Actual and Perceived Activity Levels in Polio Survivors and Older Controls: A Longitudinal Study

ORIGINAL ARTICLE Actual and Perceived Activity Levels in Polio Survivors and Older Controls: A Longitudinal Study Mary G. Klein, PhD, Leonard E. Braitman, PhD, Roberta Costello, MSN, RN, Mary Ann Keenan, MD, Alberto Esquenazi, MD ABSTRACT. Klein MG, Braitman LE, Costello R, Keenan a period of many years and others reporting a sudden deterio- that may or may not have been triggered by a specific MA, Esquenazi A. Actual and perceived activity levels in polioration survivors and older controls: a longitudinal study. Arch Physevent (eg, a fall or a particularly strenuous activity). Med Rehabil 2008;89:297-303. Polio survivors are often told to reduce their levels of activity and rest more as a way of slowing down the progression of PPS symptoms. 1 However, it is well known that a sedentary Objective: To examine factors associated with daily step activity, perceived activity, maximum walking speed, and walking speed reserve over time in polio survivors and older adults with no history of polio. Design: Longitudinal study. Setting: A research clinic and the community. Participants: Polio survivors (n 96; 65 in postpolio syndrome [PPS] group, 31 in non-pps group) and older adults (n 112) with no history of polio. Interventions: Not applicable. Main Outcome Measures: Daily step activity, perceived 2,3 lifestyle can contribute to early mortality. Therefore, polio survivors often navigate a fine line between activity levels that may be too high and cause overuse problems in their polioweakened muscles and activity levels that are too low, which can result in a further deterioration in their overall health, cardiovascular function, and muscle strength because of lack of exercise. Walking is an important aspect of daily activity for most people. 4 However, increased difficulty with walking is a commonly reported functional problem among polio survivors with activity, maximum walking speed, and walking speed reserve. PPS. 5,6 Previous studies have examined walking or step activity levels in polio survivors in the clinic or in the community Results: Results showed decreases in perceived activity over time in the PPS group. However, there was no change in 6,7 over brief 1- to 2-day periods. However, none of these average daily walking activity. Overall, polio survivors walk studies looked at activity levels in polio survivors over longer less and have a smaller walking speed reserve than controls. periods of time to assess time trends and seasonal differences Knee strength was positively associated with maximum walking speed and walking speed reserve in all groups. Weight and in activity patterns. Therefore, the objective of this study was to compare real-life, community-walking activity over time in age were associated with daily step activity in controls but not polio survivors and older controls and to identify factors associated with activity level (both actual and perceived) in these polio survivors. Conclusions: Daily walking activity did not change statistically over the 3-year study period, although perceived groups. Measurements were taken every 3 to 4 months in 5- to activity 7-day segments over the course of the 3-year study. and the walking speed reserve decreased among polio survivors Actual and perceived activity levels were expected to be with PPS. On average, polio survivors appear to function with lowest among polio survivors with PPS. We hypothesized that minimal functional reserve, as their preferred walking speed polio survivors with PPS would also walk at a speed that is was close to their maximum speed. closer to their maximum capacity than controls or polio survivors without PPS. We expected seasonal differences in the Key Words: Postpoliomyelitis syndrome; Rehabilitation. 2008 by the American Congress of Rehabilitation Medi- amount of daily walking activity in all 3 groups, with less cine and the American Academy of Physical Medicine and activity seen in the winter months. Rehabilitation METHODS OSTPOLIOMYELITIS SYNDROME (PPS) is a progressive disorder, characterized by increasing muscle weak- Participants P ness, muscle pain, and fatigue, that over time can affect functional performance and quality of life among polio survivors. Data from 96 polio survivors and 112 adults with no history The rate of symptom progression appears to vary, with some of polio (controls) were analyzed for this study. Subjects were polio survivors reporting a slow decline in muscle strength over recruited from the local community and surrounding tri-state area (Pennsylvania, New Jersey, Delaware). All subjects had to Moss Rehabilitation Research Institute, Philadelphia, PA (Klein, Costello); Albert Einstein Medical Center, Philadelphia, PA (Braitman); Moss Rehab Hospital, Philadelphia, PA (Esquenazi); University of Pennsylvania, Philadelphia, PA (Keenan); and Thomas Jefferson University, Philadelphia, PA (Klein, Esquenazi). Supported by the U.S. Department of the Army (grant no. DAMD17-01-1-0822). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Mary G. Klein, PhD, Korman 204-B, Moss Rehabilitation Research Institute, 1200 W Tabor Rd, Philadelphia, PA 19141, e-mail: mklein@einstein.edu. 0003-9993/08/8902-00139$34.00/0 doi:10.1016/j.apmr.2007.08.156 297 be able to ambulate at least 10m 30ft) ( on a level surface with or without an assistive device. Exclusion criteria were kidney failure requiring dialysis, rheumatoid arthritis, recent cancer (other than skin) with ongoing treatment, uncontrolled hypertension, unstable angina pectoris, or any other cardiac and/or respiratory conditions that were uncontrolled. Also excluded were persons with uncontrolled seizures, any disease, illness, or injury that might affect muscle strength, such as diabetes, stroke, Parkinson s disease, or neuromuscular disorders other than polio. People with any cognitive disorders that might impede the ability to understand what was involved in the study or the motivation to perform the required tests were also excluded.

298 ACTIVITY IN POLIO SURVIVORS AND CONTROLS, Klein Procedure Each testing period consisted of 2 visits to the research clinic. During the initial visit, subjects were asked to complete a subject history and polio history form (polio survivors only). The subject history form contained basic demographic and medical history information. The polio history form included questions on date of original polio infection, number of limbs affected, and new symptoms. Height (in meters) and weight (in kilograms) were measured with a standard scale. Body mass index (BMI) was calculated using the formula BMI weight 705 / height 2. Voluntary maximal isometric strength was measured in the bilateral knee extensor muscle groups using a Microfet2 handheld dynamometer a in the side-lying position with each subject s leg supported on a raised powderboard. The powderboard is a small platform used by physical therapists for rangeof-motion and strength exercises. It provided a smooth surface that allowed the lower leg to move with the hip in a neutral posture. The advantage of this testing method over those more commonly used (eg, with a Biodex or KinCom testing machine) is that it incorporates gravity-neutral positions that permit the testing of subjects with strength below grade 3 (gravity-resistant strength on the manual muscle testing scale). This allows us to get a better representative sample of polio survivors than other studies that use the other strength-testing methods. All strength measurements were collected by a single physical therapist, who had several years of experience using this method of testing with polio survivors and older adults without a history of polio. Before the strength assessment, subjects were asked to simulate the motion required for the test by pushing the lower leg against the therapist s hand. Subjects were given strong verbal encouragement during each trial and were instructed to report any pain or discomfort immediately to the therapist. Any strength data collected during trials when pain or discomfort was reported were considered invalid and excluded from all analyses. Maximum voluntary isometric force was measured in pounds. Initially, 2 strength trials were performed. If these 2 trials were not within 5% of each other, a further 2 attempts were allowed. We reviewed our previous studies involving polio survivors and a similar protocol for strength testing on the knee extensor muscle group. This showed that less than.8% of the strength trials attempted for this muscle group were either invalid or missing due to pain. 8-10 To obtain a measure of walking capacity, subjects were timed as they walked along a 10-m ( 30-ft) indoor, uncarpeted walkway twice; once at a pace that was normal and just right for them and once as fast as they could. The 10-meter walk test (10MWT) assesses short-duration walking speed. It has been used in studies of patients with neurologic movement disorders, including stroke and Parkinson s disease. 11,12 Previous research has shown that the velocities measured in the 10-m test are similar to those seen in the 6-minute walk test, which is commonly used to assess cardiovascular exercise capacity. 13 The 10MWT test has been used in at least 1 previous study with polio survivors. 