Estimating the Energy Efficiency of Gait

The same newsletter had an excellent blurb about the Energy Efficiency Index, which is one of the few simple measurements that has been shown to have some validity. The assumption behind the EEI is that the Heart Rate [HR] is an indirect measurement of how hard a person is working, so a lower HR suggests it is easier to walk after a given intervention. Furthermore, people generally walk more slowly to compensate for a disability so that their overall effort remains similar to that of non-disabled persons. Therefore, if they spontaneously walk more quickly after a given intervention, this is a very favorable sign.

The EEI combines these two factors into this formula: EEI = [Exercise HR - Resting HR] walking velocity. This is not a very sensative measurement, however, because many factors affect heart rate, including anxiety and medications. But, it often does show major trends that are likely to be pronounced enough to be clinically significant.

Thanks to the mass market created by the fitness industry, wristwatches with remote HR telemetry attachments are readily available for less than $100. [Polar is one of the major manufacturers; you can browse their site online at www.polarusa.com/polar_products/products_home.htm] Two strips of masking tape placed on the floor a specific number of meters apart turn a hallway into a measured track. A $5 digital watch to record the number of seconds it takes to go from one strip of tape to the next provides all the information needed to calculate the patient's walking velocity.

So, it is now well within any interested clinician's budget to gather the data necessary to determine the EEI, and a simple spreadsheet formula can be easily written to automate the calculation. If you feel a bit more ambitious, you can build an automated system to measure velocity for a few hundred dollars by following the instructions from the Journal of Prosthetics & Orthotics at www.oandp.org/jpo/library/2000_04_117.asp. Or, consider building an electronic walkway out of aluminum foil as described in Prosthetics & Orthotics International at www.insoft.co.uk/software/.

One caveat though: the variability of the EEI increases significantly as slower cadences. Unfortunately, many of our more involved patients walk very slowly due to the magnitude of physical limitations they face, so the EEI might not be very helpful for this population. The following graph, from the Mary Free Bed newsletter noted in the first article, illustrates this last point:


The Energy Efficiency Index is sometimes used to estimate improvements in a patient's gait. If their self-selected walking velocity increases and the EEI diminishes after receiving their new lower limb orthosis or prosthesis, this is a favorable finding. But, note that the variability of the EEI [as expressed by the standard deviations] increases at slower cadences.
The Energy Efficiency Index is sometimes used to estimate improvements in a patient's gait. If their self-selected walking velocity increases and the EEI diminishes after receiving their new lower limb orthosis or prosthesis, this is a favorable finding. But, note that the variability of the EEI [as expressed by the standard deviations] increases at slower cadences.



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