# Arm Posture Changes and Influences on Hand Controller Interaction Evaluation in Virtual Reality
## 6 Discussion  
Through a hand-controller target-acquisition experiment in a VR environment, this paper proves that the arm's biomechanical structure and its kinematic features are critical factors, having a close correlation with hand-controller interaction performance, in terms of both overall task efficiency, accuracy and user-perceived comfort. On the basis of earlier research, such as [PERSON] et al. [14] and [PERSON] et al. [21], this study extends and gains a more comprehensive understanding about arm posture change and its dependent changes in perceived arm fatigue and user experience, which is beneficial for developing a more scientific metric for quantitatively measuring user comfort and overall user experience quality. Neurophysiological studies revealed that human visual perception had a close correlation and a dominant effect on visually guided hand operations, which were widely believed to explain the hand interaction difference in upper and lower visual fields [38,39]. However, in this study, we suppose that it is the more obviously perceived arm fatigue in an upraised arm posture which generates a lower hand interaction accuracy than that through a downward arm posture. From this perspective, this study provides new evidence for explaining and predicting hand interaction differences at distinct heights. From an ergonomical evaluation perspective, this study also provides a new view of the evaluation method of hand-controller interaction. Although Fitts' law [36] is the most classical and universally accepted theoretical model in earlier HCI evaluation practices [21,22,28,35], it is little concerned with the handedness characteristic and hand choice and their influences on the interaction performance that have been revealed in this study. Therefore, the newly achieved findings will also enlighten subsequent researchers to propose an updated Fitts' law model that will be more applicable to hand-controller interaction evaluation in a 3D space. Given all the experimental findings, several featured contributions of this study are discussed, including suggestions and strategies that are universally applicable for designing more efficient and user-satisfying interactive spaces in VR, as well as implications for developing more effective and comprehensive metrics in hand-controller interaction evaluation. First, biomechanical structure and kinematic features of the user's arm, e.g., the arm's movement accuracy, comfort level and perceived fatigue level at different arm postures, were insufficiently considered in earlier ergonomical evaluations in a VR environment. However, in this study, all these characteristics were considered and measured. From this perspective, when designing 3 DUIs in VR applications, more ergonomical factors and evaluation metrics should be considered simultaneously. In particular, (a) the handedness characteristic and the hand-use choice effect should be taken into consideration in hand-controller interaction evaluations; (b) apart from the traditional metrics of interaction efficiency and accuracy, the arm's operational persistence, or task duration without hand alternations, is another performance metric which should also be measured in hand-controller interaction evaluations; moreover, (c) interaction naturalness, user immersion, physical effort and comfort level are all important metrics which should be comprehensively considered. Second, this study revealed that hand-controller interaction in some spaces is more flexible and accurate than in other spaces. In general, hand-controller interaction at waist height has a higher accuracy than that at shoulder height; hand-controller interaction at shoulder height generates a higher accuracy than that above head height. We named this result the 'hand-operational height effect'. Moreover, the left arm and hand interact more accurately on the left side of the body, but the right arm and hand interact more accurately on the right side of the body.