Persons with a physical disability experience many health-related problems,1 including functional deterioration, pain, and fatigue.2 Fatigue is one of the most common issues experienced by young adults with cerebral palsy (CP)3 and is more often reported by adolescents4 and young adults with CP5 as compared with their peers developing typically. Moreover, a study among children with physical disabilities showed that fatigue is a serious issue during daily life activities, especially for girls.6 Similarly, in clinical practice, many children and young adults with a physical disability visit a rehabilitation physician with complaints about fatigue during walking or other daytime activities.
Fatigue is described as a complex subjective7 and multidimensional phenomenon,8 with both mental and physical causes. Although measurement of fatigue is not easy, questionnaires, such as the Checklist Individual Strength (CIS), are used to measure the different dimensions of fatigue.9 A subscale of the CIS is “subjective fatigue” (CIS8R), which measures the severity of general fatigue over a period of 2 weeks (where general fatigue is described as the experience of feeling tired, weak, or lacking energy).10 A frequently reported complaint by individuals with physical disabilities is fatigue during or after walking, here referred to as walking-induced fatigue. Since children may have difficulty in recalling their perceptions related to an earlier period the Children's OMNI Scale of Perceived Exertion (OMNI) can be used (instead of a questionnaire) to measure the rate of perceived exertion in real-time for a specific task.11 Therefore, use of the OMNI after walking (ie, the OMNIwalk) can be considered a task-specific way to measure walking-induced fatigue.
A review suggested that fatigue affects the ability of children and young adults with CP to participate in school, leisure, and recreation activities.12 The authors of that study concluded that clinicians should help individuals with CP to manage fatigue to prevent loss of ambulation12; to this end, it is important to understand which factors are related to fatigue.6 Children with physical disabilities often have a pathological gait.13,14 It is well established that, associated with this pathological gait, the energy demands of walking are often increased of children and young adults with CP and spina bifida compared with peers developing typically.15,16 Furthermore, they have a decreased level of physical fitness (ie, aerobic and anaerobic fitness).16,17 The “gold standard” outcome of the aerobic fitness is the peak oxygen uptake (O2peak), the maximal amount of oxygen that a person is able to use for physical activities.18 Although it seems intuitive that higher energy demands of walking and lower physical fitness are physical causes of fatigue, the relation between fatigue and energy demands of walking has not yet been studied, and inconsistent results are reported for the relation between general fatigue and physical fitness.5,19,20 The energy demands of walking can also be expressed relative to the individual's maximal aerobic fitness, which provides insight in the relative intensity of walking (ie, the physical strain of walking).21 Higher levels of physical strain of walking may lead to faster exhaustion during daily activities.21 However, no study has investigated the relation between the physical strain of walking and fatigue in children or young adults with physical disabilities.
The present study examines whether general fatigue and walking-induced fatigue are related to the energy demands of walking and physical fitness in children and young adults with physical disabilities who experience problems with walking. Since measuring fatigue immediately after walking is a more walking-specific measure of fatigue than general fatigue measured with a questionnaire, we hypothesized that walking-induced fatigue may be explained by low physical fitness levels or high energy demands of walking, while general fatigue may more likely to be attributed to other factors. It is still unclear how different physical diagnoses are affecting fatigue and therefore we included children with a broad range of diagnoses in this study. Finally, because differences are reported in the prevalence of fatigue between children and young adults,5,22 and between boys and girls,6,23 we also investigated whether the relations differ for age and/or gender.
This cross-sectional study used data from the exercise laboratory of the outpatient clinic of the Department of Rehabilitation Medicine at Amsterdam UMC. People with walking problems (eg, reduced walking distance and fatigue during/after walking or other daily activities) were referred to the clinical exercise laboratory in the context of their regular health care. Clinical exercise test results were used to set an appropriate treatment plan to reduce their walking problems. Data were collected between January 2014 and November 2016. Participants with walking problems due to a nonprogressive physical disability were included when they were able to walk at least 5 minutes with or without walking aids, and when they were able to follow simple instructions. Exclusion criteria were (1) orthopedic or neurosurgical treatment in the past 6 months, or botulinum toxin treatment in the past 3 months, and (2) cardiac abnormalities, unstable epilepsy, or other contraindications for a maximal exercise test. The Medical Ethical Committee of Amsterdam UMC waived the necessity of official approval, since the procedures were part of a standard clinical procedure.
