The aim of the present study is to investigate the effects of different keyboard positions on the posture and 3D moments of wrist and elbow joints. Accordingly, the participants were asked to perform standard predefined computer tasks in three keyboard positions. The results showed that the keyboard distance affected the posture and the upper extremity joint moments.
The highest wrist flexion and radial deviation occurred at T1, and with increasing the distance of the keyboard from the body, these values decreased, and the lowest flexion and radial deviation occurred at T2. In this study, wrist flexion decreased by increasing the keyboard distance to 8 cm (T2). This is similar to the results of the study carried out by Kotani et al., in which by increasing the distance, the wrist flexion decreased (17).
In our study, the radial deviation decreased by increasing the distance of the keyboard from the body, as reported by Cook et al. (12). It can be concluded that at T2, the risk of MSDs in wrists and elbows is lower than at the other two distances.
By increasing the keyboard distance from T1 to T2, the ROM of left wrist flexion decreased (P-value < 0.05). In general, the ROM of left wrist flexion decreased as the distance increased (from T1 to T2 and T3), similar to the results of Kotani et al.
When the position of keyboard changed from T1 to T2, the ROM of the left wrist radial deviation decreased significantly (P-value < 0.05), similar to the results of Kotani et al. However, at distance T3, this value slightly increased compared to distance T1 (P-value > 0.05), which was not in agreement with the results of Kotani et al. (17).
By increasing the distance to T2, the left wrist radial deviation decreased significantly (P-value < 0.05). This was consistent with the results of previous studies (12, 17). However, at T3, the radial deviation increased. This part of our results was not in the same line with those of Ketone et al. (17). The study by Lindberg and Qing showed that wrist flexion and extension, even in a short time, can lead to wrist MSDs in prolonged working time (26, 27). As the keyboard distance increased, the ROM decreased, and the lowest value occurred at T2. At the other two distances, the ROM increased.
According to the study carried out by Waersted et al., elbow extension and flexion could contribute to elbow MSDs (3). In the left elbow, the ROM of flexion-extension showed a significant difference at the three keyboard distances (P-value < 0.05). Supination and pronation decreased by increasing the distance of the keyboard from T1 to T2 and form T3 to T2. Still, no significant difference was observed (P-value > 0.05), which was similar to the results of the studies by Marcus et al., Kotani et al., and Cook et al. (10, 12, 17). This can be explained by the fact that all participants were right-handed, and the left arm was passive (28).
There are differences in the wrist and elbow kinematics at the three distances as shown in Tables 1 and 2. These differences might be due to the fact that when the keyboard was moved away from the edge of the desk, the subjects placed their distal forearms on the desk to support their upper arms and shoulders, so wrist flexion, radial deviation, and pronation decreased.
When the keyboard was moved further away, the proximal forearms and the elbows were placed on the desk to support the upper arms and shoulders, so wrist extension, ulnar deviation, and supination increased (12).
The subjective assessment of discomfort showed that when the keyboard was placed on the edge of the desk, the wrists were mostly discomfort, which seemed to be due to excessive flexion of the wrists at this distance. The results of this assessment are similar to those of Marcus et al. (10).
The changes were more significant in kinematic than in kinetic variables. Therefore, the wrist and elbow moments did not show a statistically significant difference at the three distances of keyboard. This is because the active muscles are fixed (static activity) during computer tasks. In other words, by changing the horizontal distance of the keyboard from the edge of the desk, the working muscles have static activity at the beginning and end of the task cycle.
The changes in the angles of the joints, no matter how small, affect the muscles. For more changes in the angles of the joints, and for a longer duration of joint involvement, the muscles are more involved, and the risk of injuries increases. This is because no muscles were deactivated but they acted as constant mover or stabilizer. The results of this part are similar to those of the study by Kotani et al. (17).
In this study, the Opensim software was used to evaluate the joints moments in three dimensions (3D) for making it possible to calculate the moments around the coordinate axes x, y, and z. This study had some limitations, such as testing only right-handed male participants in the age range of 25–30 years, and not considering other workstation elements, e.g., monitor, desk, and chair. Moreover, due to the short duration of the tests, the results may differ in studies with more extended test duration. These limitations should be considered in future studies.