14 It was chosen for this study because it is easier to apply in a clinical setting and can accommodate people who are deconditioned, walk with assistive devices, and/or have low endurance levels. A walking speed reserve score was calculated as the difference between maximum walking speed and comfortable, normal walking speed. This variable was used to estimate how closely subjects were working to their maximum capacities when performing their usual daily activities. To obtain an objective estimate of the average frequency of walking activity, subjects were asked to wear a StepWatch activity monitor (SAM) b for 5 to 9 days during each testing period. The SAM is approximately the size and weight of a pager and stores data in consecutive time intervals over weeks to months at a time. In the current study, the SAM was worn in a cuff just above the right lateral malleolus and set to store the number of steps in 1-minute intervals. The data were transferred to a computer, where the levels and patterns of activity were analyzed. Initial studies on the accuracy and reliability of the SAM indicated that it is 99% accurate in counting steps on level surfaces and 97% on stairs and hills. 15,16 Previous research has shown that the SAM is reliable and valid for use in measuring community walking activity in healthy subjects and those with mobility restrictions related to muscular sclerosis, Parkinson s disease, and other primary muscular disorders. 17,18 At the initial visit, the activity monitor was calibrated through an infrared optical interface based on each subject s height, cadence, and gait. The sensitivity settings were then calibrated through visual inspection using a light that is programmed to blink with each step for the first 30 steps taken by the subject. Once the calibration was complete, subjects were instructed to wear the monitors continuously for 5 to 9 days, except during sleep at night and during water activities such as bathing, showering, and swimming. The total number of steps a day was collected for analysis. When subjects returned to the research clinic for their second visit, a measure of self-reported physical activity was obtained using the Physical Activity Scale for the Elderly (PASE). 19 This instrument dealt with occupational, household, and leisure activities taking place in the previous 1-week period. Previous research has supported this scale s construct validity and its use for assessment of physical activity among adults aged 65 years and older with pain and disability. 20 It has also been used in previous studies with polio survivors younger than 65 years, with the assumption that this population is more sedentary than their peers without a history of polio. 8,21 All subjects were invited to return to the research clinic for additional testing periods at approximately 3- to 4-month intervals over the 3-year study (fig 1). Statistical Analysis All analyses controlled for within-subject correlations (clustering) from multiple observations on the same subject. Multivariable mixed models with random intercepts or both random intercepts and random slopes were used to assess the independent predictors of the study outcomes. When appropriate, these models were used to compare the polio survivors with PPS, those without PPS, and the control group. All models had an independent covariance structure. The activity models were adjusted for age at baseline, time in study, sex, initial BMI, educational level, living alone or with other, season, and subject s report of osteoporosis and/or osteoarthritis at the initial visit. Product interaction terms considered were between the group variable (PPS, non-pps, control) and each of the following variables: age at entry, time in study, and initial BMI. Interactions were retained in a model if P was less than.05. In addition, the results of separate models for each group, (adjusted for strength of the weaker knee and the control variables listed above) are reported. Because the outcome variables were nonnegative and right skewed, models with logarithmic transformations of the outcomes were used and evaluated. Model fit was compared using the likelihood ratio test for nested models and the Akaike information criteria for non-nested models. Diagnostic plots, including residual plots and Q-Q plots for normality, were

