Results from this study support the hypothesis that adolescents with chronic pain have poor subjective physical functioning and lower objective physical activity levels when compared to healthy adolescents. Parents and adolescents reported similar levels of subjective physical functioning, which is consistent with previous research demonstrating concordance between adolescent and parent reports of pain-related disability.2
This study also demonstrated similar magnitude of reduced physical activity among adolescents with chronic pain compared to a previous small comparison study using actigraphy.12
Thus, impairment in physical functioning appears to be a robust finding both across reporter and across measurement method. This study also provides additional support for the use of actigraphic measures of daytime activity as a measure of physical activity among adolescents with chronic pain.
Physical functioning has been recommended for use as an outcome domain in pediatric chronic pain populations by the PedIMMPACT consensus group, which has emphasized that physical functioning is likely to include activities of everyday life 13
. Measures recommended in the PedIMMPACT consensus statement included measures of subjective disability and subjective physical health-related quality of life. Physical functioning is a multidimensional domain, which includes an individual’s subjective perception of their physical function, including perceived difficulty performing everyday activities, but also includes actual participation in everyday activities, as well as additional dimensions such as vigorous physical activity participation and physical fitness. Results from the current study highlight physical activity as a dimension that is distinct from subjective disability. Recent measurement work has also demonstrated unique factors pertaining to vigorous and routine activity limitations in the FDI9
and the CALI-2116
. In , we present an organizational framework to conceptualize three broad dimensions within the overarching domain of physical functioning: perceived disability, physical activity, and physiological function. The framework shows the potential bidirectional associations between actual and perceived function, includes the important role of physiological factors impacted by general health, and distinguishes between vigorous activities such as sports and exercise, and more habitual or routine activities. Objective and subjective measurement tools may be used to capture the different dimensions outlined within this conceptual framework. In measuring physical function in pediatric populations, parent or other adult proxy-report of perceived disability and physical activity may supplement child self-report.
A conceptual framework representing domains of physical function
As has been observed in previous studies, when associations between actigraphic measures of physical activity and subjective measures of functioning are significant, these are still relatively low correlations (e.g., .2–.3), indicating that objective measures of activity are capturing a distinct dimension or dimensions of functioning. In this study, we observed similarly small associations across reporters and with two self-report measures. Similar findings have been observed in adolescents with fibromyalgia; these data provide additional support for the importance of multi-method assessment when researching physical function.10
Assessing physical activity may provide additional useful information about an important dimension of physical functioning that has broad implications for adolescent health.
The current findings provide some support for distinctions between actigraphic measures of activity. Peak activity level or vigorous levels of activity seems to be most strongly associated with subjective disability across studies.10, 12
This may be due to the nature of this measurement, in that lower scores on actigraphic peak activity level may be an objective indicator of withdrawal from or non-participation in vigorous physical activities such as sports or physical education. Difficulty with such activities is specifically assessed on measures of subjective disability, such as the FDI,26
and the PedsQL.24
Additionally, previous work suggests that participation in physical education or sports contributes more strongly to peak rather than mean activity levels; mean levels of physical activity may be capturing the habitual level of physical activity (see ), such as daily movement and active transportation habits28
. Habitual activities may be less impacted by chronic pain for most youth, and thus less closely tied to subjective disability. Additionally, adolescent report on the CALI-21 was not significantly correlated with mean activity level (see ). This measure asks respondents to report on the level of difficulty they experience participating in activities because of pain. It is possible that adolescent reports of activity limitations are more representative of internal perceptions of difficulty, whereas parent reports are based more on observations of adolescent behaviors which could be more highly correlated with actigraphy monitoring.
Different actigraphic measures may be more or less sensitive to change over time or to treatment effects, and specific measures may be more clinically relevant based on which aspects of functioning are being targeted (for instance, in treatment). Clinical researchers may want to choose actigraphic measures based on specific hypotheses about disability or behavior change, and to serve as complementary measures to self-report tools. For instance, investigators devising an exercise intervention to increase physical activity participation might examine peak activity measures to assess whether vigorous activity increases with treatment. In contrast, investigators examining risk for the development of pain-related disability might measure baseline mean activity to see whether lower habitual activity level increased risk for disability over time.
