Physical fitness can be divided in various components, namely health-related fitness (peak oxygen uptake; VO2peak), and performance-related fitness (i.e., muscle strength and anaerobic capacity). VO2peak is most frequently assessed using progressive graded exercise tests on a cycle ergometer with respiratory gas analysis in children with JIA, although some researchers have used treadmill testing in children with JIA.[15,16]
There is a large body of evidence demonstrating that the VO2peak of children and adolescents with JIA is lower compared with healthy peers. In the recent studies in JIA, between the ages 6.7–18 years, VO2peak (L/min) and VO2peak normalized for body mass (VO2peak/kg in ml/kg/min) were, respectively, 69.8 and 74.8% in children and 83 and 80% of predicted values for VO2peak and VO2peak/kg for healthy peers.[17,18] These observations confirm the results of a previous meta-analysis demonstrating that VO2peak per kg body mass was on average 21.8% lower in children with JIA compared with healthy control subjects or reference values. We recently studied the VO2peak in a group of 12 young adults with JIA and observed a significantly reduced VO2peak (Z-score for VO2peak/kg was -1.0 ± 2.13), which was not significantly different from values observed in children and adolescents with JIA [van Pelt PA, Takken T, van Brussel M, Kruize A, Wulffraat N: Association between aerobic capacity and disease activity in adolescents and young adults with JIA. Submitted]. There is a need for longitudinal follow-up studies in physical fitness levels in JIA. The results of the previously mentioned study suggests that the physical fitness levels of JIA is not improving over time. This was also recently observed in a small group of patients with JIA who underwent autologous stem cell transplantation. As shown in Figure 2, on average there is no improvement over time. In addition, several patients had an exacerbation of the JIA following autologous stem cell transplantation, which is observed in the sudden drops in VO2peak/kg.
Changes over time in VO2peak/kg in children with juvenile idiopathic arthritis who underwent an autologous stem cell transplantation. The different symbols and lines indicate observations from individual patients. The dotted line indicates the trend line of the individual values over time.
ASCT: Autologous stem cell transplantation.
Data taken from .
From these observations, it can be hypothesized that children and adolescents remain hypoactive when disease goes into remission with or without medication. The fact that young adults treated in the prebiologic treatment era and that younger children who are currently being treated with newer biologicals are not scoring better on aerobic fitness, suggests that we have observed hardly any positive effect of the developments in medical treatment over the last decade on physical fitness. Recent unpublished data from our group indicated that JIA patients with a history of anti-TNF biological use had a lower aerobic capacity compared to nonusers [Takken T, Unpublished Data]. This might be due to the fact that in the Netherlands anti-TNF is only prescribed for patients with a polyarticular disease course who failed earlier treatment or had untreatable side effects on high-dose methotrexate [van Pelt PA, Takken T, van Brussel M, Kruize A, Wulffraat N: Association between aerobic capacity and disease activity in adolescents and young adults with JIA. Submitted].
It is probable that exercise therapy and physical activity promotion can further enhance aerobic fitness and exercise participation in patients with JIA, especially in the cases with severe disease.
Giannini and Protas also found that children with JIA had significantly lower peak work rate (amount of Watt that a subject can generate during a graded exercise test on a cycle ergometer), peak exercise heart rate (HRpeak) and exercise time than healthy control subjects matched for age, gender and body size.[20,21] Recently published observations in 91 children with JIA demonstrated that children with JIA had on average a HRpeak of 181 ± 14 bpm, while healthy children have on average a HRpeak 193 ± 7 bpm during cycle ergometry in our laboratory. Some of the children with JIA stopped the exercise test because of fatigue and/or musculoskeletal complaints, and not because of a cardiopulmonary limitation during exercise. Owing to the range in HRpeak, it is important to measure the HRpeak of a subject during a graded exercise test and not to use a general prediction, such as 220-age.
Many centers do not have the equipment to perform respiratory gas analysis to measure VO2peak. However, peak work load (Wpeak) during a graded bicycle test can be used as a surrogate measure for VO2peak, since an excellent correlation between Wpeak and VO2peak (r = 0.95, p < 0.0001) has been observed in 91 children with JIA. VO2peak can be predicted from Wpeak, weight and gender using the following equation:
This equation was established using a step-wise increased protocol with an increase of 20 W/min. In young children with a height less than 120 cm, increments of 10 W/min can be used and in children between 120 and 150 cm, 15 W increments per min can be used. These rapid increasing protocols are to be preferred above slower increasing protocols, such as the Giannini protocol, with increases of 20 W per 3 min. Slow incremental protocols often take quite long, approximately 20 min, and it is my experience that children will stop because of peripheral muscle fatigue and not because of cardiopulmonary limitation.
Impaired VO2peak does not appear to be significantly related to the severity of joint disease, but may be due to hypoactivity secondary to disease symptoms, especially in children with long-standing arthritis.[4,9,10] Physiologic factors, including anemia, muscle atrophy, generalized weakness and stiffness, resulting in poor mechanical efficiency may also limit the child's performance.
