Walking ability and predictors of performance on the 6-minute walk test in adults with spastic cerebral palsy
Abstract
Aim To describe walking ability and identify factors predicting walking capacity in adults with spastic cerebral palsy (CP) assessed with the 6-minute walk test (6MWT).
Method A cross-sectional clinical study as part of the recruitment process for a randomized controlled trial on the effects of botulinum toxin A. Data analysed were the 6MWT, Timed Up and Go (TUG) test, Borg Scale, spasticity, muscle strength, popliteal angle, pain, fatigue, type of CP, foot deformity, Gross Motor Function Classification System (GMFCS) levels, Functional Mobility Scale (FMS) scores, a gait questionnaire, interview, and demographic data.
Results In total, 126 persons were included (53 males, 73 females; mean age 39y [SD 12y]; 59 with unilateral and 67 with bilateral spastic CP; GMFCS level I, n=12; level II, n=94; level III, n=20). Mean distance on the 6MWT was 485m (SD 95m) with FMS scores reflecting independent walking performance in daily life. Multiple regression analysis identified sex, type of CP, popliteal angle, pain, and TUG values as significant predictors, with TUG values as the strongest predictor (standardized regression coefficient =−0.57, p<0.001).
Interpretation Our results demonstrate that 39% of the participants had declined one GMFCS level from adolescence to their present age, and that the TUG was the strongest predictor for the 6MWT. This implicates the importance of focusing specifically on the different elements of functional mobility in further studies.
List of Abbreviations
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- 6MWT
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- 6-minute walk test
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- 6MWD
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- 6-minute walk test distance
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- FMS
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- Functional Mobility Scale
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- SF-36
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- General Health Survey Short-Form 36
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- TUG
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- Timed Up and Go
Cerebral palsy (CP) is a group of permanent disorders of movement and posture, often accompanied by secondary musculoskeletal problems.1 Literature on children with CP supports a link between the levels of impairment and walking ability.1 However, the relation between self-reported declined walking ability and data on clinical measurements in adults needs more investigation.
Surveys of adults with CP report that contractures, spasticity, pain, fatigue, as well as reduced muscle strength and balance, are associated with declined walking ability.2–4
The Gross Motor Function Classification System (GMFCS) categorizes gross motor function into five levels, with level I describing the highest level of function and level V the lowest.5 A study of individuals with CP aged up to 21 years showed that those classified at GMFCS levels III to V have their peak motor function in childhood, whereas individuals at levels I or II have a stable gross motor function.6 However, surveys of adults with CP indicate that even persons at GMFCS levels I or II experience increasing walking difficulties in young adulthood.4
Adequate postural stability, selective motor control, muscle strength, and endurance are necessary prerequisites for performing basic gross motor skills such as walking, including the complex ability to stay upright and be moving at the same time.7 Walking in the community also requires sensory, perceptual, and cognitive abilities to anticipate and react adequately to crowds and unexpected events.8
In studies on gait and mobility/balance, the 6-minute walk test (6MWT)9 and the Timed Up and Go (TUG) test10 are frequently used measures for the activity dimension of the International Classification of Functioning, Disability and Health (ICF).11 The 6MWT, which measures the maximum distance a person can walk within 6 minutes, was originally developed as an endurance measure in chronic heart failure.12 For elderly and stroke populations, the 6MWT is considered a general measure of overall functional walking capacity, involving the integrated response of multiple body systems.9,13,14 The TUG was originally developed as a balance test for the elderly.15 By measuring, in time, the ability to stand up, walk, turn around, and sit down, the TUG captures the complex interaction between balance and movement, including planning, initiating, executing, and completing a series of linked movements that are common in daily activities.8
The purpose of this study was to describe walking ability in a population of adults with spastic CP experiencing declined walking function, and to identify clinical and demographic factors predicting walking capacity, as measured with the 6MWT.
Method
This study is based on data from the baseline screening before inclusion in a randomized controlled trial on the effects of botulinum toxin A in adults with spastic CP at the Sunnaas Rehabilitation Hospital, Oslo, Norway.
The participants were recruited through advertisements in newspapers and on the websites of the Norwegian Cerebral Palsy Association and Sunnaas Rehabilitation Hospital from 2007 to 2008. The first author was in contact with all the responders. Potential eligible responders were questioned by semi-structured telephone interview about changes in walking function from adolescence to their present age.
