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Reproducibility of walking at self-preferred and maximal speed in patients with postpoliomyelitis syndrome

      Abstract

      Horemans HL, Beelen A, Nollet F, Lankhorst GJ. Reproducibility of walking at self-preferred and maximal speed in patients with postpoliomyelitis syndrome.

      Objective

      To assess the reproducibility of walking performance, heart rate, and perceived exertion at self-preferred speed and maximal walking speed in patients with the postpoliomyelitis syndrome (PPS).

      Design

      Repeated measurement at a 3-week interval.

      Setting

      University hospital.

      Participants

      Convenience sample of 65 patients with PPS.

      Interventions

      Not applicable.

      Main outcome measures

      Walking performance: the distance walked in 2 minutes at a self-preferred speed and the time needed to walk 75m at maximal speed, heart rate, and rating of perceived exertion (RPE) on an 11-point scale.

      Results

      Test-retest reliability of walking performance was excellent for both tests (intraclass correlation coefficient [ICC] range, .94–.97). No systematic differences existed between test and retest. The smallest detectable change for an individual was 15% for both tests. Test-retest reliability for heart rate was good (ICC=.86) but moderate for RPE (Spearman ρ range, .67–.70). The smallest detectable change for RPE was between 4 and 6 scale points. The variability in walking performance was significantly correlated with the variability in heart rate at self-preferred speed (r=.36, P<.01) but not with the variability in RPE (r=.20, P=.11).

      Conclusions

      Both walking tests showed good reproducibility and may be appropriate to monitor (individual) changes in walking capacity in patients with PPS. Because of its moderate reproducibility, RPE does not seem to be suitable to monitor physical exertion. The usefulness of an objective measure such as heart rate for this purpose needs further investigation.

      Key words

      INCREASED DIFFICULTY WITH walking is among the major functional problems persons with the postpoliomyelitis syndrome (PPS) experience.
      • Lonnberg F.
      Late onset polio sequelae in Denmark. Results of a nationwide survey of 3,607 polio survivors.
      • Ramlow J.
      • Alexander M.
      • LaPorte R.
      • Kaufmann C.
      • Kuller L.
      Epidemiology of the post-polio syndrome.
      • Ivanyi B.
      • Nollet F.
      • Redekop W.K.
      • et al.
      Late onset polio sequelae disabilities and handicaps in a population-based cohort of the 1956 poliomyelitis outbreak in The Netherlands.
      • Westbrook M.T.
      A survey of post-poliomyelitis sequelae manifestations, effects on people’s lives and responses to treatment.
      In particular, the decrease in ability to walk outdoors is perceived as a major problem by patients with PPS
      • Nollet F.
      • Beelen A.
      • Prins M.H.
      • et al.
      Disability and functional assessment in former polio patients with and without postpolio syndrome.
      and may affect their level of independence and life satisfaction.
      • Burger H.
      • Marincek C.
      The influence of post-polio syndrome on independence and life satisfaction.
      In cross-sectional studies of patients with PPS, walking performance has been measured in a laboratory setting for both self-preferred speed and maximal speed.
      • Nollet F.
      • Beelen A.
      • Prins M.H.
      • et al.
      Disability and functional assessment in former polio patients with and without postpolio syndrome.
      • Willen C.
      • Cider A.
      • Sunnerhagen K.S.
      Physical performance in individuals with late effects of polio.
      • Thoren-Jonsson A.L.
      • Grimby G.
      Ability and perceived difficulty in daily activities in people with poliomyelitis sequelae.
      Whether such tests are appropriate in longitudinal studies, for instance to determine a decline in walking capacity, depends largely on their reproducibility. Although the reproducibility of walking tests at self-preferred and maximal speed is good in patients with respiratory disease,
      • Butland R.J.
      • Pang J.
      • Gross E.R.
      • Woodcock A.A.
      • Geddes D.M.
      Two-, six-, and 12-minute walking tests in respiratory disease.
      chronic heart failure,
      • Pinna D.G.
      • Opasich C.
      • Mazza A.
      • Tangenti A.
      • Maestri R.
      • Sanarico M.
      Reproducibility of the six-minute walking test in chronic heart failure patients.
      and neurologic impairment,
      • Rossier P.
      • Wade D.T.
      Validity and reliability comparison of 4 mobility measures in patients presenting with neurologic impairment.
      no data on the reproducibility of the tests are available in patients with PPS.
      In addition to walking performance, it may also be relevant to assess physical effort, which relates to the length of time that the activity can be sustained. Physical effort can be measured objectively by recording the heart rate, but also at the level of perception, by rating the perceived exertion (RPE). The RPE has been used to determine exercise levels in PPS patients.
      • Agre J.C.
      • Rodriquez A.A.
      • Franke T.M.
      • Swiggum E.R.
      • Harmon R.L.
      • Curt J.T.
      Low-intensity, alternate-day exercise improves muscle performance without apparent adverse effect in postpolio patients.
      Recently, the March of Dimes Birth Defects Foundation recommended use of an RPE scale
      • Borg G.A.
      Perceived exertion as an indicator of somatic stress.
      for daily life activities to avoid excessive fatigue in patients with PPS.
      Whether an RPE scale is appropriate for this purpose depends on its stability across measurements and on its relation with the actual effort and performance. Unfortunately, no information exists regarding the reproducibility of the RPE in PPS patients. Further, the literature has revealed inconsistencies about the strength of the relation between the RPE and physiologic criterion measures, such as heart rate.
      • Russell W.D.
      On the current status of rated perceived exertion.
      • Whaley M.H.
      • Brubaker P.H.
      • Kaminsky L.A.
      • Miller C.R.
      Validity of rating of perceived exertion during graded exercise testing in apparently healthy adults and cardiac patients.
      Knowledge of the reproducibility of RPE and its relation to walking performance and heart rate during walking, may provide supportive evidence for its use to avoid overloading in patients with PPS.
      The objective of the present study was to determine the test-retest reproducibility of 2 walking tests with respect to performance and physical effort in patients with PPS. One test measured the distance covered in 2 minutes when walking at a self-preferred speed, and the other test measured the time needed to walk 75m as fast as possible. We also investigated associations between the variability in performance and the variability in physical effort, measured by means of heart rate (only at self-preferred speed) and RPE.

