Quadriceps strength and endurance in fibrotic idiopathic interstitial pneumonia

Quadriceps muscle dysfunction is an important contributor to exercise limitation 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 endurance in patients with fibrotic IIP and healthy controls, and relate it to exercise capacity.

Quadriceps strength and endurance, as well as respiratory muscle strength, and 6-min walk distance 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 magnetic stimulation of the quadriceps over 5 min.

Both groups had comparable anthropometrics, gender proportion and respiratory muscle strength. Patients were significantly weaker than controls; 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 protocol (P < 0.001). In controls, there was a significant relationship between 6-min walk distance and quadriceps twitch force (r 0.40, P = 0.038), and quadriceps endurance (r 0.59, P = 0.016). In patients, only PaO2 and inspiratory muscle strength were retained as independent correlates of 6-min walk distance (r(2)  = 0.3 P = 0.022).

Quadriceps strength and endurance are reduced in patients with fibrotic IIP compared with healthy controls, but are less tightly correlated with exercise performance.