Walking energy expenditure in able-bodied individuals: A comparison of common measures of energy efficiency Susan Sienko Thomas a, *, Cathleen E. Buckon a , Michael H. Schwartz b , Michael D. Sussman a , Michael D. Aiona a a Shriners Hospitals for Children, 3101 SW Sam Jackson Park Road, Portland, OR 97239, United States b Gillette Children’s Specialty Healthcare, Center for Gait and Motion Analysis, 200 East University Avenue, St. Paul, MN 55101, United States 1. Introduction Assessment of walking energy efficiency via direct oxygen cost or indirect heart rate measurement is potentially useful for objectively quantifying and differentiating the gait efficiency of able-bodied individuals and those with walking disabilities. Despite this potential, there has not been widespread use of this important clinical outcome tool, especially for the assessment of walking efficiency in individuals with neuromuscular disorders. Issues regarding methodology, day-to-day variability, inter subject variability (age and size differences) and clinical relevance are barriers to the utilization of walking energy efficiency assessment as a standard assessment tool. Two primary methodologies have been utilized to assess the energy efficiency of walking. The first is measurement of heart rate that provides a proxy measure of energy efficiency and the second is the direct measurement of oxygen and carbon dioxide. Proponents of heart rate methodology believe that this method provides a simple and inexpensive method of assessing walking efficiency [1,2]. Resting heart rate, walking heart rate and velocity are used to calculate either the Energy Efficiency Index (EEI) [2] or the Physiological Cost Index (PCI) [3]. These indexes are considered reasonable alternatives to direct measurement because of the linear relationship established between heart rate and oxygen consumption at submaximal workloads [4]. In addition, the assessment of heart rate can be performed with minimal equipment and expertise making this methodology easily applic- able in a clinical setting. In contrast, direct measurement of the volume of oxygen (VO 2 ) and carbon dioxide (VCO 2 ) is accomplished with a portable unit that uses breath-by-breath or mixing chamber technology. Although the equipment is expensive [5] and requires some clinical expertise, proponents of this methodology believe that allowing the subject to walk at their own comfortable walking speed produces the optimal value of energy efficiency making valid comparisons between and within subjects possible [6]. Energy efficiency is generally reported in terms of gross or net consump- tion and cost. Gross oxygen consumption has two components. The first is the basal metabolic rate, and the second is the summation of the mechanical power required to stand, maintain balance and control, and propel the body and limbs [7]. Gross cost is commonly normalized by mass per unit distance (walking J/kg m); however, it does not take into account the non-linearity that occurs with speed, and fails to produce a variable independent of body weight [8]. Recently, the net non-dimensional (NN) scheme [9] has been Gait & Posture 29 (2009) 592–596 ARTICLE INFO Article history: Received 28 March 2008 Received in revised form 2 December 2008 Accepted 5 January 2009 Keywords: Energy efficiency measures Normalization Able-bodied Gait ABSTRACT Assessment of walking energy efficiency is potentially useful for objectively quantifying gait efficiency; however, issues regarding methodology, day-to-day variability, inter subject variability (age and size differences) and clinical relevance prevents the assessment of walking energy efficiency from being used as a standard assessment tool. Volume of oxygen (VO 2 ) and heart rate were assessed to determine the intra-subject (day-to-day) variability, the impact of age and body parameters and the clinical relevance of gross cost, Net non-dimensional Cost (NNcost) and Energy Efficiency Index (EEI) in able-bodied individuals. Gross cost demonstrated the least amount of day-to-day variability, in comparison to NNcost and EEI. Age and body parameters contributed more to the variability in gross cost than NNcost and EEI. Only net non-dimensional oxygen consumption was related to the velocity of walking. EEI did not correlate with direct measures of oxygen cost. The results from this study demonstrate that utilization of NNcost, a measure that subtracts resting oxygen consumption from walking oxygen consumption and scales for body parameters, is preferable to the traditional measures for comparing individuals of different ages and sizes and is related to the speed that the individual walks. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 503 221 3481; fax: +1 503 221 3490. E-mail address: SST@SHCC.org (S.S. Thomas). Contents lists available at ScienceDirect Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost 0966-6362/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2009.01.002