One of the major challenges with the current tools available for measuring disability in youth with chronic pain is that available measures are uni-dimensional and scored by summing or average all items, despite items representing a wide variety of activities. On the CALI-21 for instance, a total score of 18 may represent extreme difficulty attending school and doing things with friends for one child, but represent difficulty with sports, running, and physical education for another child. Thus, it is possible that with improvements in measurement tools and with use of factor scores to assess specific sub-domains of disability,16
more significant associations with actigraphic measures might emerge. Future research might utilize item-level analysis or other approaches to examine whether the content of particular self-report items or subscales is more strongly related to actigraphic measures.
Individual characteristics that impact physical activity in healthy adolescents also appear to contribute to physical activity in adolescents with chronic pain. Increasing age was associated with declines in activity and increases in sedentary time, as has been demonstrated in a number of studies.20
Female gender was related to lower peak physical activity, which is also commensurate with research showing greater declines in activity in females during adolescence.20
Higher BMI percentile was also associated with lower activity and more time spent in sedentary activity, which has been demonstrated in previous studies.23
While these associations were in the expected directions, lower levels of physical activity may be particularly problematic among adolescents with chronic pain given that pain in and of itself may limit activity and lead to weight gain, which can in turn increase discomfort and pain associated with activity.8
Greater attention might be paid to addressing modifiable risk factors in this population, particularly BMI percentile, as weight status has been noted to play an important role in chronic pain outcomes in prior research.7, 29
It may also be important to increase focus on exercise participation and overall physical activity in treatments for chronic pain, particularly for adolescents who have BMI percentiles in the overweight or obese ranges.
There are several limitations to this study that should be noted. First, although we utilized an objective measure of physical activity, there are several other dimensions of physical function that were not assessed in this study, including physical fitness or observed physical function. There are a number of laboratory-based measures that have been used in other studies to measure these dimensions, such as a timed walk and sit-to stand tasks,4
or physiological measures of fitness.1
Inclusion of these types of measures might have strengthened our ability to capture the multidimensional nature of physical functioning. Additionally, the measure of peak physical activity that was utilized, the highest level of activity achieved in a single one minute epoch, does not capture sustained moderate or vigorous activity. Other accelerometer devices and software allow for examination of the amount of time spent at higher levels of activity, which might provide more accurate information about vigorous exercise participation. While actigraphic measures provide objective information about activity, they do not provide any information about the function or meaning of these activities for individuals. Information about the function or purpose of physical activity may be important for understanding behavior change, adherence, and other aspects of functioning.
It is also important to note that selection factors may contribute to the group differences we observed between chronic pain and healthy groups, as the chronic pain group was recruited through specialty care physicians. In addition to having chronic pain, these youth may have had more concerned parents than the healthy group, or had primary care providers who were more likely to refer them to specialty care for a variety of reasons; these unmeasured factors may have contributed to the observed group differences. Another important limitation to our study was that it was cross-sectional and we did not examine associations between pain and individual characteristics and physical functioning and activity over time. Longitudinal research in this area will provide important insights into predictors of activity. We are also unable to draw causal inferences about the role of pain in reducing activity levels. It is possible that adolescents with chronic pain evidence lower levels of physical activity compared to their peers prior to developing chronic pain. Thus, low activity levels may be a risk factor for chronic pain development and/or a correlate or symptom of chronic pain.
While one of the primary goals of psychological and interdisciplinary treatments for chronic pain in children and adolescents is to improve physical functioning, few studies have demonstrated changes in physical functioning following treatment, and most have utilized measures of subjective disability to assess physical function.5, 15
It is largely unknown whether treatments for pain can improve physical activity level. Researchers aiming to improve overall physical activity or physical fitness among adolescents with chronic pain will need to carefully consider the multidimensional nature of physical functioning and choose outcome measures appropriately. Measuring physical functioning in multiple methods and across dimensions will help us better understand the impact of chronic pain on children and adolescents, as well as identify appropriate targets for intervention and measure responses to treatment.