Table 3– Relationship between exercise tolerance and physical activity in some representative cross sectional studies

First author [ref.]	Design and setting	Subjects n (males/females)	Main objective	Physical activity assessment	Exercise tolerance assessment	Correlation coefficient	p-value	Main results
Steele [39]	Single-centre convenience sample USA	47 (44/3)	Feasibility of accelerometer measurement	Accelerometer	6MWD	0.74	<0.001	Preliminary data suggesting that a triaxial movement sensor is a reliable, valid and stable measure of walking and daily physical activity in COPD patients
Belza [91]	Single-centre convenience sample USA	63 (60/3)	Evaluation of correlates of physical activity	Accelerometer	6MWD	0.60	<0.001	Physical activity is most significantly related to walking abilities
Pitta [74]	Single-centre convenience sample Belgium	50 (36/14)	Comparison of physical activity in COPD with healthy subjects	Acclerometer	6MWD % pred W_max % pred Peak V′_O₂ % pred	0.76 0.64 0.33	<0.0001 <0.0001	Patients with COPD are markedly inactive in daily life Functional exercise capacity is the strongest correlate of physical activity
Eliason [96]	Multicentre convenience sample Sweden	44 (28/16)	Associations between physical activity and clinical characteristics of COPD	Accelerometer	6MWD	0.34^#]	0.03	Mean physical activity level and physical activity of at least moderate intensity are positively associated with exercise capacity
Waschki [79]	Multicentre convenience sample UK, the Netherlands	127 (79/48)	Evaluation of compliance with wearing an accelerometer and the relationship of disease characteristics with physical activity in a multicentre study	Accelerometer	6MWD	0.47^¶]	<0.001	Excellent compliance with wearing a physical activity monitor in a multicentre study Consistent associations of physical activity with relevant disease characteristics in a multicentre study
Watz [65]	Single-centre convenience sample Germany	163 (122/41)	Various analyses of physical activity and its clinical correlates in COPD	Accelerometer	6MWD	0.46	<0.001	Relation to clinical outcomes
Van Gestel [40]	Single-centre convenience sample Switzerland	70 (49/21)	Predicting physical activity levels from 6MWD	Accelerometer	6MWD	0.69	<0.001	6MWD cannot reliably predict physical inactivity

↵6MWD: 6-min walking distance; COPD: chronic obstructive pulmonary disease; W_max: maximal workload; V′_O₂: oxygen uptake. ^#: standardised regression β-coefficient; ^¶: standardised regression β-coefficient adjusted for age, sex, study site and body mass index >30 kg·m⁻².