      Methods

      Participants

      Sixty-five subjects with PPS, according to the criteria defined by Halstead,
      • Halstead L.S.
      Post-polio syndrome definition of an elusive concept.
      • Halstead L.S.
      • Silver J.K.
      Nonparalytic polio and postpolio syndrome.
      participated in the study. Inclusion criteria were (1) symptoms of postpoliomyelitis muscle dysfunction
      • Borg K.
      Post-polio muscle dysfunction.
      (ie, new muscle weakness, new muscle fatigue, new muscle pain, or new atrophy) in at least 1 quadriceps, (2) ability to walk for at least 2 minutes, (3) age between 18 and 70 years, and (4) no significant other neurologic disorders. Use of walking aids and orthotic and orthopedic devices was allowed. The Medical Ethics Committee of the hospital approved the study, and all subjects gave written informed consent.

      Protocol

      Two walking tests were performed on 2 visits that were separated by a 3-week interval. The tests were performed on a closed, marked, 65-m indoor trajectory and were started after the subjects had been sitting on a chair to rest for 5 minutes.
      First, subjects walked at a self-preferred walking speed for 2 minutes, and the walking distance was measured. Then, after a 5-minute rest interval, they walked at their maximal walking speed (without running) over a distance of 75m, and the time needed to walk that distance was measured with a stopwatch. They were allowed to use walking aids and devices during the tests as they would when walking outdoors. Heart rate was recorded in both tests. After each walking test, the subjects rated their perceived exertion (RPE) on an 11-point scale deduced from Borg,
      • Borg G.A.
      Perceived exertion as an indicator of somatic stress.
      ranging from 0 (no exertion) to 10 (maximal exertion). The measurements on the second visit were scheduled at the same time of day as those on the first visit, and subjects walked with the same walking aids and devices on both visits.

