Physical Activity Patterns in Patients in Different Stages of Chronic Obstructive Pulmonary Disease

Abstract

It has previously been suggested that exercise capacity is decreased in COPD and that it is associated with degree of disease. The reduced exercise capacity may plausibly be due to low levels of physical activity in this patient group. The aim of the present study was to assess exercise capacity and physical activity in different stages of COPD and to examine the associations between exercise capacity, pulmonary function and degree of physical activity. A total of 44 COPD patients and 17 healthy subjects participated in the study. Exercise capacity was assessed using the 6-minute walking test and physical activity was assessed using an accelerometer worn all waking hours during 7 days. Mean exercise capacity was significantly lower in COPD patients compared with healthy subjects. Mean physical activity level and time spent at least moderately active were significantly lower in patients with moderate and severe COPD compared with healthy subjects while no differences in time spent sedentary were observed between the study groups. Pulmonary function, mean physical activity level and time spent at least moderately physically active were significantly associated with exercise capacity in the patients. We conclude that patients with moderate and severe COPD are less physically active compared with healthy subjects. Furthermore, mean physical activity level and physical activity of at least moderate intensity are positively associated with exercise capacity in COPD, while time spent sedentary is not, which stresses an important role of physical activity on exercise capacity in these patients.

Keywords: :

• COPD
• physical activity
• accelerometry
• exercise capacity

INTRODUCTION

Chronic obstructive pulmonary disease (COPD) is a complex syndrome characterized by irreversible progressive airflow limitation. In addition, a significant number of patients with COPD also suffer from extrapulmonary abnormities, so-called systemic effects, such as systemic inflammation, nutritional abnormities, weight loss, cardiovascular comorbidities and skeletal muscle dysfunction (1).

Reduced exercise capacity has been reported in many COPD patients and has been associated with increased mortality rates and decreased quality of life in this group of patients (2–3). Several explanations to the decreased exercise capacity have been suggested. Airflow limitations lead to increased work of breathing, dynamic hyperinflation and hypoxia, which may have negative effects on exercise capacity (4,5). Furthermore, changes in skeletal muscle towards a more anaerobic muscle profile have also been reported to be associated with decreased exercise capacity (1, 3, 6–11).

Table 1  Test subject characteristics

	Healthy subjects (n = 17)	Mild COPD (n = 11)	Moderate COPD (n = 21)	Severe COPD (n = 12)	p-value
Age (years)	58.4 ± 4.8	63.5 ± 5.8	63.9 ± 7.6	62.7 ± 7.6	ns
Height (cm)	172.4 ± 7.7	167.6 ± 3.2	168.8 ± 8.3	170.0 ± 9.5	ns
Weight (kg)	75.3 ± 9.5	73.1 ± 16.4	80.4 ± 21.7	76.7 ± 17.1	ns
BMI (kg/cm^2)	25.2 ± 1.8	25.9 ± 5.3	28.0 ± 6.3	26.3 ± 4.2	ns
VC (l)	3.9 ± 0.8	3.5 ± 0.5	3.0 ± 0.8*	2.7 ± 0.8*	<0.001
FEV1,0 (l/s)	3.0 ± 0.6	2.3 ± 0.4*	1.7 ± 0.3*	1.2 ± 0.3*	<0.001

All values are expressed as mean ± sd.

*Significantly lower compared with healthy subjects.

Abbreviations: COPD, chronic obstructive pulmonary disease; ns, not significant; BMI, body mass index; VC, vital capacity; FEV_1,0, forced expiratory volume in 1 sec.

However, reduced exercise capacity in COPD patients may also be due to a low level of physical activity. In a study by Pitta et al. (12), it was reported that a large proportion of COPD patients spend less time walking and standing but more time sitting and lying compared to healthy controls, and that exercise capacity is correlated to physical activity. Low levels of physical activity in COPD have further been shown by Watz et al. (13), who conclude that the number of steps per day and the physical activity level are reduced in COPD patients in “global initiative for chronic obstructive disease (GOLD)” stages II–IV and by Trooster et al. (14), who show a gradual reduction in physical activity with increasing disease severity. Apart from being associated with exercise capacity, the level of physical activity has recently been suggested to be the strongest predictor of all-cause mortality in patients with COPD (15).

