Chronic obstructive pulmonary disease (COPD) leads to a variety of pulmonary and non-pulmonary manifestations, including decreased functional status, which is the main reason why patients seek medical help.1-4 Impaired functional status further contributes to health status decline and is a predictor of exacerbations, hospitalizations, and mortality.3,5,6 Nevertheless, functional status impairment and its relationship with disease severity are still not fully understood.7-9 Thus, research on functional status and its assessment and treatment in people with COPD is a recognized priority.1,3,6,7,10

Functional status refers to people's ability to satisfy life's necessities, i.e., the activities they do in the normal course of their lives to meet basic needs, fulfill usual roles, and maintain their health and well-being.11 It comprises four dimensions: capacity, performance, reserve, and capacity utilization.11 Among these, functional capacity (FC), i.e., individual's maximum potential to perform activities people do in the normal course of their lives,11 has been recognized as a key outcome to be assessed in people with COPD.12 Although several measures have been proposed to assess the FC of people with COPD, most studies have used the 6-minute walk test (6MWT),3,6,13 which requires space, time, and trained staff.13 To overcome these barriers, simpler and quicker tests, such as the sit-to-stand tests, which need limited space and equipment, and are therefore feasible across different settings, have emerged.1,14,15 Among the sit-to-stand tests, the 5-repetitions sit-to-stand (5-STS) and the 1-minute sit-to-stand (1-minSTS) versions have been the most used in people with COPD due to their relationship with the 6MWT and ability to predict exacerbations and mortality.15-23 To date, the prevalence of FC impairment (assessed by the 5-STS and 1-minSTS), its distribution across disease severity, and the characteristics of people presenting such impairment remain unknown. We believe this knowledge would improve our understanding of a highly meaningful health domain for the daily life of people with COPD and could guide tailored interventions.

This study aimed to explore the FC of people with COPD using sit-to-stand tests. Specifically, we compared the FC of people with COPD with matched healthy individuals; explored the prevalence of FC impairment in people with COPD and its distribution across disease stages and groups; the characteristics of those with FC impairment; the correlation between clinical and patient-reported outcomes and measures of FC; and the predictive ability of clinical and patient-reported outcomes to identify FC impairment in people with COPD.

Four-hundred and eleven people with COPD and 405 healthy controls were referred to participate in the study. From these, 302 people with COPD [79.1% male; mean (SD) age: 67.5 (10.4) years] and 304 healthy controls [75.3% male; 66.2 (9.4) years] were included in the analysis (Fig. 1).

Fig. 1.

Participants’ flow diagram. COPD, chronic obstructive pulmonary disease; STS, sit-to-stand.

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People with COPD and healthy controls were similar in terms of age and sex as they were matched for these variables, but also in BMI [mean difference (95% CI): −0.4 (−1.1, 0.4)]. People with COPD presented significantly higher comorbidity index, lower physical activity levels, higher symptoms of anxiety and depression, and worse quadriceps muscle strength than healthy people. FC impairment was present in 23% and 33% of people with COPD according to the 5-STS (i.e., 5-STS>ULN) and the 1-minSTS (i.e., 1-minSTS Table 1. Characteristics of people with COPD performing above/below the LN for each sit-to-stand test are presented in Supplementary online material (tables S1 and S2).

People with COPD performed worse than healthy people in the 5-STS and 1-minSTS, independently of their GOLD grade [GOLD 1 & 2 vs healthy: mean difference (95% CI) 5-STS 1.2 (0.4, 2.1), 1-minSTS −6.5 (−8.9, −4.1); GOLD 3 & 4 vs healthy: 5-STS 1.8 (0.9, 2.8), 1-minSTS −9.4 (−12, −6.8)] (Fig. 2A) or exacerbation history [less exacerbator vs healthy: mean difference (95% CI) 5-STS 1 (0.2, 1.8), 1-minSTS −6.5 (−8.7, −4.2); more exacerbator vs healthy: 5-STS 2.4 (1.4, 3.5); 1-minSTS −10.5 (−13.4, −7.5)] (Fig. 2B). A worse performance of more exacerbator people with COPD (i.e., GOLD E) in comparison to less exacerbator (i.e., GOLD A & B) was found in both tests [mean difference (95% CI) 5-STS: −1.4 (−2.5, −0.3); 1-minSTS (4.0 (0.9, 7.1)].

Fig. 2.

Boxplots of the median values with interquartile range in healthy controls vs people with chronic obstructive pulmonary disease (COPD) on the 5-repetitions sit-to-stand test (5-STS, left side) and 1-minute sit-to-stand test (1-minSTS, right side). A) Comparisons between healthy controls and people with COPD across Global Initiative for Chronic Obstructive Lung Disease (GOLD) spirometric grades 1 & 2 and 3 & 4. B) Comparisons between healthy controls and people with COPD according to exacerbation history. Less exacerbator – GOLD groups A & B, More exacerbator – GOLD group E. C) Comparisons between healthy controls and people with COPD across GOLD groups (i.e., A, B, E). GOLD group A – Less symptomatic, Less exacerbator; GOLD group B – More symptomatic, Less exacerbator; GOLD group E – More exacerbator, independent of symptomatic level. The line in the middle of the box represents the median, the box extends from the 25th percentile to the 75th percentile, and the whiskers are drawn down to the 10th percentile and up to the 90th percentile. *p < 0.05; **p < 0.001.

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Regarding GOLD groups, only more symptomatic people with COPD (i.e., GOLD B) and exacerbators (i.e., GOLD E) presented a lower performance in the 5-STS [GOLD B vs. healthy: mean difference (95% CI) 1.8 (0.7, 3.0); GOLD E vs healthy: 2.4 (1.3, 3.6)] (Fig. 2C). People with COPD performed worse than healthy people in the 1-minSTS, independently of their GOLD group [GOLD A vs healthy: mean difference (95% CI) −3.4 (−6.4, −0.4); GOLD B vs healthy: −10.3 (−13.5, −7.1); GOLD E vs healthy: −10.5 (−13.7, −7.3)]. Additionally, more symptomatic people with COPD (i.e., GOLD B) and exacerbators (i.e., GOLD E) performed worse than less symptomatic and exacerbator people (i.e., GOLD A) in the 1-minSTS [GOLD A vs GOLD B: mean difference (95% CI) 6.9 (3.1, 10.7); GOLD A vs GOLD E: 7.1 (3.3, 10.9)] (Fig. 2C).

When combining the performance in the 5-STS with the performance in the 1-minSTS, most people with COPD (64%) were good performers in both tests, 20% were poor performers in both tests, 13% had a low performance only in the 1-minSTS, and 2% had a low performance only in the 5-STS (Fig. 3).

Fig. 3.

Distribution of people with chronic obstructive pulmonary disease according to their performance in the 5-repetitions sit-to-stand (5-STS) and 1-minute sit-to-stand (1-minSTS) tests. The vertical line represents the upper limit of normal of 11.15 s for the 5-STS. The horizontal line represents the lower limit of normal of 24 repetitions in the 1-min STS. Green points represent the good performers in both tests, red points represent the poor performers in both tests and yellow points represent the poor performers in only one of the tests.

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Poor performers on both tests were older, had higher exacerbation history, comorbidity index, activity-related dyspnea, and impact of the disease, and decreased physical activity levels, quality of life, and muscle strength compared to good performers in both tests (Table 2).

Very weak to weak correlations were found between the results in the 5-STS or 1-minSTS and the different patient-reported and clinical measures in people with COPD (Supplementary online material, Table S3). Only the mMRC (rs=–0.42), SGRQ activities score (rs=–0.45), and SGRQ total score (rs=–0.42) presented significant negative moderate correlations with the 1-minSTS.

In people with COPD, only poor to acceptable discrimination was found for the different patient-reported and clinical outcomes in predicting performance (i.e., ≤ or >11.15 s) in the 5-STS (Supplementary online material, Table S4). For the 1-minSTS, poor to excellent discrimination was found among the different patient-reported and clinical outcomes, the SGRQ activities score was the best discriminator of performance (i.e., < or ≥24 repetitions) in the 1-minSTS (Supplementary online material, Table S4).

This study found that: i) 23–33% of people with COPD exhibit impaired FC; ii) people with COPD have decreased FC in comparison to healthy controls, independently of their severity of lung function obstruction, exacerbation history, or symptomatic level; iii) in the 1-minSTS, a higher exacerbation history or symptomatic level results in further FC decline; iv) people with FC impairment are older, have a higher exacerbation history, comorbidities, activity-related dyspnea and impact of the disease, and lower physical activity levels, muscle strength, and quality of life; and v) other patient-reported and clinical outcomes are mostly poorly correlated to measures of FC and show limited discriminative ability for these measures.

Similarly to previous findings on physical activity, quadriceps muscle strength, and limb muscle function,42-45 a decline in FC is present since the early stages of COPD. This decline is independent of the level of lung function obstruction, reinforcing that lung function parameters cannot be used to predict FC.46 It also suggests that functional impairment is an early extra-pulmonary feature of COPD and, therefore, a comprehensive assessment and management of this treatable trait (i.e., functional impairment) is needed.

