James F. Knight and Chris Baber. School of Electrical Engineering. The University of Birmingham, Birmingham, UK, B 15 21T. E-mail: j .f.knight. 20 @ bham.ac.
Wearable Computers and the Possible Development of Musculoskeletal Disorders James F. Knight and Chris Baber School of Electrical Engineering The University of Birmingham, Birmingham, UK, B 15 21T E-mail: j .f.knight. 20 @ bham.ac.uk static positions (neutral, extended 3Odeg and flexed 30 deg) under different head load conditions. There were five head load conditions: unweighted, wearing a helmet, wearing a helmet +0.5Kg frontal load, +1Kg frontal load and +2Kg frontal load. The EMG data showed that changes in head position resulted in changes in trapezius muscle activity. Across the load conditions flexing the head increased the EMG activity by 14f3% from the neutral position value whereas extending the head decreased the EMG activity by 11+2% from the neutral position value. Within each head position added load to the front of the helmet resulted in an increase in trapezius EMG activity (figure 1). This increase due to load was most noticeable in the flexed and neutral head positions.
Abstract Work-related musculoskeletal disorders (WRMDs) have been shown to be caused by (anwngst other things) exposure to work tasks which involve constrained pathomechanical body postures and physical load. Current research suggests that the added weight of a wearable computer and the positions which interacting with it requires, induces acute musculskeletal stress and may lead to chronic musculoskeletal disorders. These disorders may only reveal themselves after a long period of time. Therefore it is important at this stage in wearable computer development to address these issues in the hope of diminishing their occurrence.
1. Musculoskeletal disorders
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Figure 1: Tapezius EMG due to head load and position.
3. Arm mounted loads Attaching wearable computers to the forearm is attractive as the mechanics of the arm allow the wearer to move the technology into view and offers easy access to the other arm for tactile interaction. To determine the acute effects of added load to the forearm on the musculoskeletal system EMG of the biceps brachii and the frontal and lateral aspects of the deltoid was recorded when loads were attached to the forearm. Seven subjects held their forearm horizontally with their elbow flexed at 9Odeg and their upper arm flexed and abducted at 45deg.
2. Head mounted loads The head is a common location for attaching computer equipment, primarily in the form of head mounted displays. To determine the affects of added load to the head on the musculoskeletal system of the neck electromyography (EMG) of the trapezius muscle was recorded for seven subjects as the head was held in three
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The aetiology of musculoskeletal disorders (MSDs) is multifactorial but one of the main causes is physical workload [l]. The physical workload being the internal force generated across a body joint as a muscle contracts to move a body part or hold it in a static posture working against an external force (e.g. gravity). The weight of a wearable computer and the postures adopted to enable interaction with it may generate a considerable physical workload. Through repetitive usage this increased physical workload may have a cumulative detrimental effect and lead to the development of symptoms characteristic of MSDs (i.e. pain, stiffness, tenderness, fatigue and swelling). To determine if wearable computer usage places the wearer at risk of developing MSDs short term data was collected to quantify changes in musculsoskeletal stresses due to posture and added load.
holding the arm in the test position. Perceptions of exertion and discomfort were also found to increase in the neck, forearm, elbow, and back of the shoulder.
This arm position was adopted under seven load conditions (no weight, 0.34Kg, 0.68Kg, 1.02Kg, 1.36Kg, 1.70Kg and 2.04Kg). Figure 2 shows the EMG of the biceps, deltoid-lateral and deltoid-frontal as the percentage of an EMG value recorded when the subjects performed a maximal voluntary contraction (MVC) prior to the test. A critical figure for endurance in static loading has been estimated to be below 10%MVC [2]. Levels of EMG activity above this value suggest that the muscle is contacting at a level which will fatigue it rapidly. The biceps and deltoidlateral cross this level when the load on the arm is greater than 0.7Kg. For the deltoid-frontal merely holding the arm infront of the body generates levels of EMG activity that suggest a rapid commencement of fatigue. