What are the important components of physical activity for people with fibrotic interstitial lung disease?

Abstract

Introduction

Fibrotic interstitial lung disease (fILD) is characterized by scarring of the lungs, decline in lung function and compromised exercise capacity. People with fILD participate in less daily physical activity compared to healthy controls, however, the relative importance of physical activity components (volume, intensity, duration) is unknown. This study aimed to examine the relationship between the components of physical activity and established disease severity and impact measures in fILD.

Methods

Participant data were from baseline assessment of a randomized controlled trial recruiting people with fILD and exertional desaturation during exercise testing on room air. Physical activity components including volume (steps per day), intensity (light, moderate and vigorous) and duration (time spent in different physical activity intensities) were objectively assessed for 7 days using two physical activity monitors. Associations of these components with lung function (FVC, FEV₁, TLCO), functional exercise capacity (6-min walk distance, 6MWD) and dyspnea (Dyspnea-12 questionnaire) were investigated.

Results

106 participants with mild to severe fILD were included. Physical activity volume was not associated with 6MWD (r = 0.027, p = 0.78) and weakly associated with lung function (FVC r = 0.33, p = 0.001; FEV₁ r = 0.34, p = 0.001). Physical activity intensity and duration were weakly associated with 6MWD (light r = 0.22 p = 0.02; moderate r = 0.42, p < 0.001; vigorous r = 0.33, p = 0.01). Time spent in vigorous physical activity was weakly associated with lung function (FVC r = 0.19, p = 0.05; FEV₁ r = 0.18, p = 0.006). Dyspnea scores were not associated with any physical activity variables. Relationships remained consistent after adjusting for age, gender and disease severity, with the exception of vigorous physical activity which was no longer associated with lung function or 6MWD.

Conclusion

In people with fILD, the physical activity components of volume, intensity and duration were associated with different measures of disease severity and impact. This highlights the importance of considering specific physical activity components when evaluating and promoting physical activity in this group.

Keywords

physical activity interstitial lung disease lung function exercise capacity 6-min walking test

Introduction

There is unequivocal evidence that physical activity is associated with improved health outcomes for healthy people and those with chronic disease; it is recommended that adults with chronic diseases should complete 150–300 min of moderate or 75–150 min of vigorous intensity physical activity per week, or between 6500 and 8000 steps per day.^1,2 People with fibrotic interstitial lung disease (fILD) are known to be inactive, participating in 65% less daily physical activity compared to healthy controls.³ Low levels of physical activity are a predictor of mortality and morbidity in fILD³ and are strongly related to worse respiratory function, exercise capacity and health related quality of life.^3,4

Objective monitoring of physical activity using activity monitors is well accepted and increasingly available, with a range of monitors used to assess daily physical activity in patients with fILD.⁵ Most commonly, monitors track daily physical activity using step counters or accelerometers.⁶ Physical activity components measured by wearable activity monitors include volume (e.g. step count), intensity (e.g. energy expenditure) and duration (e.g. time spent in different physical activity intensities).⁷

There is growing interest in understanding the role of physical activity in the management of fILD. Activity monitors can now capture all physical activity components, but the relative importance of these components in predicting disease outcomes such as exercise capacity, lung function and dyspnea in people with fILD has not been fully established.^8–11 Understanding the relative importance of these components may assist with physical activity monitor selection, guide the design of future interventions and monitoring strategies.¹²

Aim

The aim of this study was to evaluate the relationship of physical activity components with established clinical meaningful measures of disease severity and impact in people with fILD.

Methods

Study population

Data used in these analyses were from the baseline assessment of a multicenter randomized controlled trial of ambulatory oxygen therapy for fILD (ClinicalTrials.gov Registry NCT03737409).¹³ We recruited participants from five hospitals in Australia and two hospitals in Sweden between July 2019 and May 2023. Inclusion criteria were: age 18 years or over; physician-confirmed diagnosis of fibrosing ILD, such as idiopathic pulmonary fibrosis, connective tissue disease associated ILD, chronic fibrosing hypersensitivity pneumonitis, idiopathic non-specific interstitial pneumonia, unclassifiable idiopathic interstitial pneumonia, environmental/occupational lung disease or sarcoidosis, with features of diffuse fibrosing lung disease of >10% extent on high-resolution computed tomography, with ILD being the predominant pathologic process; stable pharmacotherapy over the last 3 months; exertional desaturation defined as oxygen saturation (SpO₂) ≤88% for ≥10 consecutive seconds during a 6-min walk test performed on room air. Potential participants were excluded if they were: currently using or eligible for long-term oxygen therapy; current smokers; predominantly obstructive lung disease with forced expiratory ratio less than the lower limit of normal; pregnant; cognitively unable to consent; non-ambulant; or admitted to an acute care hospital within the last 30 days. Participants currently in pulmonary rehabilitation were not enrolled.¹⁴