      Data analysis

      The heart rate during walking at self-preferred speed showed a steady state after 60 seconds. Therefore, the mean heart rate between 70 and 100 seconds was calculated and used in the analysis. No heart rate was analyzed for the test at maximum speed because no steady state could be observed.
      The test-retest reliability for walking distance, walking time, and heart rate was assessed by the intraclass correlation coefficient (ICC) and the 95% confidence interval (CI) of the ICC, by using a random effects 1-way analysis of variance.
      • Rankin G.
      • Stokes M.
      Reliability of assessment tools in rehabilitation an illustration of appropriate statistical analyses.
      A lower limit of the CI of at least .75 is considered as good test-retest reliability.
      • Fleiss J.
      • Lee J.
      • Koh D.
      • Ong C.N.
      Statistical evaluation of agreement between two methods for measuring a quantitative variable.
      The test-retest reliability of RPE was analyzed by calculating the Spearman correlation coefficient.
      Systematic differences between visits were tested with the Student t tests for walking distance, walking time, and heart rate, and with the Wilcoxon signed-rank test for RPE. For walking performance and heart rate, agreement of measurements was analyzed according to the Bland-Altman method.
      • Bland J.M.
      • Altman D.G.
      Statistical methods for assessing agreement between two methods of clinical measurement.
      The 95% limits of agreement (LOA) were calculated as mean (visit 2−visit 1)±2 standard deviations (SDs) to determine the smallest change that can be detected within an individual. For the RPE, agreement of measurements was analyzed by calculating the 2.5 and 97.5 percentiles of the difference between the 2 visits.
      Associations between walking performance, RPE, and heart rate were determined with respect to differences between visits by calculating the Pearson correlation coefficients. Statistical significance was defined at the P less than .05 level. For statistical analysis SPSS, version 10.0.5,
      SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.
      was used.

      Results

      Sixty-three subjects (40 women, 23 men) completed the walking tests on both study visits. Heart rate data on 3 subjects were missing because of recording failures.
      The mean age of the subjects was 52±7 years. The mean time since the onset of polio was 49±8 years, and the mean time since the onset of symptoms of postpoliomyelitis muscle dysfunction was 10±6 years. Thirty-three patients (52%) wore orthopedic footwear, 14 patients (22%) walked with an ankle-foot or knee-ankle-foot orthosis, and 9 patients (14%) used a cane or crutches. Nineteen patients (30%) did not use any orthopedic devices or walking aids.

      Walking at self-preferred speed

      According to the ICCs, test-retest reliability was excellent for walking distance and good for heart rate (table 1). Test-retest reliability for RPE was satisfactory (r=.67, P<.01). The mean level of perceived exertion corresponded to “light exertion” (score, 3). There were no systematic differences between visits in walking distance, heart rate, or RPE.
      Table 1Results for Walking at Self-Preferred Speed
      OutcomesVisit 1Visit 2đ
      Mean difference (d) between visits (second visit minus first visit).
      95% CIđ and P95% LOAICC95% CIICC
      Walking distance (m)119.2±23.9 (52.5–158.8)120.2±23.9 (60.0–166.1)1.0±8.5−1.2, 3.1; .38−16.1, 18.0.94.90–.96
      Heart rate (n=60)102±16 (77–144)100±13 (74–137)−2±8−4, 0; .06−18, 14.86.77–.91
      OutcomesVisit 1
      Median, 25th and 75th percentile, and range.
      Visit 2
      Median, 25th and 75th percentile, and range.
      dmedian
      Median difference (dmedian) between visits with 25th and 75th percentile.
      P
      Wilcoxon signed-rank test.
      95% LOA
      2.5th and 97.5th percentile of the difference.
      r
      Spearman correlation coefficient.
      Perceived exertion2 (1, 5) (0–9)2 (1, 5) (0–10)0 (−1, 1).45−5, 4.67
      NOTE. Values are mean ± SD (range) unless otherwise noted.
      * Mean difference (d) between visits (second visit minus first visit).
      Median, 25th and 75th percentile, and range.
      Median difference (dmedian) between visits with 25th and 75th percentile.
      Wilcoxon signed-rank test.
      2.5th and 97.5th percentile of the difference.
      ** Spearman correlation coefficient.
      The mean of the 2 visits against the difference between the 2 visits for walking distance are plotted in figure 1. The LOA represent the smallest change that can be detected within an individual. For walking distance, the LOA were −16.1 (−13.5% of change from the mean) and 18.0m (15.1%); for heart rate, −18 and 14; and for RPE, −5 and 4 scale points (see table 1).
      Figure thumbnail gr1
      Fig 1Bland-Altman plots for walking (A) at a self-preferred and (B) maximal speed. The solid line represents the mean difference (second visit minus first visit); the dotted lines represent the 95% LOA.
      No significant correlation was found between the difference in walking distance at the 2 visits and the difference in RPE (r=.20, P=.11). A weak relation was found between the difference in walking distance and the difference in heart rate (r=.36, P<.01). The difference in heart rate did not correlate with the difference in RPE (r=.14, P=.30).

