Multifaceted associations between walking performance, physical fitness, extremity function, health status, and depression in individuals with COPD

Abstract

Background/Objective(s)

Chronic obstructive pulmonary disease (COPD) can precipitate a deterioration of an individual’s physical performance and overall health. Evidence suggests that, along with pulmonary functions, several other factors are related to the significant impairment of walking performance in individuals with COPD. This study compared the depressive symptoms, health status, upper and lower extremity functions, and peak oxygen uptake (VO_2peak) in a group of individuals with COPD based on walking performance using a cutoff distance of 350 m in the six-minute walking test (6MWT). The study also investigated the associations between these factors and walking performance.

Materials and Methods

Participants performed the 6MWT according to the guidelines and were classified into high (>350 m; n = 40) or low (<350 m; n = 30) walking performance groups according to distance. The forced expiratory volume (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio were recorded. Participants completed the Patient Health Questionnaire-9 (PHQ-9), St. George’s Respiratory Questionnaire (SGRQ), and the Upper and Lower Extremity Functional Index (UEFI/LEFI). Predicted VO_2peak was measured using the Duke Activity Status Index (DASI).

Results

Seventy participants with a mean age of 63 ± 11 years (20% female) were enrolled in this study. Patients with high walking performance demonstrated significantly better health status than those with low walking performance (SGRQ: 49 ± 25 vs. 56 ± 21, p = 0.03). Participants with low walking performance had lower predicted VO_2peak compared to their higher performing counterparts (p = 0.002). The overall model was significant (F(8, 61) = 7.48, p = 0.0006), with PHQ-9, SGRQ, UEFI/LEFI, VO_2peak, and FEV1/FVC explaining approximately 49.5% of the variance in the 6MWT distance.

Conclusion

This study shed light on the association of depressive symptoms, health status, extremity function, and VO_2peak with walking performance, providing valuable insights that may impact the management and care of individuals with COPD.

KEY MESSAGES

• COPD is a global health issue that significantly impairs physical performance, particularly walking.

• Depressive symptoms, health status, extremity function, and predicted peak oxygen uptake can predict walking performance in patients with COPD, offering insight into potential interventions.

Keywords:

• Extremity functions
• walking, depression
• health outcomes
• COPD

Introduction

Chronic obstructive pulmonary disease (COPD) is a growing cause of illness and mortality, affecting approximately 7.6% of adults aged 30-79 years worldwide [1]. In high-income Eastern Mediterranean countries, including Saudi Arabia, the prevalence of COPD is approximately 2.4%, affecting individuals across all age groups [1]. The decline in pulmonary function from early in life to late adulthood is a hallmark of COPD [2]. This progressive respiratory impairment significantly contributes to global morbidity, and its trajectory is frequently paralleled by a decrease in physical performance and the onset of psychological distress [3]. The physically disabling features of COPD include self-reported functional performance and musculoskeletal muscle weakness [4]. One critical aspect of physical function that is significantly affected by COPD is walking performance [5].

The International Classification of Functioning, Disability, and Health developed by the World Health Organization (WHO) [6] provides a comprehensive framework for understanding the intricate dynamics of health and well-being, particularly in individuals with chronic conditions, such as COPD [7]. This classification system recognizes the multifaceted nature of disability and functioning, considering not only the physiological aspects, but also the roles of personal and psychological factors. Individuals with COPD often experience a decline in their walking performance, which can have a profound impact on their overall quality of life. The 6-min walking test (6MWT) is commonly used to estimate pulmonary function impairment in individuals with COPD and to assess the extent of the disease [8]. Researchers have shown that a distance of 350 m in the 6MWT is a threshold for classifying COPD patients at high risk of mortality, hospitalization, and exacerbation of the condition[9]. In the context of COPD, the complex interplay between personal and psychological factors and their impact on walking performance has garnered attention as an area that requires further investigation.

A critical aspect of the multifaceted impact of COPD on physical performance is the role of upper and lower extremity functions. These functions are essential for a range of daily activities, from basic self-care to more demanding tasks requiring strength, coordination, and endurance. Lower extremity muscle strength and functional impairments have been reported in patients with COPD [10,11] and can be attributed to poor oxygen uptake, which hampers the ability of muscles to meet metabolic demands [12]. Upper and lower extremity functions directly impact an individual’s ability to perform walking and other physical activities [13]. In COPD, these functions may also adversely affect walking performance. In addition, COPD is associated with limited mobility of the shoulder [14] due to changes in breathing patterns and reduced efficiency of the respiratory system [15,16]. Notably, upper extremity function has been found to correlate with the 6MWT in patients with moderate-to-severe COPD [17]. Therefore, it is necessary for clinicians to address these impairments in this population in order to enhance walking capacity. Other factors, such as depression and general health status, which are commonly seen in individuals with COPD [18], should also be considered as potentially interfering with walking performance [3,19]. Impaired walking ability could further contribute to an elevated risk of depression and decreased quality of life (QoL) [20]. Thus, determining the association of depression, upper and lower extremity function, and health status with walking is essential to inform future interventions aimed at improving walking performance in this population.

No studies have examined the interaction between upper and lower extremity function, depression, the pulmonary function profile, health status, and fitness on walking in individuals with COPD. Thus, research is needed to investigate the predictive effect of multifaceted factors on walking performance among individuals with COPD based on the 350 m threshold of the 6MWT. This study aimed to delve into these associations by comparing depressive symptoms, health status, upper and lower extremity functions, and fitness between groups of individuals with COPD based on walking performance. Specifically, we use a cutoff distance of 350 m in the 6MWT to categorize walking performance, aligning with recognized benchmarks that differentiate between varying levels of functional impairment in COPD. We hypothesized that, compared to individuals with COPD who walked more than 350 m, those who walked ≤350 m in the 6MWT would have more severe depressive symptoms, poor health status, difficulties in upper and lower limb functions, and lower peak oxygen uptake (VO_2peak). We also hypothesized that these factors would significantly predict walking performance in individuals with COPD. This comprehensive approach of examining multiple predictors, including depressive symptoms, health status, extremity function, and VO_2peak, in relation to walking performance in COPD patients allows for a more nuanced understanding of the factors influencing walking performance in this population, which may differ from previous research focusing solely on individual predictors. The findings in this study will contribute to the growing body of knowledge on COPD and its multifaceted impact on individuals’ daily functioning.

Patients/materials and methods

Setting and participants

This cross-sectional observational study enrolled a convenience sample of 70 patients diagnosed with COPD from King Saud University Medical City and King Fahad Medical City, Riyadh, Saudi Arabia. Any patients with a confirmed diagnosis of COPD according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria [21] who were able to walk without an assistive device were included. Patients who use supplemental oxygen were also included. Patients with obesity (body mass index [BMI] ≥ 30 kg/m²), any severe musculoskeletal and cardiovascular conditions that may impede balance and walking performance, or who had experienced a psychiatric or mental illness that may affect the subject’s understanding and safety in performing the test were excluded. Ethical approval for this study was obtained from the medical research and ethical committee of King Saud University, Riyadh, Saudi Arabia (IRB_017E; 22-0522).

Procedure

Prior to enrollment in the study, participants were informed of the purpose, procedures, and potential risks involved. All participants signed a written informed consent form. On the day of the test, all participants were given the Physical Activity Readiness Questionnaire Plus (PAR-Q+) to assess safety and eligibility for study participation. Following this, a battery of self-reported questionnaires on depression, health status, and upper and lower limb functions was completed. Anthropometric measurements, including height and weight, were collected.

Outcome measures

Pulmonary function test

The lung function test was performed using spirometry (Vitalograph Alpha 6000, Vitalograph Ltd., Buckingham, UK) consistent with American Thoracic Society (ATS) standards [22]. Forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) were recorded. All variables were expressed as a percentage of the predicted value.

Six-Minute walking test (6 MWT)

The 6MWT was used as a measure of walking performance and was performed according to ATS guidelines [23]. This test measures the total distance that participants can walk back and forth as far and as fast as possible in a 30-m-long, enclosed, flat corridor over a period of 6 min. Briefly, a threshold distance of 350 m was chosen to divide the participants into two groups: COPD patients with walking distance >350 m (participants with high walking performance) and COPD patients with walking distance <350 m (participants with low walking performance). This threshold was selected based on previous studies that validated a distance of 350 m as a threshold for increased mortality and poor prognosis in patients with COPD [9,24,25]. The distance covered, number of laps covered, heart rate, and pulse oximetry were recorded during the 6MWT.

