Gait Graph Deviation Index

Gait Graph Deviation has been defined as the observed difference between the right side (blue) or left side (red)  gait analysis graph curves, of the measured subject, with the mean (green) curve of the normal subject group .

Figure 1. Left  (red) & right (blue) curve deviation is defined as the distance from the mean curve of the normal subjects

Gait analysis is based on the comparison of these three curves at each moment of the gait cycle. The deviation of the right and left curves of the subject from the normal mean curve, is the main criterion for the clinical evaluation. However, if deviation from the normal mean curve falls within one normal subject's Standard Deviation, the deviation is considered non pathological or within the normal range. Contrary,  deviations larger than one SD , are interpreted increasingly pathological.

Figure 2.  Deviations within one Normal Standard Deviation from the mean normal curve  are considered within normal range. The increase of deviation is considered increasingly pathological.

A very important clinical consideration, is the fact that at each instant of the gait cycle, a different value of Standard Deviation exists around the normal subjects mean curve values. This fact indicates that normalcy range in a gait graph, fluctuates throughout the gait cycle.

Figure 3. In the above  normal subject's gait graph, the two red lines around the mean normal curve, are of significantly different length. The second is almost double the size of the first. Both lines are showing the normal range which is defined by one standard deviation at the specific instants of the gait cycle. It is obvious that this range fluctuates through out the gait cycle. 

The following example illustrates the importance of the above consideration.

Figure 4. Example values of subject curves instant deviation (red bar) from normal mean curve in degrees and normal subjects standard deviation (green bars)  at the specific three different instants of the gait cycle.  

In the above example, at one instant of the gait cycle, deviation of the subject curve from the  normal mean curve value is 20 degrees. At the same instant the Normal Subjects SD is 5 degrees. The ratio between:  

Subjects Curve Deviation from Normal Mean Curve to Normal SD =  20 / 5 = 4. 

This indicates a clinically heavy deviation at this instant of the gait cycle of 4 SDs.

If at the second instant of the gait cycle, deviation is again 20 degrees and at that instant the Normal Subjects SD is 10 degrees then 

Subjects Curve Deviation form Normal Mean Curve  / Normal SD = 20/10 = 2 

This indicates a clinically lighter deviation of 2 SDs.

Finally at the third instant of the gait cycle, deviation is again 20 degrees and at that instant the Normal Subjects SD is 15 degrees then 

Subjects Curve Deviation form Normal Mean Curve  / Normal SD = 20/15 = 1.33 

This indicates a clinically very mild deviation of 1.33 SDs.

In the above example, it was illustrated that equal deviation values may reflect different interpretations in clinical severity,  depending on the respective values of normal subjects SD. Thus the above described procedure of the ratio calculation, generates the required clinical measure for more accurate evaluation. This ratio is very useful to clinicians because they are concerned more about the severity of a deviation from normal rather than an absolute deviation value. 

This ratio for the clinical evaluation of the deviation observed in the Gait Analysis graphs is an innovation that the Gait Graph Deviation Index is bringing in Clinical Gait Analysis.

Gait Graph Deviation Index is a measure that summarizes the observed instant deviations in the gait analysis graphs in a single number for each side, using as units of measurement the Normal Standard Deviation. 

For the implementation of the index, a series of calculation steps are required. The calculation steps that are needed to create the Gait Graph Deviation Index are described here.

The potential of the GGDI has been explored over the years that has been implemented in our Lab in studying Gait Kinematics. Our experience has reinforced its use by the resulting benefits we receive for a variety of applications. One of its applications is its use in creating the Asymmetry vs Deviation Plots which is documented in this web site.

Probably the strongest advantage of the GGDI is its unit, the Normal Subject SD, which is very familiar to the clinical personnel. Experts and non experts in the field have expressed the notion that understanding the quantity that the GGDI reflects is almost intuitive.