currently , the combined prevalence of overweight ( bmi , body mass index  25 kg / m ) and obesity ( bmi  30  kg / m ) , defined by the world health organization ( who ) [ 1 , 2 ] , has ranged from approximately 50 to 65 percent in large samples of persons with ms [ 35 ] . these estimates may not differ considerably from those of nondiseased adult populations , but weight status may have important consequences for mobility and the accumulation of disability in persons with ms . mobility , assessed by a variety of measures , is compromised in nondiseased persons who carry excess body weight . compared to normal weight persons , those who were obese walked at slower speeds and covered less distance during a 6-minute walk ( 6mw ) test . a higher oxygen cost of walking ( cw ) has been established in nondiseased obese adults compared to adults of normal weight . altered gait kinematics including slower walking speed , shorter stride length , and more time spent in stance phase and double support during walking have further been observed in obese compared to normal - weight adults . such results in nondiseased populations would support the idea that weight status might similarly be a source of mobility dysfunction seen in persons with ms [ 1013 ] . our group has recently investigated the prospective relationship between self - reported bmi and disability , assessed by the patient determined disease status ( pdds ) scale , over a 24-month period in a large sample of persons with ms . interestingly , we observed a progression in disability status over time , but this change was not accompanied by , nor predicted by , a change in self - reported bmi . a comprehensive assessment of mobility may capture different information than that captured by the pdds alone , and may be affected by weight status in persons with ms , as seen in nondiseased obese populations . measures that assess mobility in persons with neurological disorders have been described by pearson et al . . these include timed walking tests , endurance tests , quantitative movement analysis , energy consumption , self - report questionnaires , and activity monitoring . we examined the effect of weight status on a comprehensive set of mobility outcomes , including ambulatory performance , cw , spatiotemporal parameters of gait , self - reported walking impairment , and free - living activity in persons with ms . based on previous research in healthy controls and mobility impairments in persons with ms , we expected that weight status would negatively impact outcomes of mobility , resulting in greater mobility impairment in persons with a higher bmi . such an examination is important given the prevalence of overweight and obesity in persons with ms and the possible impact this comorbidity may have on health and disease status . if weight status is important to mobility in persons with ms , targeted weight loss interventions may represent alterative and potential adjunct strategies for managing the consequences of this disease . participants were included in this investigation based on four criteria : a clinically definite diagnosis of ms ; free of a relapse in the past 30 days prior to completing testing procedures ; age  18 years ; and ambulatory either with or without the use of an assistive device . height and weight were collected using a calibrated , scale - stadiometer unit ( detecto model 3p7044 , webb city , mo , usa ) while participants were wearing light clothing and athletic shoes . bmi was classified as follows : normal weight , bmi = 18.524.9  kg / m ; overweight , bmi = 25.029.9  kg / m ; obese , classes i and ii , bmi = 30.039.9  kg / m . the timed 25-foot walk ( t25fw ) and the 6-minute walk ( 6mw ) assessed walking speed and endurance , respectively . the 6mw was performed in a single corridor that was 75  ft in length or in a square hallway with four corridors that were 100  ft in length , depending on the testing location . participants were instructed to walk as fast and far as possible for the duration of 6 minutes according to standardized instructions and were permitted to use an assistive device if necessary . total distance walked in meters was quantified by a member of the research team who followed 1 meter behind the participant with a measuring wheel ( stanley ms50 , new briton , ct , usa ) . oxygen consumption ( vo2 ) was assessed during the 6mw by breath - by - breath analysis using a commercially available portable metabolic unit ( k4b2 , cosmed , italy ) . the o2 and co2 sensors of the portable metabolic system and the flow meter were calibrated prior to all testing sessions . vo2 ( ml / kg / min ) values collected over the second 3 minutes of the 6mw were used to generate steady - state vo2 . cw ( ml / kg / m ) was determined by dividing steady state vo2 ( ml / kg / min ) by walking speed ( m / min ) . participants performed two walking trials at a comfortable pace using a gaitrite ( cir systems , inc . ) electronic walkway . gait parameters collected included walking velocity ( cm / s ) , cadence ( steps / min ) , step length ( cm ) , step time ( s ) , double support ( percentage of gait cycle ) , single support ( percentage of gait cycle ) , and base of support ( cm ) . the mean value of the two trials was generated for each of the 7 gait parameters . the msws-12 is a 12-item self - report scale that assesses the impact of ms on walking ability . each of the 12 items is rated on a scale ranging from 1 ( not at all ) to 5 ( extremely ) . total scores on the msws-12 range from 0100 , whereby higher scores indicate greater walking impairment . the pdds scale [ 19 , 20 ] was used to determine the level of neurological disability of the participants . the pdds is a patient - reported scale ranging from 0 ( normal ) to 8 ( bedridden ) , whereby higher scores indicate greater perceived disability . pdds scores have correlated strongly ( r = .93 ) with scores on the physician - administered expanded disability status scale ( edss ) . previous data collected by our research group supports a strong (  = .78 ) association between pdds and edss scores .   daily step and movement counts were assessed over a 7-day period using an actigraph accelerometer ( model gt3x ) ( actigraph corporation , pensacola , fl , usa ) worn on a belt around the waist . the accelerometer and this location on the body assess vertical bodily movement using a piezoelectric bender element that produces an electric signal proportional to the force acting upon it . data recorded and stored in the accelerometer were downloaded using actilife software and processed using microsoft excel . step counts and movement counts for each minute were summed for each of the 7 days and then averaged across the 7-day period . participants competed testing at one of two locations : the exercise neuroscience laboratory , university of illinois , urbana , il , usa or the illinois neurologic institute , peoria , il , usa . spatial and temporal parameters of gait were collected over two walking trials on an electronic walkway . participants then completed questionnaires assessing the impact of ms on walking impairment and self - reported neurological disability . upon completion of the testing session , participants were given an accelerometer to wear around the waist over the nondominant hip during the waking hours of the day , except while showering , bathing , and swimming , over 7 consecutive days . waking hours were defined as the moment of getting out of bed in the morning through the moment of getting into bed at night . participants were instructed to maintain typical levels of ambulatory activity during this one - week period . participants were provided with a log sheet to record times when the accelerometer was worn each day . the accelerometer was returned through the us postal service in a preaddressed , prestamped envelope provided by the investigators . values in the text are presented as mean ( sd ) , unless otherwise specified . one - way ancova controlling for age , sex , and disease duration was conducted to determine differences between groups based on bmi classification ( i.e. , normal weight , overweight , and obese ) on t25fw and 6mw performance , cw , 7 spatiotemporal gait parameters , msws-12 scores , and daily step and movement counts . the 13 measures included herein have been described by pearson et al .   as outcomes for assessing walking mobility in neurological populations . an adjusted alpha level of p  .004 was therefore used to account for the interrelated outcomes so as to avoid type i errors . effect sizes ( ess ) were calculated to determine the magnitude of the difference in mobility performance between bmi groups expressed as cohen 's d . guidelines of .2 , .5 , and .8 were used to determine small , moderate , and large effect sizes , respectively . this study included 168 persons ( 130 women , 38 men ) with predominately a relapsing - remitting disease course ( 134 relapsing remitting ms ; 19 secondary progressive ms ; 11 primary progressive ms ; 4 not reported ) . mean age of participants was 50.9 ( 10.6 ) years ( range = 2778 years ) with a mean disease duration of 11.9 ( 9.3 ) years ( range = 143 years ) . the median ( iqr ) pdds score was 3.0 ( 3.0 ) ( i.e. , gait disability ) and ranged from 0 ( i.e. , normal ) through 6 ( i.e. , bilateral support ) . mean bmi was 27.7 ( 5.1 )  kg / m ( range = 18.639.3  kg / m ) consistent with previous research involving bmi in ms [ 4 , 5 , 23 ] . the distribution of bmi scores was as follows : 52 ( 31.0% ) participants were classified as normal weight ; 61 ( 36.3% ) participants were classified as overweight ; and 55 ( 32.7% ) participants were classified as obese . bmi distribution in our study was similar to previous ms samples , although the proportion of participants classified as overweight or obese was somewhat higher in the current investigation [ 35 ] . descriptive characteristics of mobility outcomes and differences along with ess by bmi classification are presented in table 2 . mean time to