Inclusion criteria were (1) uni- or bilateral spastic CP, (2) age 18 to 65 years, (3) GMFCS levels I to III, (4) ability to walk independently continuously for 6 minutes, (5) ability to provide informed consent, and (6) a phone interview reported declined walking ability compared with adolescence such as reduced walking distance, increasing stiffness, spasticity, pain, or balance difficulties. Exclusion criteria were (1) other conditions that could affect the walking ability, (2) orthopaedic surgery within the past 18 months, (3) injections of botulinum toxin A within the preceding 3 months, (4) pregnancy, and (5) intellectual disability documented in medical records.
The Commissioner for the Protection of Privacy in Research and the Regional Ethics Committee in south-eastern Norway approved the study. All participants gave written informed consent.
Measurements
A test protocol provided detailed descriptions and instructions for all data elements collected during one visit to Sunnaas Rehabilitation Hospital. Data from medical records, classification of type of CP according to the Surveillance of Cerebral Palsy in Europe,16 functional level by GMFCS,5,17 and Functional Mobility Scale (FMS),18 6MWT, TUG test, and self-administered questionnaires were performed or collected by the first author. The first author, assisted by one of three skilled physiotherapists, used a consistent technique with standardized procedures for the outcomes of body structure/function. No formal reliability testing was performed; however, the measurements were the same as those used in the motion laboratories at Sunnaas Rehabilitation Hospital and at Oslo University Hospital, developed and adjusted through joint training sessions with consensus processes.
Activity/participation
Walking ability was determined using the GMFCS, the FMS, and a questionnaire on walking function (Appendix SI, published online). The GMFCS level at adolescence was rated on information from the participants. Present GMFCS level was rated based on observations. The GMFCS has demonstrated validity and reliability for classification of present and earlier gross motor function in adults with CP (intraclass correlation coefficient [ICC]=0.93–0.95).17 The FMS, a 6-level scale rating mobility over three distances (5m, 50m, and 500m), was used as a measure of walking performance and rated based on information from the participants. The FMS has established psychometric properties for children with CP (interrater reliability ICC=0.95; construct validity r=0.75–0.89).18 The self-administered questionnaire on walking function (Appendix SI) included information on causes of change in walking ability, walking performance in different contexts, and on limiting symptom(s) related to walking. It was developed by studying other questionnaires used in adult CP research,2,3 and through workshops with the present authors and the named person acknowledged. Applicability was assessed in the ordinary clinic before the start of the study.
The 6MWT was used to measure functional walking capacity. The participants were instructed to walk as fast and as long as possible for 6 minutes. Each participant underwent the 6MWT in a quiet hospital corridor with a 30m marked track, and chairs as turning points instead of cones to support turning if needed. The investigator repeated set phrases every minute during the walk.9 The total distance walked was recorded as the 6MWT distance (6MWD, to the nearest metre). The 6MWT has demonstrated test–retest reliability for adults with CP (ICC=0.97).19 Perceived exertion was graded immediately after 6 minutes using the Borg Scale (rating of perceived exertion, 6–20), a measure of exercise intensity with reported correlations of ratings and heart rates of r>0.80 and test–retest reliability of ICC=0.72.19,20
The TUG was used to measure functional mobility. The participants sat on a standardized armchair. They were instructed to rise, walk as quickly and safely as possible to a marker on the floor 3m away, turn around, walk back to the chair, and sit down on set cues. Timing started on the word ‘start’ and finished once the participant’s back touched the back of the chair.10 Participants performed the test three times; the fastest time measured in seconds with one decimal place was used. The TUG has demonstrated test–retest reliability of ICC=0.99 in children with CP.21 Despite some similarity with 6MWT, the TUG has several tasks involved that are potentially destabilizing, thus testing functional mobility and anticipatory aspects of postural control.8
Pain intensity and impact on daily life were measured using the ‘bodily pain’ domain of the General Health Survey Short-Form 36 (SF-36), on a scale from 0 (worst pain) to 100 (no pain).22 The SF-36 is validated in the Norwegian population,23 and the ‘bodily pain’ domain has demonstrated test–retest reliability of ICC=0.90.24
Fatigue was measured with the Fatigue Severity Scale, a questionnaire using a scale from 1 to 7.25 The questionnaire is validated in the Norwegian population, with average scores of at least 5 defined as severe fatigue,25 and has demonstrated test–retest reliability of kappa=0.74 for the average score.26
Body structure/function
Foot deformity, defined as tibiocalcaneal angle being greater than 5° varus or valgus, was determined by visual categorization with the participants in standing position.27 This assessment showed no disagreement with a secondary photograph-based assessment for 66 of the participants.
