TABLE 4

Evaluation of step count, time spent in moderate-to-vigorous physical activity (MVPA) and activity-related energy expenditure (AEE) to assess physical activity in patients with idiopathic pulmonary fibrosis (IPF)

	Step count	Time in MVPA	AEE	6-min walk distance (6MWD)
Construct validity
1. Does the end-point measure physical activity?	Most common end-points used to assess physical activity in patients with IPF Good indicator of day-to-day activity in healthy subjects; however, pure step count cannot indicate relative effort required to complete steps in subjects with respiratory diseases Subject to seasonal variation and potentially skewed by occupation [12]	Captures moderate–intense activities. Useful to capture as patients with IPF intuitively avoid intense exercise to avoid exercise-induced bronchoconstriction Patients’ long-term habits may prevent an improvement in MVPA despite efficacious treatment	Relative energy expended to perform a task above resting metabolism	Historically the most used field test to assess functional capacity Surrogate for physical activity prior to introduction of activity monitors Despite 6MWD being a strong predictor of reduced step count in IPF patients in two studies, the end-point only accounted for 42% and 31% of the step count variance, respectively, indicating this end-point is not a good surrogate for daily physical activity [28, 32] Limited functional capacity indicates muscle depletion caused by physical inactivity [19] The test is self-paced and therefore subject to motivational effects
2. Correlation to dyspnoea	Step count correlated with dyspnoea, patients with an mMRC >2 averaged 1900 steps per day, a 70% reduction compared to patients with mild dyspnoea (mMRC <2) [28]	No data available	AEE associated with dyspnoea score [27, 33]	Patients with a poor 6MWD completed a similar step count to patients with low mMRC [28]
3. Correlation to exercise capacity	Step count correlated with 6MWD [26, 27]	Time in MVPA correlated with 6MWD in 17 IPF patients [30]	AEE correlated with 6MWD in patients with IPF [26, 27]	Inherently an end-point used to indicate exercise capacity
4. Correlation to HRQoL	Step count correlated with HRQoL [28] Step count did not correlate with SGRQ and HADS score, which indicates HRQoL and anxiety/depression, respectively, within IPF cohort [26]	Time in MVPA showed moderate-to-strong correlations with the EQ-5D index score in 111 patients with fibrotic interstitial lung disease [31]	No data available	Patients with a poor 6MWD completed a similar step count to patients with low QoL (12-Item Short Form Survey, SF-12) [28]
Content validity
1. Does the end-point capture every aspect of physical activity?	Poor indicator of vigorous activity (crucial for long-term health) Poorly reflects patient experience, cannot indicate any pain experienced during essential mobility	Doesn't capture majority of daily movement, e.g. walking	Captures energy expended during physical activity in a day	Exercise capacity comprises only one of the important dimensions which determines physical activity Behaviours and environmental factors play huge roles in the amount and frequency of physical activity performed by people; exercise capacity does not directly translate to physical activity
2. Reflects respiratory disease state	Patients averaged a daily step count of 2728±2475, significantly fewer than the healthy cohort at 5953±3578 [26] Step count associated with lung function measures such as FVC % predicted normal value and D_LCO % predicted normal value [28]	No data available	Patients averaged 133±127 kcal·day⁻¹, whilst healthy controls expended 201±111 kcal·day⁻¹ [26]	No data available
3. Responsiveness to pharmacological intervention	No data available	Inhaled nitric oxide improved MVPA by 34% in patients with IPF [29]	No data available	No data available
4. Impact on survival	Step count correlated with serum KL-6, an important predictor of survival in IPF [27, 33]	No data available	Following adjustment for the prognostic factors age, sex and % FVC, AEE was the only end-point significantly associated with survival of IPF patients [26] AEE associated with serum KL-6 [27]	6MWD significantly and independently predicted mortality, with a 6MWD of >360 m having an 80% survival probability after 30 months [32]

mMRC: modified British Medical Research Council questionnaire; HRQoL: health-related quality of life questionnaire; EQ-5D: European Quality of Life Five Dimension; QoL: quality of life; SGRQ: Saint George's Respiratory Questionnaire; HADS: hospital and anxiety depression score; FVC: forced vital capacity; D_LCO: diffusion capacity for carbon monoxide; QoL: quality of life.