Physical activity components

Physical activity was assessed over 7 days using two activity monitors worn concurrently. Participants were instructed to wear both devices continuously with removal only for sleeping and showering. A valid day required at least 8 h of recorded data, and a minimum of four valid days of data was required for dataset inclusion.¹⁵

Physical activity volume was measured by step count using an ankle-worn pedometer (Stepwatch; Modus Health; Washington DC, USA). The StepWatch is a reliable and valid tool in people with chronic lung disease, and it detects slow walking speeds and small changes in step rate.^16–18 The StepWatch data was extracted using the Modus RE application software which generated Excel files with records of steps per minute for each day of monitor usage. Excel files were grouped using R studio application, excluding measures from the first and last day. A valid day was defined as at least 8 h of wear time and more than 200 step counts.^19–21

Physical activity intensity and duration was measured by wrist-worn triaxial accelerometer (GeneActiv; ActivInsights Ltd., Kimbolton, Cambridgeshire, UK). The Geneactiv has been validated for use in people with chronic lung conditions.²² Data was extracted using the Geneactiv software that generates Excel files classifying recorded data for time spent in: no wear, bed, sedentary, light, moderate and vigorous intensity physical activity. Data for time in ‘no wear’, ‘bed’ and ‘sedentary physical activity’²³ were excluded from the analysis. A valid day was defined as at least 8 h of wear time. Intensity of physical activity was processed according to validated cut-points based on energy expenditure: light (between 1.50 and 2.99 METs); moderate (between 3 and 5.99 METs); or vigorous intensity activity (≥6 METs).²⁴ Duration of physical activity was reported as time (minutes) per day spent in light, moderate and vigorous intensity physical activity, as well as combined moderate to vigorous (MVPA) physical activity.

Alignment between data from both monitors was verified using date and time stamps from extracted files; only data for days when both devices had valid recordings were included.

Measures of disease severity

Functional exercise capacity was assessed using the distance achieved in the 6-min walk test (6MWD) performed according to international standards.²⁵ Two tests were performed in room air and the best distance was recorded (meters). Heart rate and oxygen saturation were measured before, during and at least 60 s after the test and recorded every minute as per standard procedures. Standardized commands were given every minute during the test. The distance achieved after the test was recorded and the percentage of predicted value calculated.^23,26

Lung function was assessed by spirometry and pulmonary diffusion capacity measurement. Forced vital capacity (FVC), forced expiratory volume exhaled in the first second (FEV₁) and transfer factor for carbon monoxide (TLCO) were collected and used in the analysis.

The Dyspnea-12 questionnaire was used to capture both the physical and affective components of dyspnea. It is reliable and valid questionanire for use in people with fibrotic interstitial lung disease.²⁷

Statistical analysis

Descriptive statistics was used to characterize included participants. Data are presented in mean and standard deviation for normally distributed variables, and median and interquartile range for non-normally distributed variables. Pearson’s correlation coefficients or Spearman’s rho (depending on data distribution) were used to assess univariate relationships between physical activity components and measures of disease severity and were classified as strong association (0.7–1), moderate association (0.4–0.69) and weak association (0.1–0.39).^28,29 Analyses were repeated adjusting the data for age, sex, and baseline distance achieved on the 6MWT.

Results

One hundred and six participants consented and were included in the analysis. Participant characteristics are presented in Table 1. Physical activity data are presented for 102 participants as four participants did not have 4 days of valid data and were excluded.

Table 1.

Participant characteristics, measures of disease severity and physical activity.

Participant characteristics		n = 106
Age, years		70 (9.6)
Male, n (%)		73 (68)
Previous pulmonary rehabilitation participation, n (%)		26 (24)
Diagnosis, n	Idiopathic pulmonary fibrosis	54
	Unclassifiable ILD	19
	Idiopathic non-specific interstitial pneumonia	11
	Connective tissue disease-related ILD	8
	Hypersensitive pneumonitis	8
	Vasculitis induced pulmonary fibrosis	3
	Asbestosis	1
	Sarcoidosis	1
	Graft versus host disease in progressive fibrotic ILD	1
Measures of disease severity
FVC	% predicted	70.5 (16.2)
FVC	litres	2.5 (0.8)
FEV₁	% predicted	74.8 (16.2)
FEV₁	litres	2.0 (0.5)
TLCO	% predicted	44.7 (10.9)
TLCO	litres	10.6 (3.4)
6MWD, meters		426 (122)
6MWD, % predicted		66 (20)
Dyspnea-12 (total score)		14 (9)

