Aerobic Fitness and Instrumental Activities of Daily Living in People with Multiple Sclerosis: A Cross-sectional Study

Aerobic fitness, defined as the body's capacity for delivery and extraction of oxygen during physical work, is essential for maintaining physical independence, particularly in the presence of aging or chronic conditions.1,2 Those with lower levels of aerobic fitness usually have greater difficulty with everyday tasks such as housework and self-care, as everyday tasks become straining and exhausting.3 This can yield a cycle of decline whereby those with lower aerobic fitness become less active as part of daily life and, consequently, less fit and more dependent on others for completing daily tasks. The association between aerobic fitness and physical independence has been well studied in older adults,4 but less is known in those with chronic neurologic conditions and diseases. Multiple sclerosis (MS) is a life-altering chronic inflammatory disease of the central nervous system; in the United States, it commonly occurs in young and middle-aged adults.5 The progression of the disease (ie, degree and location of axonal and neuronal damage in the central nervous system) results in the accumulation of irreversible neurologic disability and further influences aerobic fitness and performance of activities of daily living (ADLs).6 

There is ample evidence supporting that aerobic fitness, measured as peak aerobic capacity or peak oxygen consumption (VO_2peak), is reduced in people with MS.7 We further note that aerobic fitness has been associated with brain structure and function,8,9 cognition, and walking in people with MS.2,10 Collectively, the accentuated decline in aerobic fitness associated with the disability progression caused by the disease could contribute to an inability to satisfactorily perform instrumental ADLs (IADLs). Therefore, we believe that aerobic fitness might be associated with the ability to perform IADLs in people with MS, but this has not been documented to date.

Usually, ADLs are separated into basic ADLs (BADLs), such as bathing, and IADLs, such as doing laundry.11 Several studies have focused on BADLs in MS.12,13 For example, investigators demonstrated that only 29% of the participants in their study (n = 248) were fully independent in all BADLs examined (bathing, dressing, grooming, and feeding).12 Another study conducted in Norway observed that 16% of their participants (n = 124) needed help with bathing, 12% with dressing, 8.6% with grooming, and 9.5% with feeding. The impact of MS on more complex tasks (ie, IADLs) has been less documented.14,15 There is evidence that the ability to perform complex tasks is reduced in people with MS, mostly due to deficits in their motor and process skills.15,16 This is a concern because the ability to perform IADLs satisfactorily is closely related to independent living (eg, cooking, shopping, managing medication, and keeping track of finances). To this end, if aerobic fitness were associated with IADLs in people with MS, it would represent an opportunity for evaluating the efficacy of experimental studies aiming to increase aerobic fitness (ie, exercise) to improve independence and participation in this population.

The present study examined the association between aerobic fitness (expressed as VO_2peak) and self-reported IADLs in persons with MS. We expected that aerobic fitness would be associated with IADLs in persons with MS such that those with lower aerobic fitness (ie, lower VO_2peak) would be more dependent. If correct, this would provide important information for the design and execution of interventions that target aerobic fitness, with a secondary influence on everyday tasks necessary for independent living in people with MS.

This cross-sectional study involved a secondary data analysis from a previous research study aiming to develop fitness testing protocols for people with MS.17,18 Briefly, prospective participants were contacted through the North American Research Committee on Multiple Sclerosis (NARCOMS) Registry. Preexisting data sets from previous studies conducted during the past 5 years at the University of Illinois at Urbana-Champaign Exercise Neuroscience Research Laboratory were also used. The project coordinator initiated contact with participants by either telephone or e-mail and then provided a general explanation about the study protocol. Those expressing an interest in study participation were then screened for inclusion based on the following criteria: 1) definitive diagnosis of MS based on physician verification (ie, a signed letter received from the participant's physician verifying the diagnosis of MS); 2) relapse free for the past 30 days; 3) ability to walk with or without an assistive device; 4) aged 18 to 64 years; 5) willingness and ability to complete in-person fitness assessments; 6) low risk of contraindications to maximal exercise testing, assured by no more than a single “yes” response on the Physical Activity Readiness Questionnaire; and 7) physician approval for participation (ie, a signed letter received from the participant's physician). The project coordinator contacted a total of 129 persons.