Procedure and Equipment
An experienced pediatric physical therapist at Amsterdam UMC tested almost all participants (n = 61). An experienced clinical exercise physiologist at Amsterdam UMC tested the other participants (n = 7). After determination of anthropometry, participants filled in a fatigue questionnaire. The energy demands of walking were then measured using a resting test and a walking test, followed by a maximal aerobic fitness test. After a 20-minute rest, the participants performed an anaerobic fitness test. All tests were performed on the same day. The participants were given specific instructions not to eat or drink (except for water) 1.5 hours prior to the measurements. Height (cm) and weight (kg) were measured with a wall-fixed measure in standing position (DGI 250D, KERN DE version 3.3 10/2004) combined with an electronic scale (Kern & Sohn GmbH, Balingen-Frommern, Germany). During the walking and maximal aerobic fitness tests, pulmonary gas exchange was measured using a portable gas analysis system (Metamax 3B Cortex Biophysik, Leipzig, Germany). Oxygen uptake (O2, mL·kg−1·min−1) and carbon dioxide production (CO2, mL·kg−1·min−1) values were measured breath by breath. The respiratory exchange ratio was calculated as CO2 divided by O2. Heart rate was monitored continuously during the tests with a heart rate monitor (Polar Vantage XL). The maximal aerobic and anaerobic fitness tests were performed on a cycle ergometer (Corival V2, Lode B.V., Groningen, the Netherlands).
General fatigue was measured with the “subjective fatigue” subscale (CIS8R) of the CIS questionnaire. This subscale assesses the severity of general fatigue in the last 2 weeks by means of 8 items (eg, “I feel tired,” “I feel fit,” and “I feel rested”).9 The participants indicated to what extent the particular statement applies to them on a 7-point Likert scale, with higher scores (range 8-56) indicating a higher degree of general fatigue. The CIS8R is a self-administered paper-and-pencil test; however, the researcher verbally reads the questions. Younger participants were assisted by a parent or caregiver (if required). The CIS was originally developed to measure fatigue in an adult population. However, in Dutch studies among healthy children aged 12 to 19 years, and among adults with physical disabilities, the CIS has proven reliable to measure fatigue.23–25 Furthermore, the CIS and the CIS8R have been used to evaluate fatigue for adolescents with CP, as well as adolescents with hereditary motor and sensory neuropathy.26,27 One of these latter studies also showed that, after fitness training for adolescents with CP, the CIS8R was sensitive to change.26
In the present study, walking-induced fatigue was measured with the OMNIwalk as the rate of perceived exertion immediately after participants had walked for 6 minutes at a self-selected speed. The OMNI scale provides a subjective method for systematically reporting perceived effort on a 0- to 10-point scale, whereby 0 indicates “not tired at all” and 10 indicates “very, very tired.” The OMNI has been validated for children developing typically aged 8 to 18 years.28 Moreover, in children with CP who were walking, aged 6 to 18 years, the OMNI is a clinically feasible and valid instrument.11
Energy Demands of Walking and Physical Fitness
Walking Test. The energy demands of walking were measured during walking at a self-selected speed for 6 minutes. The test started with a resting period during which participants watched a movie for 5 minutes in a sitting position. Participants then walked on an indoor oval track (40 m). O2 and CO2 and walking speed were measured and used to calculate the gross and net energy cost (EC) (J·kg−1·m−1). The EC was calculated from O2 of walking minus resting O2. For more information regarding these calculations, see Bolster et al.15 Participants were excluded when their walking speed was 24 m·min−1 or less, as these participants appeared not to be functional walkers in daily life. Using this protocol, EC can be reliably determined in children with CP.29