Furthermore, low levels of physical activity have been associated with co-morbidities such as systemic inflammation and left cardiac dysfunction in this patient group (16). To promote and maintain health in healthy persons aged 18–65 years, regular physical activity of at least moderate intensity for a minimum of 30 min a day at least 5 days a week has been recommended (17,18). These recommendations have also been shown to be applicable in older adults (>65 years) and in adults aged 50–64 years with clinically significant chronic conditions (19).

Motion sensors such as accelerometers have been shown to provide objective measurements of duration, intensity and frequency of physical activity in daily life in COPD patients (20). The outcome from accelerometer registrations can be used to provide cut-off levels that separate movement behaviour into level of exertion and allows physical activity to be categorized in term of sedentary, light, moderate and vigorous activity. Several previous studies have used various types of accelerometers to study physical activity in COPD patients (13, 21–23), but to our knowledge time spent at least moderately active has not previously been estimated using accelerometer cut-off levels where intensity levels corresponding to 2020–4944 counts/min are considered moderate physical activity (24).

The hypothesis put forth in present study was the existence of an association between exercise capacity, pulmonary function and physical activity in COPD patients. Therefore, the aim of the study was to assess exercise capacity, body composition and physical activity in daily life in patients in different stages of COPD and in a healthy control group and to evaluate the associations between exercise capacity, pulmonary function and physical activity. Furthermore, we aimed to use physical activity data to estimate how much time patients in different stages of disease and healthy controls spend at least moderately physically active and compare it with current health recommendations.

MATERIALS AND METHODS

Subjects

Fifty COPD patients (33 women and 17 men) recruited from primary health care centres in central Sweden were initially included in the study. Patients were selected in a stable condition and were not suffering from respiratory tract infections or exacerbation of their disease at least 4 weeks prior to the study. Exclusion criteria were malignancy, cardiac failure, distal arteriopathy and severe endocrine-, hepatic- or renal disorder. Based on the severity of airflow obstruction, the patients were divided into 3 subgroups: mild COPD (Forced expiratory volume in 1 second (FEV_1,0) ≥ 80% of expected), moderate COPD (80% > FEV_1,0 ≥ 50% of predicted) and severe COPD (50% > FEV_1,0 ≥ 30% of predicted), according to GOLD criteria from the American Thoracic Society (6). Nineteen age-matched, non-smoking, healthy subjects (13 women and 6 men) were recruited as a control group.

Six patients and two healthy subjects were excluded from the statistical analysis due to incomplete accelerometer registrations (see later). Therefore, statistical analysis included 44 COPD patients (28 women and 16 men) and 17 healthy subjects (11 women and 6 men). Of the included patients 11 (9 women and 2 men) were considered having stage I, mild, COPD, 21 (13 women and 8 men) having stage II, moderate, COPD and 12 (6 women and 6 men) having stage III, severe, COPD. Test subject characteristics are presented in Table 1. The study was approved by the regional ethical review board in Uppsala, Sweden (dnr 2004:M-355).

Table 2  Exercise capacity expressed as distance walked in 6 min, fat free mass and physical activity variables in the different study groups

	Healthy subjects	Mild COPD	Moderate COPD	Severe COPD
	(n = 17)	(n = 11)	(n = 21)	(n = 12)	p-value
Distance walked in 6 min (m)	616 ± 60	485 ± 75*	439 ± 66*	404 ± 96*	<0.001
Fat free mass (%)	36 ± 6	37 ± 6	37 ± 7	37 ± 6	ns
MVPA (min)† (range)	24.6 (10.1–105.4)	13.2 (0.9–75.3)	7.5 * (0.1–73.7)	7.0 * (0.3–82.6)	0.002
Mean physical activity (counts/min)	365 ± 134	294 ± 136	229 ± 123*	215 ± 144*	0.01
Time spent sedentary (min)	523 ± 67	433 ± 73	478 ± 93	483 ± 89	Ns

Values are expressed as mean ± sd if not otherwise indicated.