Duke Activity Status Index

The Duke Activity Status Index (DASI) is a 12-item questionnaire that measures functional capacity by estimating the VO_2peak. Patients were asked to answer ‘yes’ or ‘no’ to questions indicating their ability to perform certain activities. Each activity was scored based on specific points and the metabolic cost of the task (MET) based on its difficulties. The VO_2peak was estimated after calculating the final DASI score (total score 0 to 58.2 points) and metabolic equivalent (total metabolic equivalent 0 to 9.89 METs), with higher scores indicating a higher functional capacity. The Arabic version of the scale has an internal consistency score (Cronbach’s alpha) of 0.87 [26]. The scale is appropriate for measuring functional capacity among Arabic-speaking patients with COPD according to its test-retest reliability, which has an intra-correlation coefficient of 0.95 [26].

St. George’s Respiratory Questionnaire

The St. George’s Respiratory Questionnaire (SGRQ) measures the impact of COPD on overall health status. The Arabic version of the scale consists of a 50-item questionnaire divided into three domains: symptom level, disease activity, and the impact of disease on daily life. Scores range from 0 to 100, with higher scores indicating greater health impairment and limitations. Cronbach’s alpha for the Arabic version of the SGRQ is 0.89 for the overall scale, indicating good internal consistency [27].

Patient Health Questionnaire-9

The Patient Health Questionnaire-9 (PHQ-9) is a nine-item, self-administered questionnaire used to monitor and measure the degree of depression severity on a four-point Likert scale (0= not at all, 1= several days, 2= more than half the days, and 3= nearly every day). Scores range from 0 to 27, with higher scores indicating more severe depression. The depression severity score is categorized as follows: total score 1– 4, indicative of minimal depression; 5–9, indicative of mild depression; 10–14, indicative of moderate depression; 15–19, indicative of moderately severe depression; 20–27, indicative of severe depression. A PHQ-9 score ≥ 10 has a sensitivity of 88% and a specificity of 88% for major depression. Cronbach’s alpha for the Arabic version of the PHQ-9 is 0.85, indicating good internal consistency [28].

Upper Extremity Functional Index

The Upper Extremity Functional Index (UEFI) is a self-rated assessment of upper extremity function developed to capture a patient’s rating of the difficulty performing activities of daily living using the upper extremities. It consists of 20 items rated on a five-point Likert scale (0 = extreme difficulty/unable to perform activity, 1 = quite a bit of difficulty, 2 = moderate difficulty, 3 = somewhat difficult, and 4 = no difficulty). The total scores range from 0 to 80, with higher scores indicating the best functional status. The Arabic version of the UEFI has been shown to be a valid and reliable outcome measure for measuring upper extremity functions, with a Cronbach’s alpha of 0.96 and a test–retest reliability of 0.92 [29].

Lower Extremity Functional Index

The Lower Extremity Functional Index (LEFI) is a 20-item self-administered questionnaire measuring lower extremity function when performing certain activities of daily living on a five-point Likert scale (0 = extreme difficulty/unable to perform activity; 4 = no difficulty). Scores range from 0 to 80, with higher scores indicating a very high function. The validity and reliability of this scale have demonstrated excellent psychometric properties that efficiently measure activity limitations in people with COPD [30].

Data analysis

All statistical analyses were performed using Stata version 16 (StataCorp LLC, College Station, TX, USA). Demographic data (age, sex, weight, height, BMI, and smoking status) are presented as means ± standard deviation (SD) for both groups. Shapiro–Wilk and Kolmogorov–Smirnov tests were used to examine the normality of the outcomes. Differences between variables in the COPD groups were examined using independent t-tests. Associations between the independent variables (FEV, SGRQ, PHQ-9, VO_2peak, UEFI, and LEFI) and 6MWT as the dependent variable were adjusted for age and sex and analyzed using a forward multiple linear regression analysis. The coefficient of determination (R²) was utilized to gauge the goodness of fit of the regression model, indicating the proportion of variance in the dependent variable explained by the independent variables. Significance was set at p ≤ 0.05.