      Walking at maximal speed

      The test-retest reliability for walking time and RPE was comparable to that for walking at self-preferred speed (table 2). The level of perceived effort corresponded to “more than average exertion” (score, 6). No systematic differences existed between the 2 visits.
      Table 2Results for Walking at Maximal Speed
      OutcomesVisit 1Visit 2đ95% CIđ and P95% LOAICC95% CIICC
      Walking time (s)60.5±20.0 (38.2–167.4)60.7±18.2 (41.4–145.6)0.2±4.5−1.0, 1.3; .77−8.9, 9.2.97.96–.98
      OutcomesVisit 1Visit 2dmedianP95% LOAr
      Perceived exertion7 (4, 8) (0–10)6 (4, 8) (0–10)0 (−1, 0).14−6, 5.70
      NOTE. See note to table 1.
      Because there was no particular reason for the outlier that appeared on the scatterplot for walking time (see fig 1), we included the data of this subject in the analysis. The LOA were −8.9 (−14.6% change from mean) and 9.2 seconds (15.2%) for walking time and −6 and 5 for RPE (see table 2). No correlation was found between the difference in walking time and the difference in RPE (r=.01, P=.97).

      Discussion

      The results showed that the test-retest reliability of walking performance and heart rate was good. The ICCs for walking performance in the present study were comparable to those reported for other populations.
      • Rossier P.
      • Wade D.T.
      Validity and reliability comparison of 4 mobility measures in patients presenting with neurologic impairment.
      • Morris M.E.
      • Cantwell C.
      • Vowels L.
      • Dodd K.
      Changes in gait and fatigue from morning to afternoon in people with multiple sclerosis.
      • Connelly D.M.
      • Stevenson T.J.
      • Vandervoort A.A.
      Between- and within-rater reliability of walking tests in a frail elderly population.
      In contrast to the findings of several other studies on reproducibility of walking performance,
      • Green J.
      • Forster A.
      • Young J.
      Reliability of gait speed measured by a timed walking test in patients one year after stroke.
      • Fransen M.
      • Crosbie J.
      • Edmonds J.
      Reliability of gait measurements in people with osteoarthritis of the knee.
      • Knox A.J.
      • Morrison J.F.
      • Muers M.F.
      Reproducibility of walking test results in chronic obstructive airways disease.
      no learning effects were found. Further, the smallest detectable change in walking performance at individual level was better than that obtained by strength measurements using a handheld dynamometer (smallest detectable changes of ≥24%).
      • Nollet F.
      • Beelen A.
      Strength assessment in postpolio syndrome validity of a hand-held dynamometer in detecting change.
      Compared with such strength measurements, the walking tests in the present study will detect deterioration in performance more readily and are, therefore, appropriate for comparing conditions at the individual and at the group levels.
      It appeared that the smallest detectable change in walking performance was equal for self-preferred and maximal speed. We expected that walking at maximal speed would be less reproducible because it may be more susceptible to differences in motivation,
      • Pinna D.G.
      • Opasich C.
      • Mazza A.
      • Tangenti A.
      • Maestri R.
      • Sanarico M.
      Reproducibility of the six-minute walking test in chronic heart failure patients.
      and patients may not be used to performing activities at maximal speed. On the other hand, variability in maximal walking speed may have been limited by the use of walking aids and orthotic devices and by certain limitations in joint mobility.
      Although the reproducibility of walking performance and heart rate were good in the present study, the reproducibility of RPE was moderate. The wide LOA indicate that the individual effort that is perceived during a similar performance under the same circumstances varies considerably. Similar findings have been reported for healthy subjects.
      • Grant S.
      • McMillan K.
      • Newell J.
      • et al.
      Reproducibility of the blood lactate threshold, 4 mmol.l(-1) marker, heart rate and ratings of perceived exertion during incremental treadmill exercise in humans.
      • Lamb K.L.
      • Eston R.G.
      • Corns D.
      Reliability of ratings of perceived exertion during progressive treadmill exercise.
      Although one might expect the reproducibility of RPE to benefit from practice,
      • Buckley J.P.
      • Eston R.G.
      • Sim J.
      Ratings of perceived exertion in braille validity and reliability in production mode.
      • Mercer T.H.
      Reproducibility of blood lactate-anchored ratings of perceived exertion.
      RPE does not seem to be a reliable measure of physical exertion in walking tests in patients with PPS. Although RPE correlates with local muscle fatigue,
      • Rodriquez A.A.
      • Agre J.C.
      Physiologic parameters and perceived exertion with local muscle fatigue in postpolio subjects.
      its poor reproducibility in walking tests may result from variation in factors other than local muscle fatigue (eg, emotional state).
      • Borg G.
      To investigate whether the variability of walking performance was related to the variability in heart rate and the variability in RPE, correlations were calculated. As expected, an increase in walking performance was associated with an increase in heart rate. However, a change in walking performance did not correlate with a change in RPE. A reason for this finding may be that limited variation in walking speed is not likely to result in differences in perceived exertion.
      A limitation of the analysis of the associations among performance, heart rate, and RPE is that only a part of a full relationship could be investigated. Preferably, these variables should be compared over larger scale ranges by measuring heart rate and RPE at predetermined intervals of walking speed.
      • Chen M.J.
      • Fan X.
      • Moe S.T.
      Criterion-related validity of the Borg ratings of perceived exertion scale in healthy individuals a meta-analysis.

