Energy cost of walking: A comparison of two recognised methods for defining steady state in a group of unimpaired children and children with cerebral palsy

Measurement of VO2 based ‘energy cost of walking’ indices provide quantitative information that can be used clinically to ascertain progress and relative functional impairment in pathology. The accuracy and validity of these indices relies on being able to identify periods of steady state conditions, since it is only in steady state that there is a direct and predictable relationship between VO2 consumption and energy expenditure. This paper presents a comparison of two new mathematically based approaches that have recently gained recognition as methods for identifying steady state during an energy cost assessment. The first applies a mathematically defined threshold for steady state within a (non-treadmill) walking trial. This method is compared with a statistically based approach that relies on the calculation of the correlation coefficient, Kendall's Tau to define steady state data. To examine the impact of both methods on the calculation of the energy cost of walking, each was applied to the calculation of two oxygen based energy cost of walking parameters, the non-dimensional net oxygen cost and the net oxygen cost with speed normalised to height in a cohort of unimpaired subjects and children with cerebral palsy. The results revealed that overall there were no clinically significant differences between the two methods. It is suggested that the methods can be used interchangeably to calculate the energy cost of walking.