Physical activity	n = 102
Number of steps/days	5949 (3424)
Total physical activity time, minutes	288 (125)
Average time spent in light intensity/day, minutes	235 (72)
Average time spent in moderate intensity/day, minutes	53 (47)
Average time spent in vigorous intensity/day, minutes	1 (4)
Average time spent in MVPA/day, minutes	54 (49)

Data presented as mean (standard deviation) unless indicated; FEV₁: forced expiratory volume in 1 s; FVC: forced vital capacity; ILD: interstitial lung disease; 6MWD: distance walked on 6-min walk test; TLCO: transfer factor for carbon monoxide; MVPA: moderate to vigorous physical activity.

In unadjusted analyses, physical activity volume measured by number of steps was weakly associated with FVC (r = 0.30, p = 0.001) and FEV₁ (r = 0.27, p = 0.006) but not with 6MWD (r = 0.06 p = 0.56). Physical activity intensity and duration across all classifications was weakly or moderately associated with 6MWD (light intensity r = 0.22, p = 0.02; moderate intensity r = 0.42, p < 0.001; vigorous intensity r = 0.33, p = 0.01; MVPA r = 0.43, p < 0.001). Time spent in vigorous intensity physical activity was also weakly associated with FVC (r = 0.19, p = 0.05) and FEV₁ (r = 0.18, p = 0.006) (Table 2). When adjusted for age, sex and baseline 6MWD, the pattern of relationships remained consistent, with the exception of vigorous physical activity which was no longer significantly associated with measures of lung function or 6MWD (Table 2). There were no relationships between physical activity component measures and dyspnea scores in either unadjusted or adjusted analyses (Table 2).

Table 2.

Associations between physical activity components and measures of disease severity and impact.

Physical activity components		Measures of disease severity and impact
		Unadjusted						Adjusted
		FVC	TLCO	FEV₁	Dyspnea-12	6MWD	6MWD (% predicted)	FVC	TLCO	FEV₁	Dyspnea-12	6MWD	6MWD (% predicted)
Steps per day		0.30*	0.11	0.27*	−0.12	0.06	−0.1	0.20*	0.08	0.18	−0.11	0.05	−0.15
Time in physical activity, minutes	Light intensity	−0.11	−0.09	−0.10	−0.10	0.22*	0.08	0.02	−0.01	0.01	−0.10	0.18	0.08
	Moderate intensity	0.15	−0.19	0.14	−0.14	0.42*	0.26*	0.20*	−0.16	0.17	−0.17	0.40*	0.24*
	Vigorous intensity	0.19*	−0.04	0.18*	−0.06	0.33*	0.16	0.11	0.01	0.14	0.14	0.17	0.15
	MVPA	0.17	−0.16	0.15	−0.12	0.43*	0.27*	0.20*	−0.15	0.17	−0.15	0.40*	0.25*

Data presented as Pearson correlation coefficient; *: significance level p-value <0.05; FEV₁: forced expiratory volume in 1 s, liters; FVC: forced vital capacity, liters; MVPA: moderate to vigorous physical activity; 6MWD: distance walked on 6-min walk test, meters; TLCO: transfer factor for carbon monoxide, liters. Associations between lung function variables and Dyspnea-12 with physical activity variables were adjusted for 6MWD, age and sex. Associations between 6MWD with physical activity variables were adjusted for FVC, age and sex.

Discussion

This study has shown that in people with fILD, the physical activity components of volume, intensity and duration were associated with different measures of disease severity and impact. Relationships between the components of physical activity, respiratory function and functional exercise capacity were modest, and no relationship with dyspnea was demonstrated. These relationships remained consistent when adjusted for age, sex and 6MWD, with the exception of vigorous physical activity, where statistically significant relationships were no longer apparent. The male dominance of the sample, as well as the very low time spent in vigorous activity, may have affected these analyses.

People with fILD typically engage in physical activity levels below recommendations which is reflected in our study by the relatively low volume of physical activity (indicated by step count).^2,30 This highlights the ongoing challenge of promoting physical activity participation in this population. The average time spent by participants in vigorous intensity activity was low, limiting interpretability and suggesting little or no engagement in high intensity activities during daily life. However, participants spent an average 54 min per day in MVPA. These estimates should be interpreted with caution, as it is possible that in people with fILD, a greater metabolic effort is required even in light activities leading to a misclassification of physical activity intensity using standard values. Future studies should consider using disease-specific thresholds to capture more meaningful and relevant classification of physical activity intensity as proposed by Bianchim et al.³¹

