# Validation of a Horizontally Dynamic Armrest for Joystick Controlled Mobile Equipment
## 4 Discussion
In addition, the introduction of inward-outward dynamics to the fore-aft dynamic armrest, removed the previously observed improvements in muscle activation [27], thus negating any benefits gained via the fore-aft pendulating mechanism. Therefore, on the basis of the EMG measures, the DA was not successful in reducing muscle activation levels over the NA and SA conditions. Interestingly, regardless of armrest used, the joystick input force in the fore and outward directions was about three times larger than the force exerted in the inward and aft directions. The elevated joystick input forces observed during outward movements were likely due to participants hitting the end stops harder during outward movements than inward movements. Although the design of the dynamic armrest does not restrict range of motion, the differences in joystick input force may be due to differences in upper-limb joint range of motion dependent on joystick movement speed [36] and direction [12]. This increase in force may be due to the added mass resulting from the horizontally dynamic armrest prototype, as participants had to overcome its inertia to complete the joystick throw. This could possibly be reduced in future prototypes by reducing the mass of the horizontally dynamic armrest portion, which could be achieved by using a lightweight composite such as carbon fiber instead of aluminum. Questionnaire results showed that the DA was perceived to be significantly more comfortable than NA. Most participants preferred the DA and rated it as more effective and requiring less effort than the SA. Although participants indicated preference for the DA, these preferences may be due to individual support needs [37; 38] as improvements were not reflected in the EMG measures. Therefore, the inward-outward addition to the fore-aft dynamic armrest is still considered unsuccessful. As is the case with most investigations, there were a number of limitations. Participants were not familiar with joystick operation and a strap was used to secure the dynamic forearm support to the arm to ensure good coupling and constant positioning with the armrest. It is also probable that skilled operators would not slam the joystick to the hard end position as was observed with outward and fore movements. However, it could certainly be representative of people training to use joysticks. Furthermore, the short exposure duration to each design does reduce the value of the subjective ratings obtained from the questionnaire results. Our previous work demonstrated that fore and aft movements are the most problematic from a muscle loading perspective given the reduced support provided during fore joystick motions and constraints provided during aft movements [14]. While the experiment mimicked an excavator seat and controller setup quite well, the task was obviously not completely representative of the actual occupational task. However, prior to conducting field tests using skilled machine operators, the authors felt that it was critical to establish the efficacy of the armrest under rigorously controlled conditions. Lastly, 15 unskilled participants were utilized in this study due to their availability.