Unilateral popliteal angle, expressing ‘functional’ hamstring length,7 was used as an indicator of range of motion in lower limbs (Appendix SII, published online). Intrarater reliability of ICC=0.77 is reported for children with CP.28
The Modified Ashworth Scale, which measures the degree of resistance to fast passive movement, was used as an indicator of spasticity in lower limbs (Appendix SII).29,30 Interrater reliability with a general kappa value of 0.74 to 0.82 is reported for comparable populations.30
Manual muscle testing as described by Hislop,31 which scores muscle strength on a scale from 0 to 5, was used as an indicator of muscle strength in lower limbs (Appendix SII). This has demonstrated convergent construct validity with dynamometry (r>0.76).32 Because manual muscle testing is prone to examiner bias, all the tests were performed by the same assessor.31,32
As impairment measures for the lower limbs, the popliteal angles, Modified Ashworth Scale, and manual muscle testing scores were individually transformed into summary measures calculated as the mean of the measurements.33
Statistical analysis
Descriptive statistics were used to characterize the sample. The TUG displayed a skewed distribution and was log transformed. There were no missing data. Differences in TUG scores and 6MWD between the GMFCS levels were analysed with the Kruskal–Wallis test and the Mann–Whitney U-test. Bivariate relations between variables were explored in cross-tables, box plots, and scatter plots, with corresponding Pearson’s and Spearman’s correlation coefficients.
Relevant literature was considered to identify possible predictors for performance on the 6MWT, reflecting both patient characteristics and the different ICF domains. The regression model building included univariate regression analyses, analyses of several custom models considered to be of importance, and stepwise procedures. Every step of the analysis involved F-tests, Akaike’s Information Criterion and model diagnostics, and discussions of the clinical importance of the findings. Interdependencies between predictors were explored by principal component analysis34 and multicolinearity diagnostics. The impact of this on the regression coefficients was explored by ridge regression.34 The final multiple model was based on variables identified in these procedures, with redundant variables removed. Bootstrap methods with 10 000 replicates were used to calculate the regression confidence intervals (CIs), as the traditional formula-based CIs do not account for potential bias in the marginal distributions of the regression coefficients.35 Despite their shortcomings, bootstrap procedures are valuable tools when asymptotic properties of the estimators can be debated.36
The principal component analysis, ridge modelling, and the bootstrap routines were performed in the R language, version R.2.9.1 (http://www.r-project.org/). All other analyses were performed with SPSS software (version 15.0; SSPS Inc., Chicago, IL, USA). Probability values less than 0.05 were considered statistically significant.
Results
Of 201 respondents, 156 met the inclusion criteria and were enrolled for further testing. After clinical investigation another 30 individuals were excluded, and 126 individuals were included in the study (Fig. S1, published online). Demographic and clinical characteristics are presented in Table I.
| n (%)a | |
|---|---|
| Age in years (y), mean (SD) | 39 (12) |
| Range, y | 18–65 |
| Sex | |
| Female | 73 (58) |
| Male | 53 (42) |
| Type of cerebral palsyb | |
| Spastic unilateral | 59 (47) |
| Spastic bilateral | 67 (53) |
| GMFCS levelc | |
| I | 12 (10) |
| II | 94 (74) |
| III | 20 (16) |
| Education (duration) | |
| First level (9y) | 25 (20) |
| Second level (12y) | 49 (39) |
| Third level (>12y) | 52 (41) |
| Work/source of income | |
| Paid work ≥20% | 81 (64) |
| Student | 16 (13) |
| Disablement benefit/unemployedd | 51 (40) |
| Comorbiditiese | 61 (48) |
| Epilepsy | 1 (1) |
| Allergy | 26 (21) |
| Anxiety/depression | 16 (13) |
| Hypertension | 11 (9) |
| Hypothyroidism | 7 (6) |
| Otherf | 4 (3) |
| Orthopaedic surgery lower limbg | 62 (49) |
| Gastrocnemius/soleus lengthening | 47 (37) |
| Hamstring lengthening | 11 (9) |
| Iliopsoas lengthening | 11 (9) |
| Otherh | 9 (7) |
- aNumbers presented unless otherwise stated. bBased on data from medical records and clinical assessments. cClassified by first author. dFor many with less than 100% paid work and some of the students the income was combined with disablement benefit, so the percentages of work/studies and disablement benefit add up to more than 100. eObtained from medical records. fNamely diabetes type II, psoriasis, and hypercholesterolemia. gObtained from medical records. For several participants, this was more than one operation, so the percentages of orthopaedic surgery of lower limbs add up to more than 100. hRotational osteotomies and foot/toe corrections. GMFCS, Gross Motor Function Classification System.