Results

Seventy patients with COPD were included in this analysis. The baseline characteristics of patients with COPD are presented in Table 1. According to the GOLD staging, 5.71% were classified as stage I, 51.43% as stage II, 38.57% as stage III, and 4.29% as stage IV. Most patients were men (80%), with a mean (SD) age of 63 (11) years, predicted FEV1 of 55.98% (14.97%), and BMI of 29 (2.5) kg/m². The majority (38.57%) had a minimal depression score and moderate health-related quality of life [SGRQ:49.14 (24.50)]. Regarding smoking status, 20% of the patients were non-smokers and the majority (55.71%) were ex-smokers. The baseline prevalence of severe depression was 1.43%.

Table 1. Descriptive characteristics of the COPD patients (N = 70).

Parameters	Mean	SD	Range
			Min	Max
Age (years)	63	11	37	88
Weight (kg)	78.88	15.25	47.3	120
Height (cm)	164.87	7.90	150	181
Smoking status, n (%)
Non-smoker	14 (20)
Smoker	17 (24.29)
Ex-smoker	39 (55.71)
FEV1 (L)	1.573	0.5484	.52	2.82
FVC (L)	2.849	0.7314	1.64	4.8
FEV1/FVC ratio	54.73	11.32	26.8	70
FEV1 predicated (%)	55.98	14.97	26.3	102.8
Gender, n (%)
Male	45 (80)
Female	14 (20)
GOLD stage, n (%)
I	4 (5.71)
II	36 (51.43)
III	27 (38.57)
IV	3 (4.29)
PHQ-9	6.77	4.81	0	21
Minimal depression	27 (38.57)
Mild depression	24 (34.29)
Moderate depression	14 (20)
Moderately severe depression	4 (5.72)
Severe depression	1 (1.43)
SGRQ	49.14	24.50	4.57	93.02
Upper Extremity Functional Index	25	17.76	0	70
Lower Extremity Functional Index	40.54	16.50	0	72
Predicted VO2 max*	21.56	15.93	2.75	58.2

Abbreviations: GOLD: Global initiative for chronic obstructive lung disease FEV1: forced expiratory volume in 1 s (), FVC: forced vital capacity (); PHQ-9: patient health questionnaire-9 (); SGRQ:St. George’s respiratory questionnaire(.*Using Duke Activity Status Index (DASI).

Table 2 provides the results of the functional performance profile and health status based on walking performance levels. The study included two groups: 40 patients with high walking performance (426.97 ± 52.24 m) and 30 patients with low walking performance (233.46 ± 81.54 m). Of those with low walking performance, approximately 10% had very severe COPD (stage IV) and the majority (53.3%) had severe COPD (stage III). However, most patients with high walking performance had moderate COPD (stage II), and none of the patients had very severe COPD.

Table 2. Comparison of the upper and lower extremity-related functional performance and health status among COPD patients (N = 70).

	Walking	Performance
Characteristics	Low n = 30	High n = 40	p-value^a
6 MWT distance (m)	233.46 ± 81.54	426.97 ± 52.24	<0.001
Predicted VO2peak (mL/kg/min)^b	14.99 ± 12.03	26.50 ± 16.82	0.002
Upper Extremity Functional Index	30.9 ± 19.78	20.57 ± 14.84	0.01
Lower Extremity Functional Index	47.46 ± 14.37	35.35 ± 16.24	0.001
St George’s Respiratory Questionnaire	56.16 ± 21.73	49.13 ± 25.39	0.03
Patient Health Questionnaire
Minimal depression	5 (19)	22 (55)
Mild depression	12 (46)	12 (30)	0.01
Moderate depression	6 (23)	5 (12)
Moderately severe depression	3 (12)	0 (0)
Severe depression	0	1 (3)

Values are mean ± SD or n (%).

^a Independent t-test.

^b Using Duke Activity Status Index (DASI).

We found significant differences in functional activity between the two groups. The high walking performance group had 11.5 mL/kg/min higher levels of predicted oxygen consumption, “quite a bit of difficult” in performing the upper extremities-related activities (UEFI: 20.57 ± 14.84) and were within the range of moderate difficulty in performing lower extremity-related functions. The patients with low walking performance had significantly lower predicted oxygen uptake (p = 0.002) and approximately 10-point higher UEFI than the high walking performance group; yet, the score still corresponded to “quite a bit of difficulty’’ in performing the activities (Table 2).