Independence regarding IADLs was measured using the Lawton and Brody IADL scale.11 Despite being a scale validated for older adults in the general population,11 this scale has been widely used in studies of MS.19,20 The IADL scale included in this study is composed of eight items/IADLs. The items assess adaptive functioning versus disability in areas important for independent living (ability to use the telephone, shopping, food preparation, housekeeping, laundry, mode of transportation, responsibility for own medications, and ability to handle finances). Each item is scored as either 0 or 1. A score of 0 indicates total or modified dependence, and a score of 1 means complete independence or modified independence. For example, item 6 of the scale inquires about mode of transportation. The following options are provided: a) travels independently on public transportation or drives own car; b) arranges own travel via taxi, but does not otherwise use public transportation; c) travels on public transportation when accompanied by another; d) travel limited to taxi or automobile with assistance of another; and e) does not travel at all. Options a through c are scored 1 (complete independence or modified independence), and options d and e are scored 0 (total or modified dependence). For this study, we adopted the dichotomous-item sum score system, which is a valid approach to assess disability in IADL.21 A participant's scores on the eight items were summed to form a total IADL score that ranged from 0 to 8, with higher scores indicating greater ability or a higher level of independence.21 

Peak oxygen consumption was used as a measure of cardiorespiratory capacity and was obtained using an incremental exercise test on a recumbent stepper (NuStep T5^XR; NuStep Inc, Ann Arbor, MI) and an open-circuit spirometry system (TrueOne 2400, Parvo Medics, Sandy, UT) for analyzing expired gases.17 Participants were fitted to the stepper and received instructions on testing procedures along with instructions for reporting rating of perceived exertion (RPE). After being fitted with a mouthpiece (Hans Rudolph Inc, Kansas City, MO) for collecting expired gases, participants performed a 1-minute warm-up at 15 W. The initial work rate for the exercise test was 15 W, and the work rate continuously increased at a rate of 5 W/min until the participant reached volitional fatigue. Oxygen consumption (VO₂), respiratory exchange ratio, and work rate were measured continuously and expressed as 20-second averages. Heart rate was displayed using a heart rate monitor (Polar Electro Oy, Kempele, Finland), and heart rate, work rate, and RPE were recorded every minute during the test. The VO_2peak (expressed in mL·kg⁻¹·min⁻¹) is based on the highest recorded 20-second VO₂ value when at least one of three criteria were satisfied: 1) respiratory exchange ratio of 1.10 or greater; 2) peak heart rate within 10 bpm of the age-predicted maximum (ie, 220 – age); or 3) peak RPE of 17 or greater.

The institutional review board of the University of Illinois approved the study protocol, and informed consent was obtained from each participant before data collection. Assessments were completed in a single testing session. The testing session involved completing a demographics questionnaire, the IADLs scale, and a neurologic examination for the generation of an Expanded Disability Status Scale (EDSS) score22 that was used to describe the sample. The EDSS scores reflect function in eight functional systems, and those system scores plus ambulatory performance are combined into an EDSS score that ranges from 0 (ie, normal neurologic function) to 10 (ie, death due to MS). The neurologic examination was conducted by trained and certified researchers (eg, L.A.P.) (Level C, Neurostatus Systems AG, Basel, Switzerland), in a laboratory setting. After completing the neurologic examination, participants performed the incremental exercise test to exhaustion on a recumbent stepper to provide the measure of cardiorespiratory fitness (VO_2peak).

