The role of peripheral muscle fatigability on exercise intolerance in COPD

ABSTRACT

Introduction

Exercise limitation in chronic obstructive pulmonary disease (COPD) is multi-factorial; however, growing evidence indicates that muscle dysfunction may contribute in some patients.

Areas covered

This work outlines current evidence for and against increased peripheral muscle fatigability in COPD through a comprehensive review of relevant literature available on PubMed/MEDLINE until May 2020. The authors first discuss key methodological issues relative to muscle fatigue assessment by non-volitional techniques, particularly magnetic stimulation. The authors then provide a detailed discussion of critical studies to have objectively measured skeletal muscle fatigue in individuals with COPD.

Expert opinion

Current evidence indicates that localized (knee extension) and cycling exercise are associated with increased quadriceps fatigability in most COPD patients. Increased fatigability, however, has not been consistently found in response to walking, likely reflecting the tendency of ‘central’ respiratory constraints to overshadow potential functional impairments in the appendicular muscles in this form of exercise. Thus, addressing skeletal muscle abnormalities may be critical to translate improvements in lung mechanics (e.g., due to bronchodilator therapy) into better exercise tolerance. The positive effects of pulmonary rehabilitation on muscle fatigability are particularly encouraging and suggest a role for these measurements to test the efficacy of emerging adjunct training strategies focused on the peripheral muscles.

KEYWORDS:

• Chronic obstructive pulmonary disease
• skeletal muscle
• neuromuscular fatigue

Article highlights

• Perceived fatigue is a common complaint in patients with COPD, though it remains a misunderstood symptom in clinical practice.

• Neuromuscular fatigue is typically separated in two components (central and peripheral) and refers to any decrease in the ability to exert muscular force or power caused by exercise, whether or not the task can be sustained.

• The presence of fatigue can be objectively assessed using neurostimulation techniques such as peripheral nerve (electrical or magnetic) stimulation. These techniques allow the respective contribution of peripheral and central fatigue to be interrogated.

• Existing evidence suggests that localized (particularly knee extension) and cycling exercise are associated with increased quadriceps fatigability in COPD patients.

• The signal towards increased muscle fatigability, however, is notoriously weaker in response to walking. This likely reflects the fact that ‘central’ constraints to exercise limitation may overshadow potential functional impairments in the appendicular muscles.

• Whether increased muscle fatigability contributes to daily physical (in)activity in these patients remains to be unequivocally established.

• Performance fatigue might become a relevant outcome to objectively assess the efficacy of adjunct training strategies, such as neuromuscular electrical stimulation and ‘new’ ergogenic aids (e.g. creatine supplementation, oral nitrate as beetroot juice).

Acknowledgments

The authors thank Dr. Nicolle J. Domnik (Queen’s University, Kingston, ON, Canada) for the time she dedicated to critical reading and substantial language editing of the revised manuscript.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was not funded.