ACTIVITY IN POLIO SURVIVORS AND CONTROLS, Klein 299 Baseline Testing Period Testing Period #2 DAY 1 7 10 days later DAY 1 7 10 days later PROCESS Clinic Home Clinic 3-4 month Clinic Home Clinic 3 4 month UNDER PERIOD 2 History form(s) Step activity Strength break Height Step activity Strength break REPEATED FOR Height Walking speed Weight Walking speed TESTING PERIODS Weight PASE Strength PASE 3 8 Strength Walking speed Walking speed Fig 1. Timeline for data collection. evaluated for each multivariable model. Unless otherwise stated, P values are multivariable adjusted. Data were analyzed using the Stata c and S-PLUS d software packages. RESULTS Sixty-five polio survivors reported symptoms associated with postpoliomyelitis syndrome (PPS group) and 31 did not (non-pps group). The median duration of time since the acute polio infection was 55 years (range, 37 80y) in the PPS group and 57 years (range, 42 72y) in the non-pps group. On average, the adults with no history of polio (control group) were older and had stronger knee extensors than the polio survivors in both PPS and non-pps groups (table 1). In addition, the control group contained a higher percentage of women than the postpolio groups. The percentage of obese subjects was slightly higher in the PPS group than in the control and non-pps groups. Both the PPS and non-pps groups were more likely to use braces and/or assistive devices for walking, report a history of falls, and live with someone else than were the controls. Polio survivors in both groups also reported more formal education, on average, than the controls. Data were collected in 5- to 9-day blocks that were repeated every 3 to 4 months. The number of days that each subject participated in the study ranged from 8 to 1121 (ie, 1wk 3.1y). Daily Step Activity In the bivariate analyses, subjects in the PPS group averaged the fewest steps a day, followed by the non-pps and control groups (table 2). This ranking persisted after multivariable adjustment. When daily step count was analyzed for each of the 3 groups separately, controlling additionally for knee strength, daily step count was positively related to the strength of the weaker knee extensor for the PPS group only (table 3). Sex and education level were also associated with daily step count in the PPS group. Because the outcome variables (but not the independent variables) were transformed using natural logs, the coefficients for sex and education (and the other categoric variables) have the following interpretation: on average, men took 27% fewer steps than women. Compared with subjects who reported a high school education or less, those with some college educa- Table 1: Baseline Characteristics Baseline Characteristic Controls Non-PPS PPS Subjects (n) 112 31 65 Average time in study (y) 1.4 0.9 1.2 1.0 1.4 1.1 Women 63 52 45 White 77 90 92 Age at baseline (y) 72.3 9.3 66.3 8.5 62.5 9.0 Initial BMI 27.5 5.4 26.3 5.6 27.2 4.4 % of subjects who are obese, with BMI 25 23 29 30kg/m 2 Strength of weaker knee extensor (N) 166.8 79.2 120.5 115.2 87.2 89.9 Knee extensors: bilateral strength (N) 362.5 163.2 296.3 217.1 250.4 166.8 Normal walking speed (m/s) 1.1 0.2 1.0 0.3 1.0 0.2 % who use braces 0 39 48 % who use assistive device 0 26 45 Dizzy 14 6 20 Osteoporosis 23 23 14 Arthritis 31 39 31 History of falls 27 58 71 Live with other 64 77 83 Education level High school or less 29 10 22 Some college 30 35 22 College graduate 41 55 57 NOTE. Values are mean standard deviation (SD) or percentage or as indicated.

300 ACTIVITY IN POLIO SURVIVORS AND CONTROLS, Klein Activity Outcome Controls (n 112) Table 2: Summary of Study Outcome Variables P* (controls vs non- PPS) Non-PPS (n 31) P* (non-pps vs PPS) PPS (n 65) P* (controls vs PPS) Walking activity (steps/d) 9366 3729 NS (.04) 7996 2971.009 (.001) 6450 2794.001 (.001) Perceived activity (PASE) 157 70 NS 178 94 NS 147 98 NS Maximum walking speed (m/s) 1.5 0.2.002 1.4 0.4 NS 1.3 0.4.001 Walking speed differential (m/s) 0.4 0.2.001 (.001) 0.3 0.3 NS (NS) 0.3 0.2.001 (.001) NOTE: Values are mean SD at baseline. Abbreviation: NS, not significant (P.05). *P values in the top row are for the bivariate group comparisons between the average (over the entire study period) outcomes values. They take clustering of observations within patients into consideration. P values in parentheses (in the second row) are from multivariable mixed models. Those multivariable models controlled for age at enrollment, time in study, sex, live with other, educational level, season, and BMI, osteoporosis, and arthritis at baseline. Statistically significant interactions between the group indicator variable and age at enrollment exist. tion (uncompleted) and college graduates averaged 58% and 40% more steps, respectively (see table 3). In the non-pps group, the presence of osteoporosis was the only statistically significant factor. On average, non-pps subjects with osteoporosis took 56% fewer steps than those without osteoporosis. Among controls, the daily step count varied inversely with BMI and age at enrollment. On average, control subjects took 6% to 7% more steps in spring and fall than in winter. A similar trend was seen for fall in the PPS group, but