This study quantified the effect of screen position on the cervical and lumbar musculoskeletal system of the spine. Seventeen participants performed walking and running under two display position conditions: a standing monitor placed in front of the participant's line of sight and a monitor placed on the treadmill control panel.
R ESEARCH BACKGROUND
Postural changes due to display – static task
The postural changes and muscle activities were compared as a result of the laptop work, which suggested that adjusting the position of the screen and keyboard was effective in the cervical and thoracic flexion angles as well as the related muscle activities (Yadegaripour et al., 2021). Some studies have estimated the gravitational demand using musculoskeletal modeling (Harms-Ringdahl & Schüldt, 1989; Straker et al., 2009; Vasavada et al., 2015).
Postural changes due to display – dynamic task
The previous study showed that the vertical movement of the head is kept to a minimum during postural tasks, including walking, running, jumping and standing on one leg (Pozzo et al., 1989). Changes in postural adaptation pattern as well as tibial and head acceleration were investigated when visual feedback on gaze direction was provided (Hamill et al., 2020).
O BJECTIVE
Consequently, as the visual angle of the gaze trajectory area decreased, peak acceleration and head signal strength decreased. During running, the dynamic loading of the whole body is repeatedly exerted due to foot strike, and if additional postural deformation occurs due to external factors, the muscular demands of the runner may also change.
P ARTICIPANTS
I NSTRUMENTS
- Treadmill
- Monitor displays
- EMG system
- Motion capture system
Three-dimensional kinematics data were collected by the motion capture system (OptiTrack system, NaturalPoint Inc., Corvallis, OR). Three-dimensional data were calculated by Motive software (Motive:Body 2.2.0 Final, NaturalPoint Inc., Corvallis, OR).
E XPERIMENTAL DESIGN
Experimental variables
Average of neck flexion angle (°) Neck mean Average value of neck flexion angle in sagittal plane within each step Median of neck flexion angle (°) Neck med Median value of neck flexion angle in sagittal plane within each step Maximum of neck flexion angle (°) °) Neck max. Maximum value of neck flexion angle in the sagittal plane within each. Mean of trunk flexion angle (°) Trunk avg Average value of trunk flexion angle in sagittal plane within each step Median of trunk flexion angle (°) Trunk med Median value of trunk flexion angle in sagittal plane within each step Maximum of trunk flexion angle (°) °) Trunk max. Maximum value of the trunk flexion angle in the sagittal plane within each. Range of hip flexion angle (°) Hip ROM Range of hip flexion angle in sagittal plane within each step.
Maximum hip flexion angle (°) Hip max Maximum value of hip flexion angle in the sagittal plane within each step Knee flexion angle range (°) Knee ROM Knee flexion angle range in the sagittal plane within each step. Maximum knee flexion angle (°) Maximum knee flexion angle The maximum value of the knee flexion angle in the sagittal plane within each step. The neck and trunk flexion angles were calculated in the sagittal plane relative to a reference angle of 0 degrees with the upright posture as the reference posture (Figure 8).
Trunk flexion angle measured the forward tilt angle from the line connecting the C7 to waist back from vertical.
Experimental procedure
D ATA PROCESSING AND ANALYSIS
Motion data
EMG
Statistical analysis
The case of the same preference for display conditions under walking and running was divided into 6 panel display preferences and 6 front display preferences. When watching the video on the panel screen was significantly larger than on the front screen or without watching. The magnitude and RMS value of the head vertical acceleration was significantly greater under the front display condition than the panel display condition during running.
During the walking task, the 10th, 50th, 90th percentile, and mean EMG amplitude of the NE and ES increased significantly in the panel display condition. During the running task, NE muscle activity, except for the 10th percentile, was significantly higher in the panel display condition (p<0.05). Muscle activity was higher in the NE and ES muscles, especially when viewing the video on the panel screen during the walking task.
Walking and running were performed under two conditions: a front screen in front of the participant's view and a panel screen on the treadmill control panel.