Walking ability
Most participants (n=94) were classified at GMFCS level II. However, according to GMFCS level at adolescence, 32 had declined from GMFCS level I to level II (Table II). Examples of statements from the interviews and questionnaires were, ‘Now I need a rail while climbing stairs’ and ‘I don’t do mountaineering anymore’.
| GMFCS levela, n (%) | |||
|---|---|---|---|
| Adolescence | Present | Shift to lower GMFCS levels | |
| I | 44 (35) | 12 (10) | |
| II | 80 (64) | 94 (74) | 32 (25) from level I to II |
| III | 2 (2) | 20 (16) | 18 (14) from level II to III |
| FMS, n (%) | ||||
|---|---|---|---|---|
| Score | Description | 5m | 50m | 500m |
| 6 | Independent on all surfaces | 14 (11) | 5 (4) | 1 (1) |
| 5 | Independent on level surfaces | 103 (82) | 101 (80) | 87 (69) |
| 4 | Use one crutch or two sticks | 8 (6) | 12 (10) | 25 (20) |
| 3 | Use two crutches | 1 (1) | 7 (6) | 6 (5) |
| 2 | Use walking frame | 0 (0) | 1 (1) | 5 (4) |
| 1 | Use wheelchair | 0 (0) | 0 (0) | 2 (2) |
| Questionnaireb, n (%) | |
|---|---|
| Is able to use public transport | 98 (78) |
| Walking for exercise | 83 (66) |
| Reported changes compared with adolescencec | |
| Increased stiffness | 100 (79) |
| Decreased balance | 68 (54) |
| Decreased walking distance | 76 (60) |
| Increased pain | 81 (64) |
| Reported limitations in walking functiond | |
| Fatigue | 69 (55) |
| Stiffness | 81 (64) |
| Balance | 65 (52) |
| Pain | 73 (58) |
- aClassification of Gross Motor Function Classification System (GMFCS) level in adolescence based on interview data compared with present classification based on observation. bQuestionnaire developed for this study. c,dThe percentages add up to more than 100 owing to changes or limitations caused by more than one of the items. FMS, Functional Mobility Scale.
TUG values and 6MWD varied significantly across the GMFCS levels (p>0.001). The median scores (interquartile range) on the TUG were as follows: GMFCS level I, 5.2s (4.7–5.9); level II, 6.5s (5.9–8.4); and level III, 10.8s (9.9–13.8). Median 6MWDs were as follows: GMFCS level I, 611m (542–652); level II, 491m (435–543); and level III, 376m (317–423).
FMS scores showed that the participants were more likely to use assistive devices outdoors (Table II). Additional information from the interviews showed a trend towards increasing frequency in the use of assistive devices. Stiffness was the most frequently reported cause of declined walking ability (Table II).