SPSS Statistics for Windows, version 22.0 (IBM Corp, Armonk, NY) was used for data analysis, with statistical significance set at P < .05. We first used descriptive statistics (ie, frequency and percentage) to describe the overall sample. We then calculated Pearson correlation coefficient (r) to examine the association between aerobic fitness (VO_2peak) and total IADL score (the sum of points of each of the eight IADLs). Correlation coefficients of 0.1, 0.3, and 0.5 were interpreted as small, moderate, and large, respectively.23 We further used independent-samples t tests to compare VO_2peak in those reporting dependence versus independence in performing each of the eight IADLs assessed and by employment status (employed or unemployed). We decided to complement the IADLs with employment status because we believe that this variable is an important benchmark of independence in people with MS.24 Effect sizes based on Cohen's d were calculated and interpreted as small (0.2), moderate (0.5), or large (0.8).23 

Of the 129 persons initially contacted by the project coordinator, 26 were not interested in participating and 18 were not reached. The remaining 85 individuals were screened for inclusion: 5 did not qualify for the study, and 18 dropped out when contacted to schedule the laboratory visit. This yielded 62 persons with MS who were included and completed the testing protocol.

Table 1 presents information on demographic and clinical characteristics of the sample as well as information on IADLs and aerobic fitness. Briefly, the mean age of the participants was 52 years, with nearly 73% being female and having moderate disability (based on EDSS scores). The average disease duration was 13 years, with approximately 77% of the sample having relapsing-remitting MS. The average VO_2peak of the sample was similar to that of other MS samples but lower than that of non-MS samples.7 The average total IADL score of the sample was nearly 7 points, which is quite high. More than half of the sample was unemployed, and the two most common IADLs requiring dependence were shopping and food preparation, reported by 39% and 24% of participants, respectively.

The analysis between aerobic fitness and total IADL score yielded a positive correlation that was small in magnitude but statistically significant (r = 0.28 [95% CI, 0.03–0.49], P = .033). Those with lower aerobic fitness reported less independence.

Findings on the VO_2peak difference between persons by dependence versus independence in each of the eight IADLs and by employment status are presented in Table 2. Briefly, t test analysis demonstrated no significant differences in VO_2peak in those reporting dependence versus independence in each of the eight IADLs or by employment status, despite the small to large effect sizes observed. A marginal difference (P = .09) was observed for the IADL “responsibility for own medication,” indicating that VO_2peak is lower for those reporting dependence in the mentioned IADL compared with those reporting independence.

The present study sought to examine the association between aerobic fitness and IADLs in people with MS. The results demonstrated that aerobic fitness, expressed as VO_2peak, was weakly but statistically significantly correlated with total IADL score, suggesting that lower aerobic fitness is associated with diminished ability to perform IADLs and, hence, less dependence. This is important because we can intervene on this risk factor. The present findings revealed a marginal difference (P = .09) in aerobic fitness between dependence and independence for the IADL “responsibility for own medication,” with the dependent group showing lower aerobic fitness than the independent group. Furthermore, the difference between groups was found to be large in magnitude based on the effect size observed (Table 2). However, caution is warranted when interpreting these results owing to the small number of cases observed in the dependent group.

The weak but positive association observed between aerobic fitness and total IADL score is very important in the context of MS owing to the disease characteristics and progression; as the disease progresses, the ability to perform ADLs becomes compromised.6 It is also important because there is evidence that people with MS are able to improve cardiorespiratory capacity in a short period.25,26 For example, in a randomized crossover controlled study, investigators observed significant improvements in peak cardiorespiratory capacity (ie, VO_2peak) after 8 weeks of aerobic exercise training in persons with MS. This finding further demonstrates and reinforces the importance of peak cardiorespiratory capacity in people with MS as a fitness component that may have the potential to help prolong independence in this population. Another positive aspect of the observed association lies in that minimizing the impact of the disease regarding limitations on BADLs and IADLs is a major goal in treating individuals with MS. To that end, physical activity/exercise plays an important role. Physical activity/exercise is an established method for managing physiologic deconditioning and improving walking mobility, information processing speed, and several symptoms in MS,2,27 which, in turn, could allow individuals to better perform daily tasks by improving their endurance and maintaining their independence in everyday life. Using a randomized controlled trial design, future studies should investigate the effects of a physical activity/aerobic exercise program on the ability to perform ADLs in people with MS.