Aerobic Fitness Test. Aerobic fitness was measured with a maximal exercise test. This test started with a warming-up and submaximal exercise phase. After a 1-minute rest, the maximal phase started with 1-minute incremental exercise bouts until exhaustion.17 The test outcomes were included in the analyses if at least 2 of the following 3 criteria for achieving maximal exercise were met: (1) heart rate 180 beats·min−1 or more, (2) respiratory exchange ratio 1.00 or more, and (3) observed signs of exhaustion were present. Both the peak oxygen uptake (O2peak, expressed in mL·kg−1·min−1) and the peak aerobic power output (POpeak, expressed in W·kg−1) were used as outcomes of the maximal aerobic fitness test. The O2peak, a widely reported measure of aerobic fitness, was defined as the highest oxygen uptake over 30 seconds and the POpeak as the highest power output maintained for a minimum of 30 seconds. The test-retest reliability of this protocol is excellent for children with CP.30
Anaerobic Fitness Test. Anaerobic capacity is the maximal amount of adenosine triphosphate (ATP) that is resynthesized via anaerobic metabolism during short bursts of high-intensity exercise.31 Because it is not possible to measure the anaerobic capacity noninvasively, often the anaerobic fitness is measured while cycling or running. A 20-second Wingate sprint test was conducted to estimate anaerobic fitness in this study. For familiarization and to determine the optimal breaking torque, 3 warm-up sprints of 5 seconds each were performed at different torques, determined by the severity of the motor disorder and the body height of the participant.17 After the 3 warm-up sprints, participants performed a 20-second full-out sprint against a constant workload. The mean anaerobic power over 20 seconds (P20mean) was calculated as an estimate for the sprint capacity (expressed in W·kg−1). The test-retest reliability of this outcome is excellent for children with CP.32
Physical Strain. The physical strain of walking was calculated as the oxygen uptake of walking expressed as a percentage of O2peak.21
Descriptive statistics were applied to describe participant characteristics, general fatigue, walking-induced fatigue, energy demands of walking, and physical fitness values. Distribution of the data was checked using inspection of mean values, standard deviations (SD), and visual inspection of the histograms and normal Q-Q plots. Since the data were normally distributed, parametric statistical tests were used.
Univariate and multiple linear regression analyses were used to evaluate whether the CIS8R and the OMNIwalk (dependent variables) were related to the energy demands of walking and physical fitness. The independent variables were gross EC, net EC, O2peak, POpeak, anaerobic fitness, and physical strain of walking. To test whether these relations were different for boys and girls, interaction terms were included in the models. Because it was assumed that age could also be an effect modifier, the participants were dichotomized into 2 age groups: a younger prepubertal age group (girls <11 years33 and boys <12 years34) and an older age group. Stratified analyses were presented when the interaction terms were significant. The overall significance level was set at a P value < .05.
The study included 68 children and young adults; characteristics of these participants are in Table 1. Of the 68 participants, 2 participants were solely referred for a fitness test and not for a walking test; therefore, they did not score the OMNIwalk. Ten participants were unable to score the CIS8R because they did not understand the questions of the CIS8R; these latter participants were excluded from the analysis. Finally, 66 participants scored the OMNIwalk and 58 scored the CIS8R (Figure 1).
TABLE 1 -
Characteristics of the Study Participants
|Age, mean (range; SD), y/mo
||13/1 (7/5 to 22/7; 3/7)
|Height, mean (range; SD), cm
||153.1 (116-187; 17.1)
|Weight, mean (range; SD), kg
||46.9 (18.8-69.0; 13.9)
|Body mass index, mean (range; SD), kg/m2
||19.6 (13.7-26.9; 3.3)
|GMFCS level I
|GMFCS level II
|GMFCS level III/IV
Abbreviations: GMFCS, Gross Motor Function Classification System; SD, standard deviation.
aOther diagnoses were fatigue without any specific known cause (n = 1), psychomotor retardation (n = 1), Kabuki syndrome (n = 1), Alport syndrome (n = 1), craniosynostosis (n = 1), progressive cerebellar atrophy (n = 1), novo mutation CACNA1A (n = 1), cerebellar ataxia ECI (n = 1), and spina bifida (n = 2). Gray dots = younger participants. Black dots = older participants (girls older than 11 years and boys older than 12 years).
Mean values of general fatigue, walking-induced fatigue, energy demands of walking, and physical fitness are shown in Table 2.
TABLE 2 -
Mean (SD) of the Fatigue
Scores, Energy Demands During Walking and Physical Fitness
||GMFCS Level I (n = 13)
||GMFCS Level II (n = 34)
||GMFCS Level III/IV (n = 11)
||Other Diagnosis (n = 10)a
|CIS8R (range: 8-56)
||24.8 (9.6) [n = 12]
||28.7 (8.6) [n = 28]
||34.3 (10.5) [n = 9]
||27.8 (9.7) [n = 9]
|OMNIwalk (range: 0-10)
||3.9 (2.0) [n = 13)
||5.1 (2.3) [n = 34]
||5.7 (3.1) [n = 10]
||3.4 (2.1) [n = 9]
|Energy demands during walking
|Gross EC, J·kg−1·m−1
||5.2 (0.9) [n = 13]
||7.1 (1.6) [n = 33]
||9.6 (1.5) [n = 7]
||5.5 (1.9) [n = 10]
|Net EC, J·kg−1·m−1
||3.5 (0.5) [n = 13]
||5.3 (1.5) [n = 33]
||7.0 (1.3) [n = 7]
||4.0 (1.3) [n = 10]
||40.0 (9.9) [n = 12]
||38.1 (7.5) [n = 22]
||30.3 (7.0) [n = 6]
||32.7 (9.1) [n = 10]
||2.5 (0.7) [n = 12]
||1.9 (0.6) [n = 22]
||1.3 (0.4) [n = 6]
||2.1 (0.8) [n = 10]
|Physical strain during walking
|Physical strain, %
||48.9 (13.3) [n = 12]
||58.4 (13.0) [n = 21]
||78.0 (18.5) [n = 6]
||57.3 (19.9) [n = 10]
||6.0 (1.7) [n = 12]
||4.0 (1.2) [n = 19]
||2.5 (0.5) [n = 3]
||4.8 (2.4) [n = 6]
Abbreviations: CIS8R, Checklist Individual Strength, subscale subjective fatigue
; EC, energy cost; GMFCS, Gross Motor Function Classification System; OMNIwalk, Children's OMNI Scale of Perceived Exertion; POpeak
, peak aerobic power output; P20mean
, mean anaerobic power; O2peak
, peak oxygen uptake.
aOther diagnoses were fatigue without any specific known cause (n = 1), psychomotor retardation (n = 1), Kabuki syndrome (n = 1), Alport syndrome (n = 1), craniosynostosis (n = 1), progressive cerebellar atrophy (n = 1), novo mutation CACNA1A (n = 1), cerebellar ataxia ECI (n = 1), and spina bifida (n = 2).
Table 3 includes results of the linear regression analyses for general fatigue (CIS8R) and walking-induced fatigue (OMNIwalk). The CIS8R was not related to gross or net EC, or physical strain of walking. There were trends for a relation between the CIS8R and the O2peak (P = .055; R2 = 0.083), POpeak (P = .060; R2 = 0.082) and anaerobic fitness (P = .095; R2 = 0.076). There were no interaction effects for age and gender in the relation with the CIS8R.
TABLE 3 -
Relation Between Fatigue
and Energy Demands During Walking and Physical Fitness
||β CIS8R (95% CI)
||β OMNIwalk (95% CI)
|Energy demands during walking
||Gross EC, J·kg−1·m−1
||(n = 54)
||(n = 62)
||Net EC, J·kg−1·m−1
||(n = 54)
||(n = 45)
||(n = 49)
||(n = 45)
||(n = 49)
|Physical strain walking
Physical strainb, %
||(n = 45)
||(n = 48)
||(n = 38)
||(n = 40)
Abbreviations: CIS8R, Checklist Individual Strength, subscale subjective fatigue
; β, regression coefficient; CI, confidence interval; EC, energy cost; OMNIwalk, Children's OMNI Scale of Perceived Exertion; POpeak
, peak aerobic power output; P20mean
, mean anaerobic power; O2peak
, peak oxygen uptake.
aVariables with a significant relation with fatigue (P < .05).
bVariables that differ between the younger and older age groups.
The OMNIwalk was significantly related to gross EC (R2 = 0.093), net EC (R2 = 0.134), O2peak (R2 = 0.087), physical strain of walking (R2 = 0.213), and anaerobic fitness (R2 = 0.113). The relations show higher OMNIwalk values for children with high gross EC, high net EC and high physical strain of walking values, and with low O2peak and low anaerobic fitness values. There was a trend for a negative relation between the OMNIwalk and POpeak (P = .057; R2 = 0.076). The linear regression analyses had significant interactions of age with anaerobic fitness and physical strain of walking, indicating that these relations are different for the 2 age groups (Figure 2). For the older age group, high rates of OMNIwalk were related to low anaerobic fitness values (β = −0.854; 95% confidence interval [CI] = −1.580/−0.011; P = .038; R2 = 0.264) and high physical strain of walking values (β = 0.089; 95% CI = 0.013/0.164; P = .023; R2 = 0.350), whereas these relations were not present in the younger age group. There were no interaction effects for gender.
This study examined whether general fatigue and fatigue during or after walking (ie, walking-induced fatigue) are related to the energy demands of walking and physical fitness in children and young adults with physical disabilities who experience problems with walking. Results support that walking-induced fatigue is related to energy demands of walking and fitness in teenagers with CP, but not in younger children. In addition, general fatigue had no relationship with any of the fitness parameters in both teenagers and children.
These results indicate that complaints about general fatigue (ie, the experience of feeling tired, weak, or lacking energy) in young people with CP are not likely to be explained by low fitness levels or high energy demands of walking. Two studies, which measured fatigue in CP children and young adults with a questionnaire, had comparable findings with similar rationale.5,20 Thus, there is evidence that deviated energy demands of walking and physical fitness values do not cause general fatigue. We know, from other studies, that there are factors that are associated with general fatigue in individuals with physical disabilities such as a higher body mass index,35 more pain,36 and being physically inactive.6,35 However, not all potentially relevant factors for general fatigue have been studied in children/young adults with physical disabilities; for example, factors that are associated with general fatigue in Dutch children developing typically aged 12 to 18 years, such as unrefreshing sleep, decreased participation in sports, higher depression and anxiety scores, and concentration problems.23 Thus, future studies should examine whether these factors are associated with general fatigue in children/young adults with physical disabilities before effective interventions can be developed. This suggests that when children and young adults with physical disabilities report general fatigue in clinical practice, rehabilitation practitioners might focus on all aforementioned (possible) causes for their diagnostic evaluation and treatment, and not on the outcomes of clinical exercise tests.
In contrast to general fatigue, walking-induced fatigue was related to energy demands of walking and physical fitness in teenagers. We assume that this discrepancy reflects the different factors that cause general fatigue and walking-induced fatigue because, whereas general fatigue seems to be influenced by both mental and physical causes, walking-induced fatigue seems to be mainly influenced by physical causes. While there is no general consensus on the definition of fatigue, we suggest that general fatigue and walking-induced fatigue are different problems that should be clearly distinguished and measured separately in clinical practice.
