## 4 Discussion  
Our review presented the state-of-the-art of VR technology for training WUs, with the aim of finding how it can complement real-life training. Our first objective is to identify gaps of real-life training methods that can be addressed by VR. We found that real-life training is often very controlled, and rarely the user is exposed to environments that allow to practice everyday skills. In terms of standardised assessment methods, the WSTP has been proven to be more effective than other training programs, however the evidence only supports this for the case of manual WUs. There are fewer studies that used WSTP for powered WUs, therefore there is a need to do more research on training methods for this population. Furthermore, the WSTP does not train the user to navigate around real-life environments, and distractions and obstacles are minimal [21]. VR could mitigate these limitations given that it is a more resource efficient training system, through being more cost effective and allowing the training of driving skills in virtual \"dangerous\" environments without putting the user in danger. Furthermore, as the WSTP has been proven to be effective for manual WUs in particular [22, 23], and most VR training systems are designed for powered wheelchairs, it is reasonable to suggest that VR training systems may more suitable for powered WUs. In this regard, VR could complement real life training by developing VR systems based on the WSTP [21] for powered WUs. \begin{table}
\begin{tabular}{l l l} \hline \multicolumn{1}{c}{**Study**} & \multicolumn{1}{c}{**Limitations**} \\ \hline \multirow{4}{*}{[PERSON] et al. [55]} & \(\bullet\) & WST-Q capacity score were unexpectedly high at baseline for both groups. \\  & \(\bullet\) & Low sample size made of inexperienced users. \\  & \(\bullet\) & Lack of long-term follow up. \\  & \(\bullet\) & Lack of control group driving in the real world. \\ \hline [PERSON] et al. [38] & \(\bullet\) & Not mentioned. \\ \hline [PERSON] et al. [32] & \(\bullet\) & Not mentioned. \\ \hline \multirow{4}{*}{[PERSON] et al. [43]} & \(\bullet\) & Open-label, single-arm study design used \\  & \(\bullet\) & Lack of a control group and randomization in the study design. \\  & \(\bullet\) & Assessors were not blinded during administration of the outcome measures. \\  & \(\bullet\) & Short duration of the study. \\  & \(\bullet\) & Did not address intervention fidelity. \\ \hline \multirow{2}{*}{[PERSON] et al. [34]} & \(\bullet\) & Short study duration. \\  & \(\bullet\) & Small sample size made of non-disabled participants. \\ \hline [PERSON] et al. [35] & \(\bullet\) & Not mentioned. \\ \hline \multirow{3}{*}{[PERSON] et al. [40]} & \(\bullet\) & Small sample size. \\  & \(\bullet\) & Some comparisons of the study did not survive this procedure, mainly for the ALP \\  & & outcome measure. \\ \hline \multirow{4}{*}{[PERSON] et al. [44]} & \(\bullet\) & Small sample size. \\  & \(\bullet\) & Lack of appropriate skill analysis protocols. \\  & \(\bullet\) & The use of EPW model which does not have diagonal movements. \\ \hline \multirow{2}{*}{[PERSON] et al. [46]} & \(\bullet\) & Imbalance in the participant small in terms of numbers and gender and the lack of \\  & & participants with impairment. \\ \hline \multirow{2}{*}{[PERSON] et al. [41]} & \(\bullet\) & Lack of a better evaluation. \\  & \(\bullet\) & Bias of evaluation. \\ \hline \end{tabular}
\end{table} 
Table 5: _Cont._Our second objective is to understand the main features of the VR training system. These include the virtual environment design, the hardware and the way the effectiveness of the system is assessed. However, due to the high design heterogeneity amongst VR studies, including differences between tasks and assessment methods, it is currently unclear what methods are more effective than others. Importantly, the target audience needs to be kept in mind when developing a VR environment, in fact, different users may well have completely different disabilities and therefore needs.