# Investigating the Effect of Motion Capture Suits on the Test-Retest Reliability of Gait Parameters
## 4 Discussion
### Minimal Detectable Change  
The MDC, used here as a measure of absolute reliability, allows us to compare the reliability of the suit across literature to evaluate it as a viable alternative to current methods. It also provides a measure of the change in the variable that would need to be seen, in order to enable a research study to conclude that changes were the result of some intervention, rather than differences in marker placement.  
For ankle angle, there are significant differences in the PiG MDC with the non-suited trials (2.32\({}^{\circ}\)) compared to the suited trials (18.90\({}^{\circ}\)). The difference between the conditions, not observed in the ICC values, arises from the inclusion of the standard deviation (SD) when calculating the SEM (Equation (1)), which is in turn used to calculate the MDC (Equation (2)). We see smaller and non-significant differences between the knee (non-suited = 2.06\({}^{\circ}\), suited = 3.98\({}^{\circ}\)) and hip (non-suited = 1.88, suited = 1.71) angles across conditions.  
A systematic review of MOCAP reliability suggested that an acceptable limit of measurement error was up to 5\({}^{\circ}\)[1]. To this standard, all three PiG joint angles in the non-suited condition, the knee and hip angles in the suited condition and all CGM2 joint angles, both suited and non-suited, had an MDC below this 5\({}^{\circ}\) threshold. The PiG suited ankle MDC is almost 4\(\times\) the limit, making it inappropriate for establishing changes in ankle joint angles. Arguably, when collecting clinical data, even 5\({}^{\circ}\) is insufficient. Studies using footwear interventions [7,8] have reported subtle changes in ankle dorsiflexion of 3.9\({}^{\circ}\) due to the addition of a heel lift [8]. Another prospective study [27] compared the running data of females with lioithial band syndrome and healthy controls and found differences of 3.5\({}^{\circ}\) and 3.7\({}^{\circ}\) in peak hip induction and knee internal rotation angles, respectively [27]. These studies [7,8,27,28] highlight the need to push reliability standards of MOCAP further to ensure detection of small changes, and further highlight that the PiG model should be used with extreme caution when in conjunction with MOCAP suits. When comparing the MDC values with [PERSON] et al. (2023) [4], ankle MDC in the PiG model was reported between 3.5 and 6.8\({}^{\circ}\), which is slightly larger than the average non-suited ankle MDC in the present study and comparable to the suited alternative ankle MDC, but much smaller than the suited ankle MDC (18.90\({}^{\circ}\)). Using PiG at the knee, [PERSON] et al. (2023) [4] reported MDCangles between 4.8 and 6.4\({}^{\circ}\), which is larger than the average MDCs reported in the present study in both suited (2.32\({}^{\circ}\)) and non-suited conditions (1.20\({}^{\circ}\)). In the hip, the average MDC angle reported was 7.83\({}^{\circ}\)[4], much larger than the Hip MDCs in both conditions reported in the present study (1.71-1.88\({}^{\circ}\)).  
The CGM2 model provides a maximum MDC of 4.21\({}^{\circ}\) at the knee during HS. This is, however, an outlier, with the remaining MDC values between 0.79\({}^{\circ}\) and 2.62\({}^{\circ}\) across all joints and conditions. As a result, the CGM2 model provides improved reliability when compared to the PiG model. These MDC figures, well below the 5\({}^{\circ}\) acceptable limit of measurement error [1], allow us to recommend the CGM2 model as a suitable method for collecting ankle, knee and hip sagittal plane data with and without the use of a MOCAP suit. This can be compared to [PERSON] et al. (2023) [4], whose average MDCs of the knee and hip across all three measurements were 5.1\({}^{\circ}\) at the hip and 3.2\({}^{\circ}\) at the knee compared to an average MDC of 1.35 at the non-suited hip and 1.45\({}^{\circ}\) at the suited. At the knee, the average MDC was lower again in the present study, with the average MDC at 1.20\({}^{\circ}\) in the non-suited and 2.32\({}^{\circ}\) in the suited condition.