it was not statistically significant (see table 3). Perceived activity. In the bivariate analyses, the polio survivors with PPS had the lowest mean PASE score and the polio survivors without PPS had the highest. However, none of the differences between groups were statistically significant (see table 2). Because the interaction between group and age at baseline was statistically significant, no conclusion can be drawn regarding the multivariable adjusted comparisons of Factor Table 3: Results of the Multivariate Analyses PPS Group Non-PPS Group Control Group Coefficient P Coefficient P Coefficient P Daily step activity Strength of weaker knee.005.010.002.500.006.660 Time in study.004.510.003.510.002.490 Sex.270.030.090.590.020.780 Osteoporosis.170.330.560.005.130.120 Arthritis.070.580.080.630.030.650 Education level* Some college.580.001.070.810.040.620 College graduate.400.008.030.930.060.420 Age at enrollment.004.540.009.320.010.001 BMI at enrollment.008.540.004.770.020.001 Live alone.060.700.120.580.001.990 Season Spring.040.210.007.110.070.001 Summer.050.100.007.860.010.630 Fall.050.060.040.370.060.003 Perceived activity Strength of weaker knee.007.010.006.030.002.150 Time in study.001.040.003.740.002.001 Sex.040.810.060.690.080.390 Osteoporosis.120.580.270.140.040.680 Arthritis.100.530.290.046.006.940 Education level* Some college.270.240.140.630.120.230 College graduate.030.890.080.800.020.840 Age at enrollment.008.330.030.001.010.001 BMI at enrollment.003.850.010.280.005.510 Live alone.150.460.280.170.030.730 Season Spring.009.880.008.900.110.001 Summer.110.060.040.500.190.001 Fall.070.170.100.110.160.001 *Reference level was high school graduate or less. Reference level was winter.

ACTIVITY IN POLIO SURVIVORS AND CONTROLS, Klein 301 perceived activity between groups. When multivariate analyses were run for each group separately, perceived activity among polio survivors in both the PPS and non-pps groups was inversely associated with the strength of the weaker knee extensor (see table 3). In the PPS group, the only additional independent predictor of a lower PASE score was greater time in study. For each additional day in the study, the average PASE score for this group decreased by.01% (see table 3). In the non-pps group, lower PASE scores were reported by older subjects. In this group, we were also surprised to find that subjects with arthritis had average PASE scores that were higher than scores for subjects who did not report arthritis. In controls, the PASE score was not associated with the strength of the weaker knee extensor, unlike in polio survivors (see table 3). PASE scores were inversely associated with age at enrollment in the control group and also decreased with time in study (.02% a day). Control subjects perceived themselves to be less active in winter than in other seasons. On average, compared with winter, their PASE scores were 11%, 19%, and 16% higher in the spring, summer and fall, respectively. Maximum Walking Speed and Walking Speed Reserve On average, maximum walking speed was slightly lower among polio survivors than controls (see table 2). Maximum walking speed was directly related to strength in the weaker knee extensor in all 3 groups (see table 3). However, there was no evidence of a change in maximum walking speed or strength over the course of the study. Maximum walking speed was inversely associated with age in the PPS and control groups but not in the non-pps group. Surprisingly, the average maximum walking speeds for all 3 groups were 1% to 2% lower in fall than in winter. College graduates had higher average maximum walking speeds than subjects with no college experience in the control and non-pps groups. Maximum walking speed was inversely related to BMI in the control group. Non-PPS subjects with osteoporosis had average maximum walking speeds that were 39% lower than subjects without osteoporosis. The walking speed reserve was also lower among polio survivors than controls, with subjects in the PPS and non-pps groups having similar walking speed reserve (see table 2). When the strength of the weaker knee was controlled for in separate group models, walking speed reserve was directly associated with knee strength in all 3 groups (see table 3). In the PPS group, walking speed reserve was also inversely associated with BMI and decreased over time (.007% a day). However, there were no variables, other than knee extensor strength, that were associated with walking speed reserve in the non-pps or control groups. DISCUSSION Previous studies on disability in polio survivors have indicated that the impact of PPS has mainly been on mobilityrelated activities, such as walking on level surfaces and stairclimbing. 22,23 Halstead and Rossi 24 reported that 82% of postpolio patients with new symptoms report progressive loss of ability to ambulate and 81% have more difficulty with stairs. Progressive mobility impairments and disabilities may affect polio survivors abilities to perform various activities of daily living. As a result, they may adapt lifestyle changes that result in reduced activity levels and/or increased assistive device use. These changes, which may include less walking, reduced social life, and reduced or stopped physical recreation, may mean more than just a change in lifestyle but may also result in an increase in disability. The goal of the current study was to examine different measures of activity level over time in polio survivors with and without symptoms of PPS and in older adults without a history of polio. Because the day-to-day variability in daily activity for individual subjects was expected to be high, the testing was repeated multiple times over the course of each year to get a representative picture of activity patterns for each group and to capture any seasonal differences. As hypothesized, daily activity level was lower in the PPS group than in the control and non-pps groups. According to the previously published classification of pedometer walking activity, 25 subjects in the non-pps and control groups would be considered somewhat active on average, which means their daily activities were likely to include some exercise and walking. However, subjects in the PPS group would be considered low active, which means their daily activities were not likely to include any regular exercise. As a result, subjects with PPS are more likely to be deconditioned because of their relatively low physical activity levels. We observed that a higher proportion of subjects in the PPS group were obese compared with the other 2 groups. We also expected the PPS group to have substantially lower perceived activity scores. However, although this group had the lowest average PASE score, it was not statistically significantly different from the scores for the other 2 groups. There were decreases in perceived activity over time in the PPS group. However, this did not correspond to a statistically significant decline in average daily walking activity over the 3-year study period. It may be that a longer study is needed before measurable changes in the actual walking activity level are apparent. On the other hand, the PASE is designed to measure frequency of participation in specific activities, such as housework, home repairs, gardening, sports and recreation, and occupation, not just activities that involve walking. It is possible that the changing PASE scores for polio survivors in the PPS group represent a decline in participation in activities that do not involve a great deal of walking. This may be a reason why there was no corresponding decrease in daily step activity. It may also explain why there were significant group differences in average daily step activity levels but not in PASE scores. Sex and education level were associated with daily step activity level in the PPS group. This supports the results of a previous study 7 in which researchers concluded that actual walking activity in polio survivors was largely determined by factors other than those related to walking ability. The researchers listed social behavior and personal lifestyle as potential factors, which could be associated with sex and education level. Similar results for sex were seen in a previous study on activity levels in Parkinsonian patients. The researchers reported lower activity levels among men than women 26 but provided no explanation for this finding. One tentative explanation could be that women traditionally are more likely to do the shopping for the family, which could result in higher activity levels. Similarly, it is possible that polio survivors with higher education might be more likely to have a lifestyle that includes involvement in social and/or fitness activities than polio survivors with a high school education or less. However, it is also possible that education and sex may be markers for something else that was not measured in this study. Further research is needed to clarify the relationship between these variables and daily step activity. Strength of the weaker knee was not an independent predictor of actual or perceived activity levels in the control group. However, knee strength was directly related to actual and perceived activity levels for the PPS group, and was associated with perceived activity level in the non-pps group. Age and BMI were independent predictors of daily step activity among controls but not for either of the postpolio groups.