Recent studies in people with chronic obstructive pulmonary disease have demonstrated a strong correlation between physical activity intensity and volume by means of light intensity physical activity and number of steps per day.^32,33 Accordingly, it might be expected that these two measures would show similar associations with measures of disease severity and impact. However, this was not observed in our study. In people with fILD, previous research has shown that physical activity volume, measured by step count, is strongly associated with lung function and functional capacity. In contrast, physical activity intensity and duration may not show the same relationships. Interestingly, Wallaert et al.³ observed that in people with fILD, time spent in light activity was correlated with the number of steps per day. However, only steps per day were significantly associated with disease severity indicators such as FVC, TLCO, and FEV₁. They suggested that people with fILD who showed more physical activity volume - walked more daily steps - did not necessarily perform more light physical activity in daily life. This suggests that physical activity volume and physical activity intensity may reflect distinct behavioral constructs, potentially explaining their different associations with disease severity and impact in this population.³

Physical activity is complex; volume, intensity and duration of physical activity are important and distinct components that may relate to different aspects of disease-related impairment in people with fILD. These analyses demonstrated that people with fILD and better-preserved lung function took more steps in daily life. Better functional exercise capacity, measured by 6MWD was associated with physical activity of higher intensity and longer duration in the daily life in people with fILD.³ Physical activity volume (measured by step count) may relate to lung disease impairment, while physical activity intensity and duration might relate to functional and physical capacity. This may be important to guide the design of future interventions aiming to increase physical activity and improve outcomes for people with fILD.

Whilst it is likely that physiological capacity and physical fitness influence physical activity in daily life in people with fILD, behavioral factors are also important. It is possible for someone to have high exercise capacity (e.g., to undertake exercise of moderate to vigorous intensity) but to spend little time moving during daily life (fewer steps per day). Conversely, a person can have a high step count but never undertake physical activity above light intensity. Whether these components of physical activity confer different health benefits for people with fILD is unclear. Further work is needed to understand how physical activity components change over time and with disease progression in fILD. It is key that sociodemographic factors such as age, employment type and status, access to transportation and residential environment are considered when assessing physical activity engagement. Functional influences, including the severity of dyspnea, may determine physical capability and psychological factors, such as the presence of anxiety or depression, can affect motivation and perceived barriers to exercise. Lifestyle factors, such as whether an individual is generally active or sedentary, should be factored in when considering how targeted interventions might be developed and delivered.^34,35

The associations described in this study are broadly consistent with limited earlier work,^36,37 and extend this work by elucidating the differences between physical activity components. Two studies have demonstrated moderate correlations between step count and 6MWD in people with fibrotic idiopathic interstitial pneumonia (r = 0.45, p = 0.001)³ and ILD (r = 0.413, p = 0.01),³⁸ however, potential inaccuracies of the monitors used in these studies for this outcome has since been demonstrated, highlighting the importance of monitor selection appropriate to the outcome of interest. Moreover, the correlations reported in that study were stronger than those observed in the present study.^39,40 Over time, it has become clear that physical activity monitors have different levels of accuracy in detecting different components of physical activity subject to variables such as monitor position and gait speed.⁴¹ Previous studies have not made a direct comparison of physical activity components and their relationship to important markers of disease severity and impact. Identification of the specific physical activity component(s) of interest ensures appropriate monitor selection for accurate assessment.

The strength of this study is the use of two activity monitors validated for assessment of different components of physical activity. However, one study limitation is that only people with fILD and exertional desaturation during a 6-min walk test performed on room air were eligible for study inclusion, and therefore results may not be representative of the larger population of people with fILD. Additionally, as is common practice, the thresholds used to classify physical activity intensity were based on an older healthy population²⁴ and may not be the same for people with fILD.

Conclusion

In people with fILD who experience desaturation on exertion, modest but statistically significant associations were observed between physical activity components (volume, intensity and duration) and different measures of disease severity and impact. Future research should examine whether these components of physical activity confer different health benefits for people with fILD both with and without exertional desaturation. The impact of interventions aimed at modifying specific physical activity components on fILD clinical outcomes should be further investigated. When selecting a monitor for objective measurement of physical activity in fILD, consideration should be given to whether it measures the physical activity component of interest.

Footnotes

ORCID iD

Mariana Hoffman

Author contributions

MH contributed to conceptualization, data curation, formal analysis and writing the original draft; CM contributed to data curation, formal analysis and reviewing and editing final draft; ATB contributed to conceptualization, formal analysis and reviewing and editing final draft; AEH contributed to conceptualization, supervision, resources, project administration, reviewing and editing final draft.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study is part of a multicentre randomized controlled trial supported by the National Health and Medical Research Council, Melbourne, Australia [Grant number 1139953].

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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