On the effectiveness of rehabilitative exercise training in COPD: why does contractile muscle fatigue matter?

Extract COPD is characterised by exercise limitation, dyspnoea being patients' cardinal symptom and typically occurring from "excessive breathing" (referred to as ventilatory inefficiency) and "constrained breathing" (i.e., how well, in mechanical terms, ventilation is produced) [1]. It is now increasingly acknowledged that impairment in locomotor muscle structure and function may also hold a relevant contributory role [2–5]. This includes, in particular, heightened muscle susceptibility to fatigue [6] owing to structural derangements (e.g., a muscle fibre shift toward a lower oxidative profile [7] or reduced muscle capillarisation [8]). In this context, it might seem rather counterintuitive at first that contractile muscle fatigue may be used as a "biomarker" of effectiveness of rehabilitative exercise training in COPD.