Subjective fatigue
Fatigue is a common feature in patients with chronic respiratory diseases, both as a local muscle
phenomenon and general fatigue. This is reflected during the 6MWT, where COPD patients experienced
more fatigue compared to healthy elderly subjects [60]. Patient-reported fatigue is commonly measured at
the beginning and end of the 6MWT using the Borg scale [59], which exhibits moderate reliability [9, 61].
Fatigue measured during the 6MWT using the Borg scale is associated with lower 6MWT, slower gait speed,
more severe airflow obstruction, more dyspnoea on exertion and lower HRQoL [1]. Notably, low-frequency
muscle fatigue in the quadriceps is seldom found with the 6MWT, although low-frequency fatigue in distal
leg muscles has been reported to be greater in COPD patients compared with controls