The reliability and validity of the physiological cost index in healthy subjects while walking on 2 different tracks

Objective: To investigate the reliability and validity of the Physiological Cost Index (PCI) scores, as a measure of energy expenditure, when healthy subjects walk on 2 different tracks (20-m and 12-m figure eight tracks). \ Design: Intra- and interrater reliability and construct validity. Setting: Physiotherapy division of a university in London, UK. Participants: Forty healthy subjects (15 men, 25 women; mean age +/- standard deviation, 34.5 +/- 12.6y). Interventions: Not applicable. Main Outcome Measures: Heart rate (in beats/min) and speed (in m/min) were used to calculate the PCI (in beats/m). Rate of oxygen consumption (VO2, in mL(.)kg(-1.)min(-1)) and oxygen cost (EO2 in mL(.)kg(-1.)m(-1)) were used as criterion estimates of energy cost EO2. Pearson correlation coefficients between the PCI, components of the PCI, EO2, and VO2 were used to quantify validity. Intrarater reliability was assessed in all participants and interrater reliability was assessed on a subset of 13 subjects using intraclass correlation coefficients and Bland-Altman plots. Results: Intrarater (r = .73, r = .79) and interrater (r = .62, r = .66) reliability were acceptable between PCI scores from 20-m and 12-m tracks, respectively. Correlations between VO2 and EO2 with PCI were weak. PCI scores from the 20-m track were significantly lower than those on the 12-m track (P = .002). Subjects walked significantly faster on the 20-m track (P < .001). Results suggest a large difference in PCI scores would be necessary to indicate a "true" alteration in performance (52% for 20-m track, 43.4% for the 12-m track). Conclusions: The PCI is reliable but not valid as a measure of the energy cost of walking in healthy subjects, on either track. The 20-m track is recommended for clinical use because it enables subjects to walk at a faster pace.