In terms of health status, we found a significantly better health status in the high walking performance group compared with the low walking performance group (SGRQ: 49.13 ± 25.39 vs. 56.16 ± 21.73, p < 0.001). In addition, the percentage of patients who reported minimal depression was higher in the group with high walking performance than in the group with low walking performance (55% vs. 19%). These findings support the notion that reduced functional activity is associated with poorer health outcomes and decreased overall well-being.

Table 3 presents the results of the multiple linear regression models. The overall model was significant (F(8, 61) = 7.48, p = 0.0006), indicating that the predictors (health status, predicted oxygen consumption, and the upper and lower extremity-related activity, and depression level) collectively explained a significant proportion of the variance (49.5%) in the 6MWT distance. The results indicated that higher FEV1, DASI, and SGRQ values were associated with increased 6MWT distance, whereas higher LEFI, UEFI, and PHQ values were associated with decreased 6MWT distance. These findings suggest that pulmonary function (FEV1), physical fitness (DASI), health status (SGRQ), UEFI and LEFI, and depression symptoms are important factors influencing 6MWT performance among the study participants.

Table 3. Multiple linear regression analysis with 6MWT distance as the dependent variable.

Model	Predictors^a	Coefficients						R^2
		β	SE	T	p-value	95% CI
						Lower	Upper
Model*	(Constant)	316.56	91.74	3.45	0.001	133.11	500.01	0.495
	FEV1	56.48	26.56	2.13	0.03	3.34	109.53
	EEV1_ratio	1.55	1.16	1.33	0.18	−0.77	3.89
	VO2peak	3.32	0.75	4.42	<0.001	1.81	4.82
	SGRQ	−1.46	0.50	−2.93	0.005	−2.46	−0.46
	LEFI	−2.30	0.77	−2.97	0.004	−3.85	−0.75
	UEFI	−2.58	0.73	−3.53	0.001	−4.04	−1.12
	PHQ-9	−6.99	2.42	−2.89	0.005	−11.82	−2.15
	F	F(8, 61) = 7.48
	P-value	0.0006

Abbreviations: FEV1: forced expiratory volume in 1 s (; SGRQ: St George’s Respiratory Questionnaire ; UEFI: upper extremity functional index (); LEFI: lower extremity functional index (); PHQ-9: patient health questionnaire-9 .

* Regression model adjusted for age, COPD severity, and gender.

Discussion

This study aimed to highlight the differences in upper and lower extremity functions, VO_2peak, depressive symptoms, and health status between two distinct groups of COPD patients categorized by walking performance. The study also investigated whether upper and lower limb functions, VO_2peak, depressive symptoms, health status, and pulmonary functions predict the walking performance of individuals with COPD. We found that 35% of moderate to severe depressive symptoms were among patients who walked <350 m in the 6MWT, compared to only 15% in those who walked ≥350. The overall health status was also lower among patients who walked <350 m. Patients with higher walking performance exhibited notably increased predicted oxygen uptake and encountered considerable difficulty in upper extremity functions and moderate difficulty in lower extremity functions. In contrast, patients with low walking performance had significantly lower predicted oxygen uptake and higher UEFI and LEFI scores, corresponding to "quite a bit of difficulty" in activity performance.

The study also highlights the intricate relationship between depressive symptoms, health status, upper and lower extremity function, VO_2peak, and pulmonary functions with walking performance in patients with COPD. Given that the variance in distance walked during the 6MWT was explained by 49% of the above-mentioned factors, the significance remained even after adjusting for age and sex. Along with the pulmonary function, these results underscore the importance of considering various factors, including depressive symptoms, overall health status, predicted VO_2peak, and functional impairment of the extremities in understanding the complexity of walking performance in this population. These findings contribute valuable insights and may inform targeted interventions aimed at enhancing functional activity in patients with COPD.

Among our sample of individuals with COPD, pulmonary functions were impaired compared to the average values reported in healthy patients [31]. In addition, our results were in agreement with previous research in which pulmonary function was a significant predictor of the 6MWT distance in a COPD population [32–34]. Although the primary effects of COPD on lung function are well documented [35], emerging research has shed light on its intricate impact on upper and lower extremity function [4,36,37]. Individuals with COPD often exhibit a reduced gait speed, decreased muscle strength, and altered biomechanics during walking. These impairments can result in diminished mobility, increased risk of falling, and compromised overall functional independence [38,39]. In this study, it was apparent that patients with COPD reported difficulty in performing the upper and lower extremity functions and presented with moderate difficulty in executing the task. Muscle wasting and weakness, often attributed to systemic inflammation and deconditioning, can affect the muscles of the upper limbs [4,40]. Although our study did not delve into the precise mechanisms underlying the deterioration of upper and lower extremity functions, it is reasonable to infer that the findings from previous research could be applicable to our study population and contribute to the observed decline in extremity functioning. However, further investigations are warranted to validate this hypothesis.