†Values are expressed as median and range, ANOVA performed on natural logarithm (ln) MVPA.

*Significantly lower compared with healthy subjects.

Abbreviations: COPD, chronic obstructive pulmonary disease; ns, not significant; MVPA, time spent moderately or vigorously active.

Pulmonary function tests

All patients underwent a post-bronchodilatory spirometry using a SPIRARE® spirometer with a SN 15020 sensor (Diagnostica, Oslo, Norge).Vital capacity (VC) and FEV_1,0 were assessed according to guidelines of the American thoracic Society and the European Respiratory Society (25) with the highest value from at least three technically acceptable assessments being used. To determine VC and FEV_1,0 in the healthy subjects spirometry without administration of bronchodilators was performed according to current guidelines (25) and the highest value from at least three acceptable assessments was used (Table 1).

Exercise capacity test

To assess exercise capacity the 6-min walking test was used. The test was conducted according to current guidelines (26) with the exception that a 20-meter corridor was used instead of, as recommended, a 30-meter corridor.

Body composition

Body weight was measured on a calibrated scale (Seca, model 713) to the nearest 0.5 kg with the test subjects wearing light clothes and without shoes. Height was measured with a stadiometer (Hyssna mätutrustning AB, Sweden) to the nearest 0.5 cm with the test subjects standing barefoot. Body mass index (BMI) was calculated as weight (kg)/height (m)² (Table 1). Skin fold thickness was measured with a Harpenden® caliper at 4 different sites of the body: the triceps, the biceps, the subscapula and the suprailiaca. Body fat content was estimated from the sum of the skin folds using the body density equation by Durnin and Womersley (27) and the Siri equation for body fat (28) and fat free mass was calculated as (total body mass)-(body fat content).

Physical activity assessment

To assess physical activity the uniaxial accelerometer ActiGraph, model GT1 M (Manufactoring Technologyy IC, Fort Walton Beach, Fl, USA) was used. This accelerometer records vertical accelerations as “counts” and provides both duration and intensity of physical activity presented as counts/time unit (epoch).

The test subjects wore the accelerometer on an elastic belt around the waist all waking hours except during water activities, during 7 days, and data were sampled in 60 s epochs. Data were reduced using the ActiGraph analysis software MAHUffe (available from http://www.mrc-epid.cam.ac.uk/ Research/PA/Downloads.html). Continuous periods of zero values exceeding 20 min were regarded as “accelerometer not worn” and were not included in the calculation of total registered time. Five hundred min of registration per day was required for a day to be considered as valid and at least 3 valid days of which at least one was a Saturday or Sunday were required for the registration to represent levels of habitual physical activity and be included in the statistical analysis (29).

To estimate the mean physical activity level of the test subjects the mean number of counts/minute during total registered time was used. Based on previously reported validation studies activity count cut-off points applied to assign accelerometer outcomes to physical activity categories were as follows:

Time spent sedentary was defined as where counts/min were <100 (30), light activity was defined as activity resulting in 100–2019 counts/min (31), moderate activity was defined as activity resulting in 2020–4944 counts/min (24) and vigorous activity was defined as activity resulting in more than 4944 counts/min (32).

Table 3  Standardized regression coefficients (β), explanatory power (R² adjusted) and p-value when examining the associations between exercise capacity and of percent of predicted FEV_1,0, MVPA, mean physical activity level and time spent sedentary in the patients.

Association between exercise capacity and	β	R^2 adjusted	p-value
FEV1,0	0.363	0.110	0.02*
MVPA	0.335	0.090	0.03*
MVPA after controlling for FEV1,0 and gender	0.381	0.298	0.01*
Mean physical activity	0.336	0.091	0.03*
Mean physical activity after controlling for FEV1,0 and gender	0.333	0.261	0.02*
Time spent sedentary	-0.094	-0.015	0.55
Time spent sedentary after controlling for FEV1,0 and gender	-0.155	0.176	0.32

*Significant association (p ≤ 0.05).

Abbreviations: FEV_1,0, forced expiratory volume in one second; MVPA, time spent moderately or vigorously active.