The marginal difference in VO_2peak observed between those reporting dependence and independence in the IADL “responsibility for own medication” (Table 2) deserves some attention. Managing one's own medication requires adequate information processing speed, as well as executive-attentional abilities and memory, which are impaired in people with MS.28,29 People with MS normally take several different types of medication that may include medications to modify the disease course (disease-modifying therapy), to treat relapses and exacerbations, and to manage symptoms (eg, sleep problems, pain, fatigue, spasticity, bladder control, depression, anxiety).30,31 There is evidence in the literature of the association between aerobic fitness (ie, VO_2peak) and cognition in people with MS.10 To that end, it is possible that those with higher aerobic fitness have better cognitive skills that enable them to remember, prepare, and take correct doses at the correct time, or to take the medication at the correct time when prepared by others. The nonsignificant difference in VO_2peak in the IADLs investigated based on dependence status was contrary to the hypothesis. We expected a significant difference in aerobic fitness based on dependence in IADLs, particularly in IADLs requiring physical effort (shopping, housekeeping, laundry). Of note, individuals who report being independent in IADLs requiring physical effort had a higher VO_2peak compared with those reporting dependence; however, such differences were not significantly different (Table 2). Furthermore, the sample was high functioning based on the total IADL score (ie, a median score of 8.0), which may explain why some of the relationships that we expected between fitness and different IADLs were not demonstrated.

This study also observed partial or full dependence level in different IADLs. Approximately 40% of the participants were dependent in shopping and 24% in food preparation. Partial or full level of dependence in all other IADLs (Table 1) ranged from 0% in ability to use the phone, mode of transportation, and handle finances to 8.6% in laundry. These findings are similar to those found in other samples of people with MS with moderate-to-severe disability15 but slightly different from those presenting with mild disability.14 Although moderate MS seems to affect the ability to perform IADLs, based on the associations between aerobic fitness and IADLs, fitness levels may have the potential to sustain an important level of independence in this population.

This study has some limitations. The first limitation relates to the cross-sectional (ie, nonexperimental) nature of the study, which does not allow for the assessment of cause-effect relationships. Furthermore, the sample was primarily composed of persons with relapsing-remitting MS. We do not know whether similar associations would occur in people presenting with different courses of the disease (eg, primary and secondary progressive MS). Fitness was assessed only in terms of cardiorespiratory capacity (ie, VO_2peak). As known, fitness encompasses other components, such as muscular strength and body composition. These other fitness components may also affect the ability to perform IADLs in people with MS. Future studies should have a more comprehensive assessment of fitness and examine its association with IADLs in a more diverse sample of people with MS. Moreover, the scale used to measure IADLs may represent a limited number of tasks and may not fully capture all IADLs performed by people with MS. In line with that, we used one of seven different ways of scoring the scale.21 Hence, we do not know whether analysis using other scoring systems would provide the same results. Despite the limitations, we provide preliminary data on the associations between aerobic fitness, expressed as VO_2peak, and IADLs in individuals with MS presenting with moderate disability.

In summary, aerobic fitness was associated with IADLs (ie, total IADL score) in persons with MS. Despite a marginal difference in VO_2peak between those reporting dependence versus independence regarding responsibility for their own medications, no significant difference in VO_2peak was observed between groups reporting dependence versus independence on other IADLs. These findings support and extend previous studies on ADLs (BADLs and IADLs) in MS by examining the association between IADLs (ie, complex tasks) and aerobic fitness. These findings also underscore the need for additional studies on this matter and advocate for the potential use of interventions that improve aerobic fitness (ie, exercise training) as a means for improving IADL performance in people with MS, thereby maintaining independence in daily living.

The authors declare no conflicts of interest.

NARCOMS is supported in part by the Consortium of Multiple Sclerosis Centers (CMSC) and the Foundation of the CMSC.