perform the t25fw was 6.7 ( 4.1 ) seconds and did not differ significantly by bmi classification , f(2 , 160 ) = 0.93 , p = .40 . participants in the sample walked a mean of 420.0 ( 138.5 ) meters during the 6mw test which was not significantly different based on bmi classification , f(2 , 161 ) = 3.11 , p = .05 . small ess were observed for the difference between normal and obese groups on t25fw performance ( .29 ) , and between normal and overweight groups on 6mw performance ( .27 ) . mean cw in the sample was 0.225 ( 0.119 )  ml / kg / m . there was no significant difference between groups with respect to cw , f(2,157 ) = 3.32 , p = 0.04 , although a moderate es ( .62 ) was observed for the difference in cw between normal weight and obese groups . the magnitude of the difference in cw between normal and overweight groups ( .27 ) , and between overweight and obese groups ( .20 ) was small . none of the spatiotemporal gait parameters examined differed based on bmi : walking velocity , f(2 , 159 ) = 2.53 , p = 0.08 ; cadence , f(2 , 159 ) = 1.64 , p = 0.20 ; step length , f(2 , 159 ) = 1.70 , p = 0.19 ; step time , f(2 , 159 ) = 1.00 , p = 0.37 ; percentage of gait cycle spent in double support , f(2 , 159 ) = .50 , p = 0.61 ; percentage of gait cycle spent in single support , f(2 , 159 ) = .49 , p = 0.61 ; base of support , f(2 , 159 ) = 0.37 , p = 0.69 . the magnitude of the difference between groups on gait parameters ranged from no effect to small ( range = .02 to .34 ) . mean msws-12 score for the sample was 42.8 ( 28.5 ) and was not significantly different based on bmi classification , f(2 , 162 ) = 1.38 , p = .26 . mean daily step and movement counts were 4215.5 ( 2469.6 ) and 144725.7 ( 84828.8 ) , respectively . there was no difference between bmi groups in free living mobility assessed by daily step , f(2 , 159 ) = 2.43 , p = 0.09 , and daily movement counts , f(2 , 159 ) = 1.15 , p = 0.32 . the magnitude of the difference in daily step counts between groups was small when normal weight and overweight groups were compared ( .28 ) , and when overweight and obese groups were compared ( .31 ) . this is the first study to provide descriptive and comparative data regarding objectively measured weight status and its association with a comprehensive set of mobility outcomes in a large sample of ambulatory persons with ms . almost 70% of the sample were identified as overweight or obese , classes i and ii ( i.e. , bmi = 25.039.9 ) ; however , weight status did not appear to have a consistent , large , or significant impact on mobility outcomes . nevertheless , the high prevalence of overweight and obesity in this sample may still have important health or disease consequences for persons with ms , but these seemingly do not involve mobility dysfunction . further , this emphasizes the importance of identifying and focusing on other factors which may be important to mobility in persons with ms . we did not establish any significant difference between bmi status on t25fw and 6mw performance , cw , spatiotemporal gait parameters , msws-12 scores , and free - living mobility outcomes . this was supported by generally small ess when comparing the magnitude of the difference between groups on mobility outcomes . overall , these findings are contrary to our expectations based on the influence of excess body weight on mobility outcomes established in nondiseased adult populations [ 79 ] . that body of research suggests superior ambulatory performance in normal - weight individuals without a chronic disease or condition . factors other than weight status might account for ambulatory problems in persons with ms and such factors are different from adults without a disease . factors which may contribute to impaired walking in persons with ms as outlined by pearson et al .   include muscle weakness , spasticity , loss of proprioception and coordination , vestibular and visual dysfunction , cognitive and mood disturbances , and pain . to further evaluate and confirm the relationship between weight status and mobility , we conducted partial correlations between bmi and outcomes of mobility controlling for age , sex , and disease duration ( data not presented ) . there was a mean partial correlation coefficient ( sd ) of .01 ( .12 ) which ranged from .21 to .16 . this further supports the evidence presented herein indicating weak , inconsistent associations between bmi and mobility in persons with ms ; such associations indicate that , at best , less than 4% of variance is shared between bmi and mobility outcomes . our findings are further in accordance with cross - sectional [ 5 , 24 ] and prospective   data which do not support a relationship between weight status ( determined by bmi and percent body fat ) and disability ( determined by the edss , pdss , and a self - report functional ability ) in persons with ms . the present study further provides a comprehensive assessment of the functional impact of weight status on both clinical and community mobility outcomes in a