Fitness in other Rheumatic Conditions
Of the other pediatric rheumatoid conditions, physical fitness levels in children with juvenile dermatomyositis, systemic lupus erythematosis and recently the fitness levels among children with mixed-connective tissue disease were reported. Their results are shown in Figure 3. As reference, the values as observed in children with chronic fatigue syndrome (significant overlap with fibromyalgia) is provided.
The Z-score for VO2peak/kg of patients with juvenile dermatomyositis, systemic lupus erythematosis, mixed-connective tissue disease and chronic fatigue syndrome.
CFS: Chronic fatigue syndrome; JDM: Juvenile dermatomyositis; JIA: Juvenile idiopathic arthritis; MCTD: Mixed-connective tissue disease; SLE: Systemic lupus erythematosis.
Data taken from [17,26–29].
It is imminent that a rheumatic disease that primarily attacks the muscle will result in the lowest score in physical fitness, an overlap syndrome, such as mixed-connective tissue disease and JIA turns out to show almost equal outcomes in physical fitness. Interestingly, recently diagnosed children with chronic fatigue syndrome have on average a normal score in physical fitness compared with healthy peers Figure 3.[17,26–29]
As previously mentioned, performance-related fitness includes components, such as muscle strength and anaerobic capacity. Impairments of muscle strength include weakness in hip extension and abduction, knee extension, plantar flexion, shoulder abduction and flexion, elbow flexion and extension, wrist extension and grip. Muscle bulk, strength and endurance should be examined at disease onset and monitored regularly. Bilateral measurements of circumference quantify asymmetries in muscle bulk. Functional muscle strength can be estimated in young children by observing their performance of age-appropriate motor tasks or activities of daily living. In older children, manual muscle testing can be done to measure isometric strength, especially if the child has pain while moving the limb against resistance. Instrumented measurements using a handheld or isokinetic dynamometer or modified sphygmomanometer [van Pelt PA, Takken T, van Brussel M, Kruize A, Wulffraat N: Association between aerobic capacity and disease activity in adolescents and young adults with JIA. Submitted] provide consistent and reliable information in patients with arthritis. Both isometric and isokinetic strength have been shown to be valid and reliable measures of muscle strength in normal children. In JIA the reliability of isometric strength measurement of the lower limb muscles has been assessed and reported to have a sufficient intra- and inter-rater reliability. Several studies have demonstrated that muscle strength is significantly reduced in children with JIA compared with healthy peers.[31–35]
Lindehammar and Sandstedt reported in a longitudinal study that muscle strength diminishes rapidly near an inflamed joint. This is probably caused by atrophy of the muscle, which is influenced by local arthritis. One study suggested that muscle weakness may contribute to activity restrictions in children with arthritis. Fan and colleagues found a significant relationship between 50 m run times and lower extremity Childhood Health Assessment Questionnaire (CHAQ) scores in girls with JIA.
However, muscle strength testing in children with JIA, especially hand-held dynamometry using the 'break' technique, might be problematic in some cases, because children might give way due to pain instead of the limits in muscle strength. Moreover, in some cases children can experience increased knee pain and swelling.
Anaerobic capacity can be assessed using short-term exercise tests. Usually anaerobic capacity is measured using a 30 s all-out cycle ergometer test, the Wingate Anaerobic Test. Children have to cycle as fast as they can against a fixed resistance, and based on the number of revolutions per second, the power output is recorded.
Two recent studies investigating the anaerobic capacity in children and adolescents with JIA reported significantly lower values of anaerobic capacity in subjects with JIA.[6,7] Anaerobic capacity was reduced to the same extent compared with aerobic fitness (VO2peak). Previously it has been found that the reduced anaerobic capacity was significantly correlated to CHAQ scores in 18 children with JIA, ages 7–14 years. This is not surprising, since the typical physical activity behavior of children – short bursts of intense activities separated by periods of rest – is anaerobic in nature. Given the apparently similar deficits in anaerobic capacity of youth with JIA, exercise training of the anaerobic energy system (e.g., high-intensity interval training) might be equally valuable as training of the aerobic system and, therefore, warranted in children with arthritis. However, this training modality has not yet been studied.
Another widely used performance-related fitness test is the 6-min walk test (6MWT). In this test children have to cover as much distance as they can in 6 min while walking (not running). This test is used in different patient groups, such as JIA, spina bifida, cerebral palsy and hemophilia.[40–42] Lelieveld et al. found a low correlation between walking distance and VO2peak in children with JIA. In addition, Paap et al. found that children with JIA were exercising at 80–85% of their HRpeak and VO2peak during the 6MWT, indicating that it is an intensive, submaximal exercise test to measure functional exercise capacity in children with JIA. Furthermore, these data indicate the exercise intensity at the end of the 6MWT can be used for the programming of exercise intensity during aerobic exercise training in children with JIA, because this exercise intensity is sufficient to improve fitness levels. The HR at the end of a 6MWT can thus be used for exercise programming. However, the 6MWT distance cannot be used as a measure of VO2peak in children with JIA.
Pediatr Health. 2010;4(5):499-507. © 2010 Future Medicine Ltd.
Cite this: Physical Fitness, Activity and Training in Children with Juvenile Idiopathic Arthritis - Medscape - Oct 01, 2010.