Prevalences of functional impairment ranging from 22.3 to 64.4%,42,43,47-49 depending on the outcome measure, have been previously reported in people with COPD. In this study, a prevalence of FC impairment of 23–33% was found, which falls within that range. Several factors might contribute to this wide range in prevalences. First, studies include different populations of people with COPD, with different symptomatic levels and exacerbation history, parameters known to impact FC.50-52 Moreover, the use of different reference values and/or equations and different methodologies to establish the limit of normality influences the determination of the presence/absence of impairment.53-56 Lastly, the use of different outcome measures to assess FC also results in different classifications regarding the presence/absence of impairment.47 This further highlights the need for an accurate identification of the presence/absence of this trait to guide treatment strategies.

Interestingly, a higher prevalence of functional impairment was found using the 1-minSTS in comparison to the 5-STS, and the 1-minSTS was the only test able to discriminate between people with COPD with different exacerbation history and symptomatic levels. Moreover, only 2% of people with COPD presented impairment in the 5-STS without also showing impairment in the 1-minSTS. Thus, it seems that the 1-minSTS is a more suitable test to accurately discriminate amongst people with stable COPD. In fact, the literature has shown that the 1-minSTS is a more stressful test to the cardiorespiratory system (inducing a similar stress to the cardiopulmonary exercise test) and demands a higher physical effort than the 5-STS.1,57 The 5-STS is part of the screening measures to identify sarcopenia, a good measure to identify frailty, and a predictive factor for hospitalization due to exacerbation.19,58-62 It is, therefore, likely that the 5-STS is more suitable for more frail contexts, such as during exacerbations and hospitalizations, where a higher symptomatic burden and impact of the disease is experienced.52 Furthermore, although both tests are measures of FC, the 5-STS has been more associated with muscle strength and power, coordination, and posture control; while the longer duration of the 1-minSTS has been more related with muscle endurance and higher hemodynamic and symptomatic demands, contributing to the use of anaerobic processes and muscle fatigability.1,15,63,64 Considering the specificities of training, it is therefore likely that individuals with impairments in the 5-STS may benefit from resistance training, individuals with an impaired 1-minSTS may benefit from endurance training, and individuals with impairments in both tests will need a program combining resistance and endurance training.65,66

People with functional impairment were found to be older, have a higher exacerbation history, comorbidities, symptoms, and impact of the disease, and lower physical activity levels, muscle strength, and quality of life. Similar characteristics in people with functional impairment have been previously reported.7,67,68 Because most of these characteristics are preventable and modifiable, attention to interventions in this population should be given. In fact, different stakeholders have stated that people with functional impairment should be prioritized to pulmonary rehabilitation,69 and these programs have shown to be effective in improving the FC of people with COPD.47,70,71 Nevertheless, there is a large number of non-responders on FC and the effects of pulmonary rehabilitation are not sustained after the intervention.47,70 Thus, future studies should investigate how to personalize pulmonary rehabilitation to maximize and sustain its effects on FC.

Limited discriminative ability and overall weak correlations were found between the different patient-reported and clinical outcomes and the sit-to-stand tests, which is consistent with previous findings.37,72,73 This emphasizes the complexity of COPD and how different measures reflect different constructs, reinforcing the need to perform comprehensive functional assessments.

This study has some strengths and limitations that need to be acknowledged. We included a relatively large sample, which is representative of our population considering a 95% confidence interval and a 5% margin of error. Moreover, we collected data across different settings and geographic regions, reinforcing the representativeness of real-world data. Functional impairment in people with COPD was defined based on the results of a matched healthy Portuguese population instead of other population-based reference values or predictive equations, which also strengthens our results. Nevertheless, our sample has an unbalanced proportion of male/female and is mostly composed of people with moderate to severe COPD. Although these characteristics are commonly reported in this population,38,39 we acknowledge that it might limit the generalization of our findings to all people with COPD. Additionally, being healthy was self-reported by participants according to how they felt. Although there was no formal medical appointment, this is consistent with the World Health Organization's definition of health,26 and the satisfactory results (within normality) obtained by the healthy controls attest their health status. Lastly, functional status is a comprehensive outcome that can be assessed with several measures,3,6 thus the use of only sit-to-stand tests, which focus on the assessment of FC, might have limited the identification of impairment and characterization of the population. Future studies should try to integrate a combination of measures of FC (e.g., 6MWT, sit-to-stand, stair climbing) with measures of functional performance (e.g., accelerometer, Clinical COPD questionnaire functional status domain, Canadian occupational performance measure), or use combined measures of capacity/performance (e.g., Glittre activities of daily living, Londrina activities of daily living) if time and space constraints allow it.3

People with COPD exhibit decreased FC, independently of the severity of lung function obstruction, exacerbation history, or symptomatic level. This decline in FC might be better reflected by the 1-minSTS than the 5-STS (as a slightly larger proportion of individuals presented impairment in the 1-minSTS than in the 5-STS) and affects 23–33% of people with stable COPD. Because impaired FC is a treatable trait not accurately reflected by other patient-reported and clinical outcomes, a comprehensive assessment and management of this early extra-pulmonary feature of COPD is needed.