Idiopathic pulmonary fibrosis (IPF) is a debilitating disease that is characterized by progressive impairment in lung function and degradation of daily activity. Lung function, such as forced vital capacity (FVC), is now recognized as one of the important outcomes in clinical trials and cohort studies of IPF, because it significantly predicts survival [1]. Exercise capacity, such as a 6-min walk distance (6MWD) and maximal oxygen consumption measured by a cardiopulmonary exercise test, is decreased in IPF and is considered a significant predictor of survival [2–۴]. However, the actual daily life physical activity in patients with IPF has not been adequately elucidated.

Recently, physical activity has been of focal interest in chronic obstructive pulmonary disease (COPD) [5]. Patients with COPD have reduced physical activity, which is not completely reflected by the disease severity when determined via the lung function [6,7]. More importantly, physical activity is reportedly a stronger predictor of survival than is forced expiratory volume in 1 sec (FEV1) and exercise capacity assessed by a 6-min walk test (6MWT) [8].

The question remains as to whether the reduction in physical activity due to IPF is similar to that in COPD and is associated with the survival rate. To address this, patients with IPF and age-matched healthy participants were prospectively recruited, and physical activity was measured to determine its relationship with physiological parameters and survival rate.