Predictors of 6MWT
During the model-building process, we noted that type of CP, spasticity, muscle strength, popliteal angle, FMS, GMFCS, and the TUG test were interrelated, with bivariate correlation coefficients ranging from r=0.31 (0.14–0.46) to r=0.63 (0.51–0.73; GMFCS/FMS), all factors having a correlation with 6MWD of r>0.38 (0.22–0.52). Both the GMFCS and FMS were significantly associated with 6MWD in the univariate analysis (r=0.58 [0.45–0.69] and r=0.56 [0.43–0.67]) respectively. Because most participants were at GMFCS level II and FMS 5, and because of colinearity in models with the TUG and FMS, GMFCS and FMS were not included in the multiple regression analysis. Descriptive statistics of the variables in the final multiple model are presented in Tables I and III.
| Variablea | Mean (SD)b | Range |
|---|---|---|
| Dependent variable | ||
| 6-minute walk test distance (m) | 485 (95) | 260–730 |
| Potential predictor | ||
| Borg Scalec (6–20) | 13.5 (2.1) | 9–20 |
| Foot deformityd, n (%) | 75 (60) | Yes/no |
| Popliteal anglee (degrees) | 51 (13) | 25–90 |
| Spasticityf | 1.2 (0.8) | 0.1–3.3 |
| Muscle strengthg | 4.4 (0.4) | 3.4–5.0 |
| Timed Up and Goh (s), median (Q1–Q3) | 6.8 (5.9–9.6) | 3.6–24 |
| Log-transformed Timed Up and Go | 0.86 (0.14) | 0.56–1.38 |
| Bodily paini (0–100) | 54.8 (25.2) | 0–100 |
| Fatigue severity scalej (0–7) | 4.8 (1.4) | 1.1–7.0 |
- aAge, sex, and type of CP are described in Table I. bNumbers presented unless otherwise stated. cPerception of physical exertion, higher scores indicating more exertion. dOccurrence of equinovarus or -valgus deformity. eUnilateral popliteal angle for the right and left side calculated as the mean of the measurements, higher score indicating reduced ‘functional hamstring length’. fModified Ashworth Scale (0–4) for eight different muscle groups calculated as the mean of the measurements, higher score indicating more spasticity in lower limb. gManual muscle testing (0–5) for 14 different muscle groups calculated as the mean of the measurements, lower score indicating less muscle strength in lower limbs. hHigher score indicating more functional mobility impairment. iGeneral Health Survey Short-Form 36 (SF-36) domain of bodily pain, higher score indicating less pain and less effect of pain on daily activities. jHigher score indicating more fatigue. Q1–Q3, interquartile range.
Univariate analyses (Table IV) showed that neuromuscular and musculoskeletal impairments, such as increased spasticity, reduced muscle strength, increased popliteal angle, equinovarus or -valgus foot, as well as increased pain, fatigue, and time used in the TUG, were related to decreased 6MWD. Sex was included in the multiple regression analyses as a known predictor for 6MWD.
| Unadjusted estimates | Adjusted estimates | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Bootstrap | Bootstrap | |||||||||
| Potential predictors | B a | p | Betab | B a | 95% CI | B a | p | Betab | B a | 95% CI |
| Sex | −29 | 0.089 | −0.15 | −29 | −63; −4 | −29 | 0.008 | −0.15 | −29 | −50; −9 |
| Age | −2.3 | 0.001 | −0.31 | −2.3 | −3.6; −1.0 | |||||
| Type of cerebral palsy | −98 | <0.001 | −0.52 | −98 | −126; −70 | −31 | 0.007 | −0.16 | −31 | −55; −9 |
| Foot deformityc | −43 | 0.011 | 0.22 | −43 | −9; −78 | |||||
| Popliteal angled (degrees) | −2.6 | <0.001 | −0.39 | −2.6 | −3.6; −1.4 | −1.5 | 0.001 | −0.20 | −1.5 | −2.3; −0.6 |
| Spasticitye | −40 | <0.001 | −0.45 | −40 | −53; −28 | |||||
| Muscle strengthf | 133 | <0.001 | 0.51 | 133 | 96; 173 | |||||
| Borg Scaleg | −12.8 | 0.001 | −0.29 | −12.8 | −20; −6 | |||||
| Log-transformed TUGh | −508 | <0.001 | −0.77 | −508 | −606; −424 | −377 | <0.001 | −0.57 | −377 | −478; −291 |
| Bodily paini | 1.3 | <0.001 | 0.34 | 1.3 | 0.7; 1.9 | 0.5 | 0.009 | 0.14 | 0.5 | 0.1; 1.0 |
| Fatigue Severity Scalej | −16.2 | 0.008 | −0.24 | −16.2 | −28; −4.9 | |||||
- Eleven clinically relevant potential predictors were included. The adjusted estimates are those from the final model, using multiple linear regression analysis (R2=0.67). aRegression coefficient. bStandardized regression coefficient. Coding of categorical variables: male=0, female=1; spastic unilateral=0, spastic bilateral=1. cOccurrence of equinovarus or -valgus deformity classified as no=0, yes=1. dUnilateral popliteal angle for the right and left side calculated as the mean of the measurements, higher score indicating reduced ‘functional hamstring length’. eModified Ashworth Scale (0–4) for eight different muscle groups calculated as the mean of the measurements, higher score indicating more spasticity in lower limb. fManual muscle testing (0–5) for 14 different muscle groups calculated as the mean of the measurements, lower score indicating less muscle strength in lower limbs. gPerception of physical exertion, higher score indicating more exertion. hHigher score indicating more functional mobility impairment. iGeneral Health Survey Short-Form 36 (SF-36) domain of bodily pain (0–100), higher score indicating less pain. jHigher score indicating more fatigue. TUG, Timed Up and Go.
