A review of developments in gait asymmetry quantification

Bilateral asymmetry in gait is a topic that has important implications on injury, performance and data collection. Asymmetry research studies have investigated various forms of gait ranging from sub-maximal velocity walking to maximal velocity sprint running. There have been several recent advancements in the methods used to quantify asymmetry. This chapter critically discusses some of the main developments in asymmetry calculation. Initial asymmetry studies presented comparable values for left and right sides of the body, but did not calculate explicit measures of asymmetry magnitude. Methods of quantifying asymmetry were developed to meet the need for a dimensionless measure of asymmetry that could be used to compare asymmetry for different discrete variables. Early asymmetry measures, such as the Ratio Index and Symmetry Index, were based on ratio of left and right values, which could be normalised by a common value. Whilst these methods provided a dimensionless quantification of asymmetry, it has been noted they can lead to the under or over-inflation of results and can be difficult to interpret. overcome the problem of over-inflated values, more recent methods have been proposed as alternative measures of gait asymmetry. Recent methods such as the Symmetry Angle provide values that are truly limited to ±100%, overcoming the problem artificial inflation. However, a limitation associated with the Symmetry Angle is the failure to included intra-limb variability within the asymmetry measure. Since the method requires mean left and right values as input variables, large standard deviations around these mean values may reduce the significance of the reported asymmetry. Recent work focussed on developing the Symmetry Angle along with other methods used to calculate asymmetry over complete gait cycles to provide a dimensionless asymmetry measure that also incorporates intra-limb variability in the quantification of asymmetry. The link between kinematic and kinetic asymmetry has been investigated in lowerlimb amputees, revealing important asymmetry interactions between the different joints. Recent work has also focused on investigating the relationships between associated kinematic and kinetic variables for individual participants. This work has suggested that compensatory kinetic mechanisms may exist to reduce asymmetry in the kinematic outcome variables. Further work investigating asymmetry of amputee runners and the asymmetry interaction between key gait variables would increase understanding of this topic.