Upper extremity function can play a major role in predicting a patient’s physical quality of life. In a previous study, the association between upper extremities, unfavorable discharge disposition, and all-cause 30-day readmission was significant [38]. Furthermore, in a meta-analysis, a significant effect of upper extremity exercise on dyspnea and overall activity was reported in patients with COPD [39]. Addressing upper and lower extremity functions is integral to comprehensive COPD management. Therapeutic interventions that address both upper and lower extremity functions can contribute to enhancing quality of life and promoting functional independence in individuals living with COPD. Further research is warranted to explore novel strategies that mitigate extremity-related limitations and improve the overall well-being of this patient population.

Compared to the general population, patients with COPD have low levels of cardiorespiratory fitness and physical activity [41]. Our results are in accordance with those of another study that reported a low VO_{2 peak} in patients with COPD [42]. Furthermore, impaired physical activity of the upper and lower extremities has been reported to manifest as a reduced number of steps per day, a lower peak oxygen consumption, early anaerobic threshold, and reduced distance in the 6MWT [43]. Recent studies have demonstrated that the aerobic capacity of stable hypoxemic COPD patients is impaired, and slow oxygen uptake kinetics at submaximal workloads have been reported [42]. Conversely, improved walking performance through targeted interventions enhances extremity function, encourages greater physical activity, and positively influences VO_{2 peak}. This indirect pathway underscores the pivotal role of walking performance in shaping cardiopulmonary fitness.

Walking performance has been shown to be diminished in individuals with COPD compared to the healthy population. A 6-min walking distance of less than 350 m has been shown to have prognostic value in patients with COPD, which could be useful for identifying patients who are at high risk of future complications [44–46]. In addition, a distance less than 350 m has been associated with increased mortality [24]. In the present study, almost 43% of the participants demonstrated a walking distance of less than 350 m in the 6MWT. These results are in line with previous evidence showing a decline in exercise tolerance and changes in gait and overall walking economy in the COPD population [32,44]. With respect to the severity of the disease, efforts have been made to explore the determinants of such a reduction in walking performance in this population [32,47]. Our results showed that patients with low walking performed exhibited more airflow limitation by GOLD stage compared to those who walked ≥ 350 m, as well as a poor overall health status and worse depressive symptoms. Consistent with our results, a previous study demonstrated that disease-related symptoms can directly influence walking performance [48]. In addition, airflow limitation in people with COPD only partially explains the reduction in the distance walked in the 6MWT [32,49,50]. However, more severe conditions were also included in these studies. In general, pulmonary-related factors, such as dyspnea [34], dynamic hyperinflation [33], and central hemodynamics [51] have been sown to impact walking performance in COPD patients and require further investigation.

Non-pulmonary factors that may influence walking distance have also gained attention in COPD. We found that patients with low walking performance had a poorer overall health status than those with high walking performance, which was supported by lower DASI scores, representing poor physical fitness (predicted VO_{2 peak}). In addition, the SGRQ and DASI scores were significant predictors of walking distance. These results are supported by the results of previous research [52,53] in which those who had better SGRQ scores walked longer in the walking test. To further confirm this claim, Pongor et al. revealed an association between COPD patients’ health-related quality of life, physical activity participation, and the distance walked in the 6MWT [54]. These results indicate that physical activity level could moderate the relationship between health status and walking distance. However, investigating the moderating role of physical activity level and its related mechanisms of action in our population was beyond the scope of this study and requires further research.