Statistics

The distribution of all continuous values was checked using the Shapiro-Wilk test for normality. All values except time spent moderately or vigorously physically active (MVPA) showed a satisfactory pattern. After natural logarithm (ln) transformation time spent MVPA also fitted within the normal distribution and parametrical tests were applied for all analysis. The physical activity data that was statistically analysed was the mean physical activity level, time spent MVPA and time spent sedentary.

Mean ± standard deviation (SD) was calculated to describe the characteristics of the subjects, exercise capacity, body composition and physical activity data. For comparison between the study groups analysis of variance (ANOVA) was used. When significant differences between the groups were observed Tukey's post-hoc test was applied.

Linear regression analyses was used to determine the associations between exercise capacity expressed as distance walked in 6 min, degree of airflow obstruction expressed as percent of predicted FEV_1,0, gender and physical activity data. All data were analyzed using PASW Statistics version 18.0 (former SPSS) and p ≤ 0.05 was considered to be significant.

RESULTS

Exercise capacity expressed as distance walked in 6 min was found to differ significantly between the study groups (p < 0.001) with all patient groups walking a significantly shorter distance in 6 min compared with the healthy subjects (Table 2). An interaction effect of gender on exercise capacity was observed in the whole study group; therefore, gender was included as an independent factor in further statistical analyses where exercise capacity was included.

No significant differences in fat free mass were found between the study groups (Table 2) and this variable was therefore not included in further statistical analysis. When studying physical activity the mean time registered using the accelerometer was 749 ± 104 min/day and the number of valid days was on average 6 days.

Time spent moderately or vigorously physically active (MVPA) was found to differ significantly between the study groups (p = 0.002) with patients having moderate and severe COPD spending on average less time MVPA compared with the healthy subjects (Table 2). The recommendation of 30 min of at least moderate physical activity/day was reached by 47% of the healthy subjects, 27% of the subjects with mild COPD, 10% of the subjects with moderate COPD and 17% of the subjects with severe COPD. No interaction effects of gender on time spent MVPA were observed.

The mean physical activity level was found to differ significantly between the study groups (p = 0.01). Patient with moderate and severe COPD had on average lower mean physical activity level than the healthy subjects (Table 2) and no interaction effects of gender were observed. Time spent sedentary did not differ significantly between the study groups (Table 2) and gender was not found to have any interaction effect.

When studying only the patients (n = 44) a significant association between exercise capacity and percent of predicted FEV_1,0 was observed (Table 3), yet when studying only the healthy subjects (n = 17), no association between exercise capacity and percent of predicted FEV_1,0 was shown. The significant association between exercise capacity and FEV _1,0 in the patients was found to remain also after controlling for gender (Table 3).

In the patients significant associations between exercise capacity and time spent MVPA as well as between exercise capacity and mean physical activity level were observed (Table 3). In the healthy subjects a significant association between exercise capacity and mean physical activity level but not between exercise capacity and time spent MVPA was shown. No associations between exercise capacity and time spent sedentary were observed in the patients (Table 3) or in the healthy subjects.

The significant association between exercise capacity and mean physical activity in the healthy subjects was found to remain also after controlling for FEV _1,0 and gender. Likewise, the significant associations between exercise capacity and time spent MVPA as well as between exercise capacity and mean physical activity level in the patients were found to remain after controlling for percent of predicted FEV_1,0 and gender (Table 3).

DISCUSSION

In the present study we show that patients with moderate and severe COPD are significantly less physically active compared with healthy subjects, yet patients with mild COPD do not differ significantly from healthy subjects regarding physical activity patterns. The patients in the later stages of disease have a lower mean physical activity level and spend less time moderately to vigorously physically active compared with the healthy subjects. A significant association between exercise capacity expressed as distance walked in 6 min and airway obstruction expressed as percent of predicted FEV_1,0 is also shown. This is in line with previous studies where it has been suggested that exercise capacity is decreased in COPD patients and that it is associated with degree of disease (2, 9, 33).

