Bipedal gait requires movements from the right and left sides of the body, which can be measured and analyzed to determine the quality of a person’s gait. A common method of analyzing the quality of gait is with spatial temporal gait parameters (STP). These measurements (discussed in previous posts) can be analyzed looking at magnitude, asymmetry, and variability.
Gait asymmetry is measured according to the differences between the right and left sides of the body during a walking task. Spatiotemporal variables can be used to quantify the difference between the two sides. These asymmetry measures reveal the quality of the walking pattern and are an important metric in gait analysis.
Small asymmetries are not necessarily indicative of the onset of disease or musculoskeletal conditions, as they are commonly observed even in asymptomatic populations. Patterson, et al. (2010) proposed asymmetry thresholds for gait asymmetry. Values exceeding these thresholds are considered asymmetric. Asymmetries often occur outside of these thresholds in stroke, unilateral amputees, hemiplegic cerebral palsy, and countless other conditions that affect gait.
Asymmetries can affect gait both in spatial ways, such as a difference between right and left step length; and in temporal ways, such as differences in right and left single support time. It is important to quantify these asymmetries by calculating the differences between the right and left STP, as these asymmetries can lead to increased energy costs in walking, balance deficits, and abnormal joint loading. For instance, when one lower limb is paretic, subjects often attempt to reduce the support on this side. In these cases, increased swing and decreased stance times are seen on the paretic limb side, while typically, there is also a shortened step length on the non-paretic side.
How to calculate gait asymmetry
The first way to calculate gait asymmetries is to measure the difference between the right and left mean values for a given measure. In this method, perfect symmetry would have a value of 0. When looking at left/right difference, the value indicates the direction of the asymmetry, while the absolute value indicates the magnitude of the asymmetry.
Left/Right Difference = Left mean – Right mean
Asymmetries can also be expressed as a percent difference, as with the Asymmetry Index (ASI). This variable is calculated by dividing the difference between the left and right mean values with one half of the sum of the left and right means. Multiplying this value by 100 expresses the asymmetry as a percent difference between the left and right mean values.
ASI = [Left mean – Right Mean]/ [0.5*(Left mean + right mean)]*100
The second way to calculate gait asymmetry is to calculate a ratio between the left and right mean values. The left/right ratio is determined by dividing the left mean value by the right mean value. The value of 1 represents perfect symmetry; if the left mean is larger, the L/R ratio will be greater than 1; if the right mean is larger, the ratio would be less than 1.
L/R Ratio = Left mean/Right mean
Patterson KK, Gage WH, Brooks D, Black SE, McIlroy WE (2010) Evaluation of gait symmetry after stroke: a comparison of current methods and recommendations for standardization. Gait Posture 31:241–246.