large sample of persons with ms which may provide different information than that captured by disability measures alone . collectively , this suggests that the effect of weight status on mobility is limited in persons with ms and that this relationship may differ from that established in the general nondiseased population . the lack of an association between weight status and mobility is important in suggesting that we should focus on other factors that may impact mobility in persons with ms . interestingly , a potential role for weight status in the development of ms has been suggested . before the age of 20 , a high bmi ( > 27.030.0 ) has been associated with a greater than twofold increase in the risk of developing ms [ 25 , 26 ] . no association , however , was established between adult bmi and risk of ms [ 25 , 26 ] . taken together with our current and previous findings , this suggests that weight status may be more influential during disease development than following disease onset , although this concept requires further investigation . the combined prevalence of overweight and obesity , classes i and ii ( i.e. , bmi = 25.039.9 ) in the current sample was 69.0 percent . this percentage is somewhat higher than values that have been observed by other investigators but is comparable to the general nondiseased adult population . for instance , 55.6 percent of individuals were classified as overweight and obese in a large sample of persons with ms ( n = 8983 ) . in a sample of veterans with ms , 53.0 percent of women and 63.3 percent of men were classified as overweight and obese . similarly , 53.2 percent of persons with ms were classified as overweight or obese in a previous investigation by our research group . those studies all used self - reported height and weight which may have led to an underestimation of overweight and obesity in these investigations . furthermore , our findings are consistent with those of the general nondiseased adult population in which 68.0 percent of persons were identified as overweight and obese . height and weight were assessed by the investigators using a scale stadiometer , rather than self - reported , in that cross - sectional , nationwide sample of healthy adults without disease . our results suggest bmi may be somewhat higher than previously thought in persons with ms , but similar to estimates in nondiseased adult populations . we observed that weight status does not appear to impact mobility outcomes , but the prevalence of excess body mass may still pose considerable health risks for persons with ms . obesity is a risk factor for a variety of chronic health conditions including diabetes , hypertension , high cholesterol , stroke , heart disease , certain cancers , and arthritis . obesity further is associated with cognitive outcomes in the general population   and this might account , in part , for the extensive prevalence and burden of cognitive impairment in ms . persons with ms and health care providers should be aware and address additional health risks imposed by excess body weight . the present study provides comparative information on weight status and its effect on a comprehensive battery of mobility outcomes in a large sample of persons with ms . nevertheless , the generalizability of our conclusions is mainly limited to ambulatory , community - residing persons with ms who have a relapsing - remitting disease course . other outcomes such as waist circumference , body shape and composition , or fat distribution may have had different associations with mobility outcomes than those observed using bmi alone . future studies should include gold standard outcomes for the assessment of body composition , such as dual - energy x - ray absorptiometry ( dxa ) , to further establish the relationship between weight status and mobility in persons with ms . this study might benefit from the inclusion of longitudinal data to determine the possible impact of changes in weight status on mobility outcomes over time , although our previous data using self - report measures would argue against such a prospective association between variables . the inclusion of symptomatic outcomes known to be influenced by obesity , such as pain and fatigue , may further impact mobility and should be included in future investigations . we provide the first evaluation of the effect of bmi on a comprehensive battery of mobility outcomes including ambulatory performance , cw , spatiotemporal parameters of gait , self - reported walking impairment , and free - living activity . there does not appear to be a consistent nor strong pattern of association between weight status and mobility , which differs from results observed in nondiseased adult populations . this highlights the importance of focusing on other factors that may impact mobility in persons with ms . the high proportion of persons with ms classified as overweight or obese in the current sample , nevertheless , remains a considerable health concern that requires better management .