GPS and/or GDI? Part 3 – A decision?

In the last two posts I’ve looked at the differences between the GDI (Schwartz & Rozumalski, 2008) and GPS (Baker et al., 2009). The broad conclusion is that they are very minor. Much less important than I thought when we published the GPS paper in 2009. The derivation of the GPS is much simpler than that of the GDI and this is what I really like about it (but takes no less time to calculate once you’ve got the software). I prefer the way the GPS is scaled but now accept that this is simply personal preference and the requirement to log transform data before performing statistical tests is clumsy (and I suspect is being ignored in many applications). On balance we should probably have published an alternative derivation of the GDI rather than proposing an alternative index.

The other issue, of course, is the Movement Analysis Profile. This is an obvious and extremely useful extension of the GPS. There are technical reasons why an equivalent doesn’t flow from the derivation of the GDI. If we’d published an alternative derivation of the GDI in 2009 as opposed to a different index, however,  then it would have been the easiest thing in the world to define similarly scaled variables for the different joints and different planes. We’d then have a MAP that was equivalent to the GDI.

In conclusion what we should have done in 2009 was to propose an alternative derivation of the GDI and extended that to develop a MAPGDI. Hindsight is a wonderful thing – we didn’t. We’re thus stuck with two indices that are very much the same but different. Some research groups have understood the subtleties of the differences and opted for one or the other for a good reason. Others may have tossed a coin. Other groups have chosen to report both.  There is an argument that we should try and roll back the clock by proposing MAPGDI now but I don’t know what stomach there is for even more new indices in the community. It might be interested see what comments this post prompts from you. Should we try and tidy this up by doing what we should have done in 2009 or will this just cause even more confusion?

What is clear is that there is little point any study reporting the results of both GPS and GDI (they are too similar for this to be useful). There is absolutely no point performing statistical tests on both because they should give identical results (as long as the log transform is applied to the GPS). People should choose which they prefer and report that. Mike and I have submitted a joint paper to GCMAS next year which includes the formulae required to convert one index to the other.

Perhaps even more important – research should not focus solely on either measure. Whilst the indices provide a simple number that describes how different the kinematics are from normal there is a lot more to walking than looking like everyone else. Gait speed is an extremely important measure of gait function and how this depends on cadence and step length provides basic clinical insight. Simply taking the rather poorly defined “self-selected” walking speed used in most gait analysis tests may be the easiest measure of speed but it is not necessarily the best. Endurance is also important and the 6-minute walk test that is becoming standard in other areas of exercise physiology is combines elements of speed and endurance that have considerable face validity. In many cases it is also important to assess dependence on walking aids (as in the FMS, Graham, Harvey, Rodda, Nattrass, & Pirpiris, 2004) and orthoses. Whilst GDI and GPS greatly enhance our ability to characterise gait kinematics they should not be used to the exclusion of other measures.

Graham, H. K., Harvey, A., Rodda, J., Nattrass, G. R., & Pirpiris, M. (2004). The Functional Mobility Scale (FMS). J Pediatr Orthop, 24(5), 514-520.

Schwartz, M. H., & Rozumalski, A. (2008). The gait deviation index: A new comprehensive index of gait pathology. Gait Posture, 28(3), 351-357.

Baker, R., McGinley, J. L., Schwartz, M. H., Beynon, S., Rozumalski, A., Graham, H. K., & Tirosh, O. (2009). The gait profile score and movement analysis profile. Gait Posture, 30(3), 265-269.

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