physical activity was measured in both studies by objective and self-reported instruments that have been validated in persons with MS (13, 14). We hypothesized that physical inactivity would be associated with reduced levels of cardiorespiratory fitness in the first study, and that physical inactivity and neurological disability would be independently associated with reduced levels of cardiorespiratory fitness in the second study.