Interestingly, scores indicating difficulty on the UEFI and moderate difficulty on the LEFI were more pronounced among those who walked ≥350 m than those who walked <350 m. Yet, UEFI and LEFI were negatively associated with the distance walked. This could be explained, in part, by the number of exacerbations that are known to affect walking distance [55,56]. However, we did not include the number of exacerbations in our study. In contrast to previous findings from a study conducted to investigate the potential determinants of exercise capacity (as measured by 6MWT), isometric peripheral muscle force, such as quadriceps force, was an important determinant of the exercise capacity in COPD [40]. Furthermore, with regard to upper limb function, upper limb muscle (middle deltoid) has been shown to have a higher fatigability compared to lower limb muscles in the COPD population [57]. Upper limb muscle and thoracic muscle strength were also shown to be important in the distance measured during the 6MWT [58]. However, such results should be interpreted with caution. Further investigation is needed among the sample to better understand the variations observed in the lower and upper extremity functions.

Depressive symptoms have been significantly associated with the distance walked by people with COPD [19,43], but others contradict our results [59]. The present study showed that 35% of moderate to severe depressive symptoms were found among patients who walked <350 m, compared to only 15% in those who walked ≥350. In addition, depressive symptoms were significantly associated with walking distance during the 6MWT. Despite the severity of depressive symptoms among participants in our study, almost all patients had at least minimal symptoms of depression. This is well documented in the COPD literature, as depressive symptoms are an important factor that could impact physical functioning and performance in COPD [43,60] and are highly prevalent among this population [61,62]. Possible mechanisms underlying the relationship between depression and walking distance in the COPD population could be depression playing a barrier role to both pharmaceutical and non-pharmacological treatment compliance. Depression may also reduce adherence to oxygen masks, causing a cascade of negative events, and it can cause a person to become less motivated, more despondent, or have decreased self-esteem and self-efficacy [63,64]. Collectively, all of these possibilities may contribute to activity participation and walking performance [3]. Thus, our findings indicate the importance of early assessment of the psychological health of patients with COPD, including depression. Investigating the benefits of psychotherapy for depression severity, such as cognitive behavioral therapy on walking distance, in the COPD population could be promising. Determining the factors that predict the severity of depression among the COPD population is important but beyond the scope of this study and future research is needed.

Limitations and future directions

Our study has several limitations that need to be acknowledged. The cross-sectional design of the study may exclude the judgment of a clear causal relationship. Further studies are required to ensure a rigorous methodology and prospective longitudinal follow-up. Our results were also limited by the self-reported measures, which can lead to reporting bias. Consequently, the interpretation of our findings may have been improperly influenced. Using objective measures would be more precise and appropriate in future studies. It is also beyond the scope of this study to comprehensively investigate and consider other factors associated with poor 6MWT distance. Fatigue, sleep, pain, nutritional status, respiratory and skeletal muscle strength, habitual physical activity, and level of hypoxemia may contribute to the walking distance but were not considered in the present study. Lastly, our sample mainly consisted of males, which may affect the generalizability of the study findings among women. Further studies are needed to include an equal number of both sexes to overcome this limitation.

Conclusion

In conclusion, the factors associated with the distance walked in the 6MWT are complex and multifactorial. Factors other than disease-related factors, such as physical and psychological factors, should be considered to explain the reduction in the walking distance among the study population. In addition to pharmacological protocols that mainly target pulmonary function in the COPD population, the results of this study could be beneficial for healthcare providers to evolve better non-pharmacological, rehabilitative management strategies for patients with COPD to improve walking performance, functionality, and overall health outcomes. The results of this study will add to the fundamental knowledge on impaired walking distance in the COPD population. Such results will guide healthcare providers regarding the benefits of early assessment of non-pulmonary factors that may impact walking distance and help in developing new rehabilitative strategies targeting potential factors other than disease-related factors that could play major roles in the physical health outcomes of patients with COPD and in ameliorating their impaired functional capacity.

Authors’ contributions

All authors contributed substantially to this manuscript. M.I.A. and A.A. contributed to the conception and design of the study. A.A. and R.N. contributed to data collection, and M.I.A. contributed to data analysis. M.I.A., B.A.B., M.D.A., M.A., and A.A. contributed to the interpretation of the analyzed data and writing the original manuscript. All authors have reviewed the manuscript for important intellectual content. All of the authors critically read, revised, and approved the final draft of the manuscript.

Acknowledgements

We would like to thank Princess Nourah Bint Abdulrahman University for supporting this project through Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R 286), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data for the current study will be available upon reasonable request from the principal investigator or corresponding author.

Additional information

Funding

This research was funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R 286), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.