P REFERENCES
Electromyographic sensors were attached to the neck, shoulder, and lower back to confirm the difference in muscle load for each condition. This study is important because it is a comprehensive study of changes in upper body posture, including neck and trunk flexion, and the resulting physical strain in a treadmill environment that is similar to the actual use environment.
K INEMATICS VARIABLES
In previous studies, the bending of the lower back was calculated for the angle of thoracic kyphosis and lumbar lordosis. Except for the angle of neck flexion, no significant difference was found in spatiotemporal gait parameters and kinematics parameters between viewing conditions in the walking task. There were differences in the variability of the spatiotemporal variables for each lower limb and torso on the treadmill compared to ground walking, but the average.
The visual angle of the screen was calculated based on the dimensions of the screen and the position data of the marker on the top edge of the monitor and the marker attached to the subject's forehead. However, in this study, the effect of the display on the change of lower limb and spatiotemporal gait parameters may not have been fully identified because it was a short endurance run of three minutes. It is consistent with the results of the previous study that head acceleration was greater while participants stared at the wall compared to the treadmill display (Lucas-Cuevas et al., 2018).
Depending on the relative position of the neck and head, the moment and dynamic loads transferred to the neck and head are also likely to be different.
EMG VARIABLES
The weakening of impact loads is achieved by the deformation of soft tissues or contraction of muscles associated with the movement. There was a significant difference between the neck angle under the front panel display and the panel display, and the RMS value of the head under the panel display was significantly lower than the front display during running, with larger impact loads occur than walking. It could be interpreted that the neck muscles play a major role in reducing the dynamic loads associated with the vertical sway imposed on the neck when the user looks head-down at the panel display.
On the contrary, during running, trunk flexion was significantly higher under the panel display condition, and ES muscle activity had no significant difference. The contradictory results of trunk flexion and ES muscle activity during walking and running indicate the correlation with the ES muscle activity and the maintenance of trunk flexion posture in the sagittal plane. Walking and running are exercises performed by many people, from amateurs to experts, and they are often performed consistently over a long period of time, not temporary exercise.
Therefore, it is important to understand the additional muscle load due to the display position from a long-term perspective.
R ECOMMENDATIONS
L IMITATIONS AND FURTHER STUDY
Moreover, walking and running are not performed temporarily, but are performed continuously over a long period of time. The task performance with the longer time is necessary to assess the changes over time in the development of muscle fatigue or musculoskeletal discomfort caused by prolonged walking and running under given conditions. Although the results of this study suggest that display position conditions may affect the user's musculoskeletal system, there is a lack of information on the stability of walking and running on the treadmill with the displays.
Variability in kinematic and spatiotemporal gait parameters during walking and running is a factor that may reflect the user's balance ability and postural stability, therefore further research is needed. Examining the coordination of each joint will allow a more comprehensive understanding of walking and running strategies. Finally, the results of this study hypothesized that increased impact was absorbed at the neck and lower back to maintain head stability.
The tibia is considered to accommodate the increased impact to maintain head stability, with the results of a reduction in vertical head accelerations and an increase in vertical tibial accelerations in the same direction (Mangubat et al., 2018).
A PPLICATION
The main purpose of this study is to identify the effect of screen position on walking and running posture and neck, shoulder and back muscle loads on the treadmill. Although both the standing display position in front of the user and the treadmill panel display position are commonly used together in many current environments, it has been confirmed that the display installation position of the typical indoor walking and running environment makes users take different positions physically. The main outcomes of this study are apparent neck flexion and neck muscle activation while using a panel display on a treadmill.
The display attached to the treadmill control panel when walking and running induces neck flexion, suggesting that this change in upper body posture requires additional demands on the neck extensor muscles. During walking, the trunk flexion angle did not differ significantly and the ES muscle activation level was higher in the panel display condition. During running, the trunk angle was higher in the panel display condition, and the ES muscle activation level had no significant difference except for the 10th percentile value.
The conflicting results of trunk flexion and ES muscle activity during walking and running suggest a role for the ES muscle in maintaining trunk flexion posture in the sagittal plane. It is also expected to provide a rationale for selecting the location and design of an optimal treadmill screen. Adjustments of eye posture and viewing parameters to changes in visual display terminal screen height.