302 ACTIVITY IN POLIO SURVIVORS AND CONTROLS, Klein The only factor associated with daily step activity in the non-pps group was the presence of osteoporosis. None of the variables associated with daily step activity in either the control or PPS groups came close to statistical significance in the non-pps group. More research is needed on the factors that affect activity level in this subgroup of polio survivors. Decreased levels of walking activity were expected in the winter months for all groups because of cold temperatures, shorter daylight hours, and potential for ice and/or snow. However, season was a statistically significant predictor of walking activity in controls only, with less activity occurring in the winter months. The average normal walking speeds recorded for the postpolio groups in this study were slightly lower than the average walking speed reported in a previous study on 24 patients with PPS (1.08m/s). 7 However, the subjects in the current study were, on average, approximately 10 years older than the patients in the earlier study. Both the normative and maximum walking speeds were lower, on average, for postpolio subjects than for controls, and the difference between those speeds (ie, the walking speed reserve) was smaller. This suggests that polio survivors have less reserve capacity available. It may help explain why polio survivors, especially those with PPS, are more likely to experience discomfort and fatigue with daily activity. 14 Previous research on subjects with incomplete spinal cord injury (SCI) used a similar measure to assess walking capacity in patients and age-matched control subjects. 13 The researchers reported that the preferred walking speed was closer to the maximum walking speed for subjects with incomplete SCI compared with control subjects. Similarly, Willen and Grimby 21 reported a positive association between daily pain experienced by polio survivors and their spontaneous walking speed as a percentage of maximal speed (ie, walking speed ratio). When we analyzed that walking speed ratio over time for each of the 3 groups in this study, we reached the same substantive conclusion of declining reserve that we found using the difference (walking speed reserve). Study Limitations The major limitations of this study were that the follow-up time was relatively short (up to 3y) and varied between subjects. However, this study is powerful statistically in that it found many small differences to be statistically significant. For example, for each additional day of follow-up, average perceived activity decreased by.02% in the control group (P.001) and.01% in the PPS group (P.04) (see table 3). Although follow-up was limited to a maximum of 3 years, by using multivariable mixed models we were able to include all available outcome data (from all subjects) and quantify outcome changes over time. The statistical model commonly used for longitudinal studies, repeated-measures analysis of covariance (ANCOVA), lacks those advantages of mixed models. In repeated-measures ANCOVA, missing data at intermediate time points result in the total exclusion of that person s data from the analysis. The repeated-measures ANCOVA model provides no information on the magnitude of changes over time or of differences between groups. Furthermore, it cannot be used for outcome data containing varying time points for different subjects as in our data set. CONCLUSIONS This study suggests that polio survivors function with minimal physiologic functional reserve, because their normal walking speeds are very close to their maximum speeds. Although polio survivors overall perceived themselves to be as active as controls, the average amount of daily walking activity was lower among polio survivors. Daily walking activity levels did not change statistically over the 3-year study period in either postpolio group, even though the perception of activity level and the walking speed differential decreased among polio survivors who reported symptoms associated with PPS. Future analyses will focus on patterns of activity among polio survivors specifically, the absolute and relative amounts of moderate and high levels of activity as a percentage of total walking activity. We will also investigate the association between walking activity and reported pain and fatigue symptoms over time to get a better understanding of the role activity level plays in the symptoms commonly associated with PPS. Acknowledgments: We thank Jennifer Kuehl, PT, Ann Louise Simone, and Gemma Baldon for all their help in implementing this project and managing the data. References 1. Agre JC, Rodriquez AA. Intermittent isometric activity: its effect on muscle fatigue in postpolio subjects. Arch Phys Med Rehabil 1991;72:971-5. 2. Pate RR, Pratt M, Blair SN, et al. Physical activity and public health. A recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA 1995;273:402-7. 3. Franco OH, de Laet C, Peeters A, Jonker J, Mackenbach J, Nusselder W. Effects of physical activity on life expectancy with cardiovascular disease. Arch Intern Med 2005;165:2355-60. 4. Esquenazi A, Talaty M. Gait analysis: technology and clinical application. In: Braddom RL, editor. Physical medicine and rehabilitation. 3rd ed. Philadelphia: Elsevier; 2007. p 93-110. 5. Lonnberg F. Late onset polio sequelae in Denmark. Results of a nationwide survey of 3,607 polio survivors. Scand J Rehabil Med Suppl 1993;28:1-32. 6. Nollet F, Beelen A, Prins MH, et al. Disability and functional assessment in former polio patients with and without postpolio syndrome. Arch Phys Med Rehabil 1999;80:136-43. 7. Horemans HL, Bussmann JB, Beelen A, Stam HJ, Nollet F. Walking in postpoliomyelitis syndrome: the relationships between time-scored tests, walking in daily life and perceived mobility levels. J Rehabil Med 2005;37:142-6. 8. Klein MG, Keenan MA, Esquenazi A, Costello R, Polansky M. 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