      Conclusions

      Reproducibility of walking at different speeds was good in patients with PPS. Therefore, both walking tests are appropriate to monitor walking performance over time. Because no difference existed in reproducibility between the tests at different speeds, the choice of test must be based on other criteria. Testing at maximal speed might be more sensitive because one may assume that a strength decline in the muscles that are responsible for locomotion will affect walking performance at maximal speed more readily than at self-preferred speed. On the other hand, if the interest is primarily to study changes in actual walking performance, it might be preferable to use the walking test at self-preferred speed because it conforms more to the normal daily situation.
      From the moderate reproducibility of RPE at the individual level, we conclude that this is not an appropriate tool to monitor the physical performance of patients with PPS to avoid overload of their physical capacity. Heart rate was a better indicator of physical effort. However, the appropriateness of heart rate as a way to monitor physical effort in patients with PPS must be determined in further studies.
      Supplier

      References

        • 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
        • Ramlow J.
        • Alexander M.
        • LaPorte R.
        • Kaufmann C.
        • Kuller L.
        Epidemiology of the post-polio syndrome.
        Am J Epidemiol. 1992; 136: 769-786
        • Ivanyi B.
        • Nollet F.
        • Redekop W.K.
        • et al.
        Late onset polio sequelae.
        Arch Phys Med Rehabil. 1999; 80: 687-690
        • Westbrook M.T.
        A survey of post-poliomyelitis sequelae.
        Aust J Physiother. 1991; 37: 89-102
        • Nollet F.
        • Beelen A.
        • Prins M.H.
        • et al.
        Disability and functional assessment in former polio patients with and without postpolio syndrome.
        Arch Phys Med Rehabil. 1999; 80: 136-143
        • Burger H.
        • Marincek C.
        The influence of post-polio syndrome on independence and life satisfaction.
        Disabil Rehabil. 2000; 22: 318-322
        • Willen C.
        • Cider A.
        • Sunnerhagen K.S.
        Physical performance in individuals with late effects of polio.
        Scand J Rehabil Med. 1999; 31: 244-249
        • Thoren-Jonsson A.L.
        • Grimby G.
        Ability and perceived difficulty in daily activities in people with poliomyelitis sequelae.
        J Rehabil Med. 2001; 33: 4-11
        • Butland R.J.
        • Pang J.
        • Gross E.R.
        • Woodcock A.A.
        • Geddes D.M.
        Two-, six-, and 12-minute walking tests in respiratory disease.
        Br Med J (Clin Res Ed). 1982; 284: 1607-1608
        • Pinna D.G.
        • Opasich C.
        • Mazza A.
        • Tangenti A.
        • Maestri R.
        • Sanarico M.
        Reproducibility of the six-minute walking test in chronic heart failure patients.
        Stat Med. 2000; 19: 3087-3094
        • Rossier P.
        • Wade D.T.
        Validity and reliability comparison of 4 mobility measures in patients presenting with neurologic impairment.
        Arch Phys Med Rehabil. 2001; 82: 9-13
        • Agre J.C.
        • Rodriquez A.A.
        • Franke T.M.
        • Swiggum E.R.
        • Harmon R.L.
        • Curt J.T.
        Low-intensity, alternate-day exercise improves muscle performance without apparent adverse effect in postpolio patients.
        Am J Phys Med Rehabil. 1996; 75: 50-58
        • Borg G.A.
        Perceived exertion as an indicator of somatic stress.
        Scand J Rehabil Med. 1970; 2: 92-98
      1. March of Dimes. Post-polio syndrome. identifying best practices in diagnosis and care. March of Dimes, White Plains2001
        • Russell W.D.