Furthermore, the association between physical activity and exercise capacity that has previously been shown (12, 34) is confirmed in the present study using an objective technique for assessment of physical activity. It has previously been suggested that disuse as a consequence of inactivity may be an important factor involved in skeletal muscle alterations such as muscle wasting and changes in muscle morphology in COPD patients (1, 35) and that changes in skeletal muscle may contribute to decreased exercise capacity in this patient group (9).

Interestingly, no significant differences in time spent sedentary were observed between the study groups and exercise capacity was not associated with time spent sedentary in the present study. A plausible explanation to these results is that the accelerometer was only worn all waking time why total registered time does not cover full 24 h and that the patients may have worn the accelerometer less time/day compared with the healthy subjects, which could result in less registered time spent sedentary. However, the accelerometer was used in accordance with current guidelines and for a day to be considered valid, at least 500 min of registration was required (29).

Our findings of a positive association between exercise capacity and time spent MVPA and between exercise capacity and mean physical activity level indicate that the level of physical activity plays an important role for the lower exercise capacity in COPD and may contribute to changes in skeletal muscle of COPD patients. However, as fat free mass did not differ between the patients and the healthy subjects in the present study no indications of muscle wasting are present in the studied population. Yet, changes in muscle morphology may be present without the presence of muscle wasting and as it has previously been shown that physical activity is associated with muscle morphology (36), low levels of physical activity may contribute to the previously reported morphological changes in skeletal muscle of COPD patients. Thus, further studies are needed to confirm these speculations.

International health recommendations state that 30 min of at least moderate physical activity/day is preferable for promoting and maintaining health (19, 20). In clinical practice physical activity in COPD patients is often evaluated using not fully objective measurements such as questionnaires and observation. From these evaluations it may be assumed that COPD patients have a low physical activity level and do not reach the stated health recommendations that may contribute to the decreased exercise capacity in this patients group. In the present study we confirm this assumption using objective measurements.

Here, we show that time spent MVPA as well as mean physical activity level are positively associated with exercise capacity in COPD. Thus, reaching the health recommendations is of importance also for this group of patients. Previously Watz et al. (13) and Trooster et al. (14) have shown that patients in the later stages of COPD spend less time moderately physically active compared with control subjects and may not reach the stated health recommendations. Our study further strengthen these results as we show that a low number of patients with moderate and severe COPD spend at minimum 30 min, at least moderately physically active/day.

In the present study the median value for time spent MVPA does not reach 30 min in any of the study groups. However, 47% of the healthy subjects reach recommended levels of physical activity and we show that patients with moderate and severe COPD spend significantly less time MVPA compared with healthy subjects. An explanation to the low physical activity levels seen in all study groups may be that the study was carried out during the winter season. It has previously been discussed that seasonal variations may affect physical activity patterns (37).

It can be speculated that higher levels of physical activity could have been registered during the summer season. However, as data sampling for all subjects was carried out during the same season, seasonal variation do not explain the differences shown between the different study groups. Furthermore, the subjects in the different study groups reaching the recommended level of at least 30 min of at least moderate physical activity per day did not differ in other matters than physical activity level compared with the other subjects in the same study group, which otherwise could have influenced the results.

As decreased exercise capacity is associated with increased mortality and decreased quality of life in COPD patients (2,3), it is of great importance to maintain an adequate exercise capacity in this group of patients. Our results highlight the need of focusing on physical activity and preferably activity of at least moderate intensity on regular basis in rehabilitation of COPD patients. Furthermore, low levels of physical activity may contribute to the changes in skeletal muscle seen in the later stages of disease why it is of importance to promote increased physical activity in treatment already in the early stages of disease to inhibit or delay these changes. However, to confirm these speculations an interventional study is necessary.

In conclusion the present study shows that patients with moderate and severe COPD are significantly less physically active compared with healthy subjects. We also show that physical activity of at least moderate intensity (MVPA) as well as mean physical activity level are positively associated with exercise capacity in COPD patients, while time spent sedentary is not associated with exercise capacity in this patient group. These findings stress an important role of physical activity on exercise capacity in COPD patients.

DECLARATION OF INTEREST

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.