The present study revealed clear differences between children and teenagers. An explanation for the differences between the 2 age groups might be that particularly younger children tend to underestimate fatigue; for example, children with CP reported less fatigue than their parents via proxy reports.22 In the present study, younger participants had slightly lower scores for the OMNIwalk (4.2; SD = 2.1; range 0-7) compared with older participants (5.0; SD = 2.6; range 0-10). Alternatively, walking-induced fatigue might be less severe in younger children, or they may experience fatigue differently. A good understanding regarding the differences in the experience of fatigue between younger children and adolescents/young adults is required before effective interventions for both groups can be developed.
Among teenagers, lowering the physical strain of walking may result in less fatigue during or after walking, and this may positively influence daily life activities. According to training guidelines for individuals developing typically, walking around 46% to 63% of the O2peak (physical strain of walking) is a moderate-intense activity and values around 64% to 90% represent vigorous-intense activities.37 Our participants (with Gross Motor Function Classification System levels II, III/IV, and other diagnoses) walked at intensities of 60% or more, whereas children and adolescents developing typically have values around 40%.38 Walking represents an intense or even vigorous activity for most of our participants. It is therefore not surprising that walking-induced fatigue is related to physical strain of walking because walking at higher intensities is more fatiguing. The different causes of high physical strain values (increased energy demands of walking and/or reduced O2peak) require different intervention strategies. Orthotics and orthopedic surgery have the potential to reduce EC in ambulatory children with CP,39,40 while fitness training has the potential to improve the O2peak in children and young adults with physical disabilities.41,42 However, whether individuals will have less walking-induced fatigue when the physical strain improves (either by lowering energy demands of walking or by improving the aerobic capacity) is yet to be determined.
Improving the anaerobic fitness might also result in less fatigue during or after walking in teenagers. Due to the short and intermittent patterns that characterize physical activity in children, adequate anaerobic fitness is assumed to be an important prerequisite for performing daily activities in children, such as walking.43 Fitness training has the potential to improve the anaerobic fitness in children and adolescents with CP.44 However, studies are required to reveal whether increasing the anaerobic fitness leads to clinically important improvements in walking-induced fatigue.
Since all participants reported walking problems, the results of this study cannot be generalized to individuals without walking problems. Prepubertal status was solely based on age and not objectively measured with clinical tests.
There were missing values for both the energy demands of walking and for physical fitness. Although some participants were unable to perform a walking test, or an aerobic or anaerobic fitness test, most missing data on the energy demands of walking and physical fitness were because participants were specifically referred for one of the tests (and not the other), as they had already undergone too many tests in the context of their regular health care. These missing values may have caused a type II error.
The psychometric properties of the CIS have been evaluated for individuals developing typically and physically disabled aged 12 years or older, and not for younger children.23–25,27 However, another questionnaire (with items similar to those in the CIS8R) designed to measure fatigue (ie, the Pediatric Quality of Life Inventory [PedsQL]), has good psychometric properties for children with CP aged 5 to 18 years.45 Furthermore, younger participants were assisted by a parent or caregiver (if required) and, when the researcher doubted the ability of the participant to score the CIS8R (both children and teenagers), these outcomes were excluded from the analyses (n = 10). Therefore, in the present study, it was assumed that the younger children were also able to adequately fill in the CIS8R.
Due to the small sample size, we did not investigate whether the observed relations between fatigue and the energy demands of walking and physical fitness differ between the diagnostic subgroups. Additional studies with a larger number of participants are required to further examine these differences. Although the heterogeneous nature of the population tested could be considered a disadvantage, this variety in characteristics will also be beneficial to generalizability.
The results of this study show that walking-induced fatigue was related to energy demands of walking and physical fitness in teenagers with physical disabilities, but not in younger children. In addition, general fatigue had no relationship with any of the parameters in both teenagers and younger children. These results suggest that interventions aimed at improving anaerobic fitness and reducing physical strain might result in less fatigue during or after walking and therefore warrant further investigation.
The authors thank all the children, adolescents, and parents for their willingness to participate in this study.
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