In the final multiple regression analysis, sex, type of CP, popliteal angle, pain, and TUG values were identified as significant predictors of 6MWD. The adjusted estimates and CIs are shown in Table IV. Females had a shorter 6MWD than males, as did bilateral CP compared with unilateral CP. An increase of 1° in popliteal angle resulted in a 2.6m shorter 6MWD; transferring this result to clinical practice, a 10° increase in popliteal angle would have resulted in 26m shorter 6MWD. The regression coefficient for pain reflected a 1.3m longer 6MWD, with a higher score on the 0 to 100 scale indicating less pain. The regression coefficient for a log-transformed TUG of −377 reflected a shorter 6MWD with increasing TUG values (in seconds). Participants with a TUG value at the lower quartile for the group (5.9s) walked on average 100m further than those at the upper quartile for the group (9.6s). Log-transformed TUG became the strongest predictor, with a standardized regression coefficient of −0.57 (Table IV). The coefficient of multiple determination (R2) showed that this model explained 67% of the variance of the 6MWD. These results were similar to the results from ridge regression models.
Discussion
This study investigated walking ability and the influence of demographic, clinical, and self-reported factors on functional walking capacity in adults with CP who reported a declined walking function compared with their function during adolescence. We identified CP-related neuromuscular deficits, pain, and sex as important factors predicting functional walking capacity. Self-reported walking function in adolescence compared with current GMFCS classification indicated a shift in GMFCS level for 39% of the participants.
As expected, type of CP was a significant predictor for 6MWD, which implies that individuals with spastic bilateral CP have more neurological impairments.1 Because CP is a multifactorial disorder, and there is conflicting evidence on how motor impairments relate to each other and to function for children with CP,29,33 it seems logical that our results indicate a complex interrelation between the variables investigated.
Increased mean popliteal angle was associated with decreased 6MWD. By choosing unilateral popliteal angle instead of bilateral popliteal angle,7 we measured the ‘functional hamstring length’ with the potential effect of a contralateral hip flexor tightness, a factor considered relevant when choosing a lower-limb range of motion measure related to walking function. The effect of mean spasticity in the adjusted analyses may be confounded by other effects, such as the mean popliteal angle. This is supported by the association between reduced range of motion and increased spasticity/more impairment in spastic CP,7,29 the ‘increased stiffness’ as the most frequently reported cause of declined walking function, and the fact that the modified Ashworth Scale does not discriminate between dynamic spasticity and spastic contractures.29
When adjusted, mean lower-limb muscle strength was not a significant predictor. This may reflect the narrow range of muscle strength and the clinical impression that the participants had satisfactory muscle strength for walking.37 However, we recognize that manual muscle testing has limited ability to discriminate muscle strength scores.31,32 Thus, we hypothesize that muscle strength is important for maintaining functional mobility in more demanding mobility tasks than walking in a corridor.8,38 Foot deformity was excluded in the adjusted analysis as a consequence of other variables being more important.
As the TUG is a measure of functional mobility, the interrelations between the TUG, GMFCS, FMS, and 6MWD were expected. We considered it important to use a measure with a potential ability to detect anticipatory aspects of postural control, as well as sensory and perceptual abilities required in functional mobility,8 thus challenging the complex impairments mentioned in the definition of CP.1 The literature is conflicting about the ability of the TUG to differentiate between the different GMFCS levels.21,39 We found a significant difference in TUG values across these levels. Thus, we suggest that the TUG can give useful information about functional mobility in daily activities in comparable populations. Recognizing the limitations of the TUG as a specific measure of balance, it may be sufficient for assessing changes of functional balance in follow-up programmes. Although the 6MWT and TUG are different measures, there is some overlap in tasks, which may overestimate the effect of the TUG on 6MWD. Further, it is recognized that the performance of the TUG is related to age, possibly because of its dependence on muscle strength as well as anticipatory postural control, both of which decrease with age.8 Thus the TUG may confound the effects of age and muscle strength in our final model.
Our finding that pain was a significant predictor of functional walking capacity, confirms the earlier reported impact of pain on declined ambulation.2,4 In contrast, fatigue was adjusted out as consequence of other variables being more important. However, it is reasonable to hypothesize a greater impact from pain and fatigue if walking performance had been investigated.4
Despite the normally distributed age range from 18 to 65 years, we did not find any effect of age. This was surprising,3 but may be due to the impact of the TUG or to other CP-related impairments being more important predictors than age. Moreover, we recognize the fundamental challenge of age effects in a cross-sectional study.
As with samples of healthy populations, sex was a significant predictor in our final model; height and body mass index are likely to contribute to this finding.40 The Borg Scale was not a significant predictor, which probably reflects that most of the participants were more limited by neuromuscular impairment than by cardiovascular factors. This has also been demonstrated for stroke populations.14
The walking capacity in these high-level ambulant participants was better than in a population of adults with CP recruited from a rehabilitation centre,19 but reduced compared with the general population.40 The results show that even persons at GMFCS level I experience declined walking ability. As others have demonstrated stability until young adulthood,6 our results demonstrate the need for studies on adults. In addition, there are few studies exploring different mobility aspects that, potentially, protect from or predispose to declined walking in individuals at GMFCS levels I and II. Thus, our results support the need for a measure differentiating the children at GMFCS level I.41
We recognize that a limitation of this study is the selected group of high-level independent walkers. The recruitment through advertisements also led to more female participants than males. Though widespread in clinical use, the lower limb assessments have limitations to their reliability.28,29,32 To compensate for this we used a strict protocol, and one experienced clinician conducted all the assessments. The use of mean scores may have limited the measurements for popliteal angle, spasticity, and muscle strength. However, the purpose was to have clinically relevant scores in spastic lower limbs, and therefore sum scores were judged to be sufficiently precise.33
Although we have attempted to explore a wide range of different subsets of predictor variables to support our final model, subset selection procedures tend to be slightly overfit, and the effect size estimates may, therefore, be biased.42 Our bootstrap estimation ensured that possible biases in the distribution of the estimators were accounted for in the calculations of the CIs. However, a potential slight overestimation of effect sizes remains, with other subsets of predictor variables potentially being equally good or superior to those we have presented.34,42,43 Therefore, our model requires further validation in studies on similar samples. The CIs in the final model may reflect both measurement errors and the known heterogeneity in spastic CP.7 The results support the need for more studies with adequate sample sizes addressing subgroups, as well as studies with a longitudinal design exploring the effects of ageing on walking ability.
Conclusion
The present study demonstrates that in this population of independent adult walkers reporting declined walking ability compared with adolescence, 39% had changed GMFCS level. The multiple regression analyses identified sex, type of CP, popliteal angle, pain, and performance on the TUG as significant predictors, with the TUG as the strongest predictor. This implicates the importance of focusing specifically on different aspects of functional mobility in further studies with an adequate sample size and a longitudinal design.
What this paper adds
- •
Thirty-nine per cent of ambulant adults with spastic CP had declined one GMFCS level from adolescence to their present age.
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6MWD and TUG values varied significantly over the GMFCS levels.
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The TUG test was the strongest predictor of 6MWT.
- •
Age effects and TUG elements need specific investigation.
Acknowledgements
This study was performed as part of a randomized controlled trial, and the project was supported by the East Regional Health Administration and Sunnaas Rehabilitation Hospital (grant number 206 24 503). We are grateful to all the adults with CP who participated. We acknowledge Professor Katharina S Sunnerhagen, and the staff at Sunnaas Gait laboratory.
