Feasibility of the physiological cost index as an outcome measure for the assessment of energy expenditure during walking

Objective: To determine if the Physiological Cost Index (PCI) can be recommended as an outcome measure in clinical trials. Design: Three assessments were performed, 2 with shoes, 1 without. The difference between walking with shoes and walking barefoot was used to study the ability of the PCI to detect a change in the criterion standard. Setting: A research department affiliated with a rehabilitation hospital in the Netherlands. Participants: Twelve children with cerebral palsy. Interventions: During the first and third assessments, the children walked with shoes. During the intermediate assessment, the children walked without shoes. Main Outcome Measures: Breath-by-breath oxygen uptake, heart rate (HR), and walking speed were measured at a self-selected comfortable speed. Oxygen Cost (EO2) and the PCI were subsequently calculated offline. Feasibility judgments were made regarding the ability of the PCI to detect changes in a criterion standard and the statistical power of the outcome measure. Results: Pearson correlation coefficients were .66 and .62 for HRwalking-HRbaseline and HRwalking, respectively. The smallest detectable difference of the PCI and EO2 were 69% and 32%, respectively. A difference of at least 69% or 32% should be found before one can conclude a difference with a certainty of 95%. Conclusions: The reproducibility of the PCI and the ability to show small differences in EO2 were moderate. Subtracting HRbaseline when calculating the PCI is probably not useful because it only increased within-subject variability. With respect to statistical power of a new clinical trial, we recommend using EO2 instead of the PCI. (C) 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.