        On the current status of rated perceived exertion.
        Percept Mot Skills. 1997; 84: 799-808
        • Whaley M.H.
        • Brubaker P.H.
        • Kaminsky L.A.
        • Miller C.R.
        Validity of rating of perceived exertion during graded exercise testing in apparently healthy adults and cardiac patients.
        J Cardiopulm Rehabil. 1997; 17: 261-267
        • Halstead L.S.
        Post-polio syndrome.
        in: Munsat T.L. Post-polio syndrome. Butterworth-Heinemann, Stoneham1991: 23-38
        • Halstead L.S.
        • Silver J.K.
        Nonparalytic polio and postpolio syndrome.
        Am J Phys Med Rehabil. 2000; 79: 13-18
        • Borg K.
        Post-polio muscle dysfunction.
        Neuromuscul Disord. 1996; 6: 75-80
        • Rankin G.
        • Stokes M.
        Reliability of assessment tools in rehabilitation.
        Clin Rehabil. 1998; 12: 187-199
        • Fleiss J.
        Statistical methods for rates and proportions. 2nd ed, 10th print ed. Wiley, New York1988
        • Lee J.
        • Koh D.
        • Ong C.N.
        Statistical evaluation of agreement between two methods for measuring a quantitative variable.
        Comput Biol Med. 1989; 19: 61-70
        • Bland J.M.
        • Altman D.G.
        Statistical methods for assessing agreement between two methods of clinical measurement.
        Lancet. 1986; 1: 307-310
        • Morris M.E.
        • Cantwell C.
        • Vowels L.
        • Dodd K.
        Changes in gait and fatigue from morning to afternoon in people with multiple sclerosis.
        J Neurol Neurosurg Psychiatry. 2002; 72: 361-365
        • Connelly D.M.
        • Stevenson T.J.
        • Vandervoort A.A.
        Between- and within-rater reliability of walking tests in a frail elderly population.
        Physiother Can. 1996; 48: 47-51
        • Green J.
        • Forster A.
        • Young J.
        Reliability of gait speed measured by a timed walking test in patients one year after stroke.
        Clin Rehabil. 2002; 16: 306-314
        • Fransen M.
        • Crosbie J.
        • Edmonds J.
        Reliability of gait measurements in people with osteoarthritis of the knee.
        Phys Ther. 1997; 77: 944-953
        • Knox A.J.
        • Morrison J.F.
        • Muers M.F.
        Reproducibility of walking test results in chronic obstructive airways disease.
        Thorax. 1988; 43: 388-392
        • Nollet F.
        • Beelen A.
        Strength assessment in postpolio syndrome.
        Arch Phys Med Rehabil. 1999; 80: 1316-1323
        • Grant S.
        • McMillan K.
        • Newell J.
        • et al.
        Reproducibility of the blood lactate threshold, 4 mmol.l(-1) marker, heart rate and ratings of perceived exertion during incremental treadmill exercise in humans.
        Eur J Appl Physiol. 2002; 87: 159-166
        • Lamb K.L.
        • Eston R.G.
        • Corns D.
        Reliability of ratings of perceived exertion during progressive treadmill exercise.
        Br J Sports Med. 1999; 33: 336-339
        • Buckley J.P.
        • Eston R.G.
        • Sim J.
        Ratings of perceived exertion in braille.
        Br J Sports Med. 2000; 34: 297-302
        • Mercer T.H.
        Reproducibility of blood lactate-anchored ratings of perceived exertion.
        Eur J Appl Physiol. 2001; 85: 496-499
        • Rodriquez A.A.
        • Agre J.C.
        Physiologic parameters and perceived exertion with local muscle fatigue in postpolio subjects.
        Arch Phys Med Rehabil. 1991; 72: 305-308
        • Borg G.
        Borg’s perceived exertion and pain scales. Human Kinetics, Champaign1998
        • Chen M.J.
        • Fan X.
        • Moe S.T.
        Criterion-related validity of the Borg ratings of perceived exertion scale in healthy individuals.
        J Sports Sci. 2002; 20: 873-899