# Use of Wearable Sensor Technology in Gait, Balance, and Range of Motion Analysis
## 5 A Taxonomy for Gait, Balance, RoM Analysis
### Gait Analysis
#### 5.1.5 Gait Analysis: Asymmetry  
Gait asymmetry can be expressed in two different ways: _temporal asymmetry_ and _spatial asymmetry_[104]. In spatial asymmetry, the step length values are unequal, while in temporal asymmetry there is a difference in time spent in swing and/or stance phase between the two feet. In a temporal symmetric walking pattern, the steps and strides are equal. In the case of a temporal asymmetric walking pattern, step lengths are different between the two legs but stride lengths are equal. Two common ways to represent symmetry are by differences or ratios [37,48]. In differences, the values are subtracted (i.e., left-right) from each other, in which 0 represents perfect symmetry. Using ratios, the values are divided (e.g., left/right), in which 1 represents perfect symmetry.  
Another two approaches have been used to represent symmetry: Dynamic Time Warping (DTW) algorithm and Autocorrelation. DTW is popular because it is extremely efficient in measuring  
Figure 4: Step Deviation Calculation Visualization.  
Figure 3: FGA Walking Path with 4-Level Deviation Scale.  
time-series similarities, thus minimizing the effects of shifting and distortion in time, and allowing transformation of time series in order to detect similar shapes with different phases [120]. The algorithm can be applied by comparing two acceleration signals of different steps to see if there are any differences between one step and the other.  
Autocorrelation is the correlation between a signal with a delay copy of itself and has been widely used to find repeated patterns in a signal [118; 119]. Accelerometer signals from wearable sensors provide information to be used in the algorithm. Researchers have demonstrated that low values in the coefficient of the first and second dominant period represent a low regularity between steps and cycles and the ratio of both coefficients represents symmetry between left and right steps [121]. Researchers used these techniques to check similarities between different steps and to capture a walking pattern problem if an impediment was present [54; 118; 119].  
Gait asymmetry has shown to be an important marker of mobility impairment [64]. Recently, a Gait Symmetry Index (GSI) that uses one accelerometer placed at the lower back was proposed [64]. GSI uses autocorrelation coefficients of vertical (\(AR_{\var  
the subject's locomotion quantitatively. By combining both techniques, researchers can conduct these tests by using instruments, such as the Romberg Test or Limits of Stability Test with force platforms, in order to have a complete assessment with both components: rating scales/scores and quantitative data about the subject's locomotion. Force platforms are the most common sensor for instrumented balance assessment. They quantitatively measure center of pressure and center of mass displacement of a subject while the subject is standing on the force platform performing static and/or dynamic tasks, such as Romberg Test. The problem with force platforms is that they are often expensive and are not portable.  
Researchers have begun to integrate the use of wearable sensors for balance assessment because they are more portable and less expensive. Non-wearable instrumented tests, such as dynamic posturography, and non-instrumented tests are still the gold standard methodologies to assess balance. They are often used to evaluate performance of the wearable sensors systems. For example, researchers used wearable sensors to measure subject's balance as they were performing the BBS [60]. Another study determined a smartphone could measure static postural stability and distinguish elderly at risk to fall, and they validated the performance of the smart device using a force platform [68]. Additionally, other balance studies have used wearable sensors during gait activities to provide information about subject's dynamic balance [24; 49; 50].