ORIGINAL ARTICLE Quadriceps strength and endurance in fibrotic idiopathic interstitial pneumonia LAURA MENDOZA, 1,3 A THENA GOGALI, 3 DINESH SHRIKRISHNA, 3 GABRIEL CAVADA, 2 SAMUEL V. KEMP, 3 SAMANTHA A. NATANEK, 3 ABIGAIL S. JACKSON, 3 MICHAEL I. POLKEY, 3 A THOL U. WELLS 3 AND NICHOLAS S. HOPKINSON 3 1 Hospital Clínico, 2 Escuela de Salud Pública, Universidad de Chile, Santiago, Chile, and 3 National Institute for Health Research Respiratory Biomedical Research Unit at Royal Brompton and Harefield NHS Foundation Trust and Imperial College, London, UK ABSTRACT Background and objective: Quadriceps muscle dys- function is an important contributor to exercise limi- tation in chronic obstructive pulmonary disease, but little is known about skeletal muscle function and its impact on exercise capacity in patients with fibrotic idiopathic interstitial pneumonia (IIP). The aim of the study was to compare quadriceps strength and endur- ance in patients with fibrotic IIP and healthy controls, and relate it to exercise capacity. Methods: Quadriceps strength and endurance, as well as respiratory muscle strength, and 6-min walk dis- tance were compared among 25 patients with fibrotic IIP,forced vital capacity mean (standard deviation) 78.7 (14.0) %predicted, carbon monoxide transfer factor 40.3 (10.9) %predicted and 33 age-matched healthy controls using non-volitional measures. Quadriceps strength was assessed using magnetic femoral nerve stimulation (quadriceps twitch force), and endurance using the decay in force in response to repetitive mag- netic stimulation of the quadriceps over 5 min. Results: Both groups had comparable anthropome- trics, gender proportion and respiratory muscle strength. Patients were significantly weaker than con- trols; quadriceps twitch force 10.1 (3.0) kg versus 8.0 (2.4) kg (P = 0.013). Quadriceps force in response to repetitive magnetic stimulation declined significantly more rapidly in patients during the endurance proto- col (P < 0.001). In controls, there was a significant rela- tionship between 6-min walk distance and quadriceps twitch force (r 0.40, P = 0.038), and quadriceps endur- ance (r 0.59, P = 0.016). In patients, only PaO2 and inspiratory muscle strength were retained as inde- pendent correlates of 6-min walk distance (r 2 = 0.3 P = 0.022). Conclusions: Quadriceps strength and endurance are reduced in patients with fibrotic IIP compared with healthy controls, but are less tightly correlated with exercise performance. Key words: exercise, interstitial lung disease, magnetic stimu- lation, muscle, pulmonary fibrosis. Abbreviations: 6MWT, 6-min walking test; IIP, idiopathic interstitial pneumonia; IPF, idiopathic pulmonary fibrosis; SNiP, maximum sniff nasal pressure. INTRODUCTION Peripheral muscle dysfunction is a common feature of chronic cardiorespiratory diseases, including chronic obstructive pulmonary disease and heart failure. It is an important factor contributing to exercise limita- tion in these conditions, can occur early in the course of disease, 1,2 and is associated with a worse quality of life and increased mortality. 3–7 However, few data exist regarding skeletal muscle in patients with interstitial lung disease. 8–10 Fibrotic idiopathic interstitial pneumonia (IIP) includes two categories of idiopathic pneumonia, which are characterized by a predominance of pul- monary fibrosis and share a similar clinical presenta- tion and prognosis: idiopathic pulmonary fibrosis (IPF) and fibrotic nonspecific interstitial pneumo- nia. 11,12 Patients with fibrotic IIP experience progres- sive exertional dyspnoea and reduced exercise capacity, both attributed to pulmonary function deterioration. 13–17 However, it has been demonstrated that some patients with interstitial lung disease report leg fatigue as the symptom limiting their exercise capacity, 18 and Nishiyama et al. found a correlation between quadriceps strength and maximum oxygen uptake in patients with IPF, 10 suggesting that locomo- tor muscle dysfunction may contribute to exercise Correspondence: Laura Mendoza, Hospital Clínico Univer- sidad de Chile, Santos Dumont 999, Independencia, Santiago, Chile. Email: lmendoza08@gmail.com Received 4 June 2013; invited to revise 4 July 2013; revised 16 July 2013; accepted 3 August 2013 (Associate Editor: Amanda Piper). SUMMARY AT A GLANCE This study demonstrates that quadriceps strength and endurance are significantly reduced in patients with fibrotic idiopathic interstitial pneu- monia compared with healthy controls. © 2013 The Authors Respirology © 2013 Asian Pacific Society of Respirology Respirology (2014) 19, 138–143 doi: 10.1111/resp.12181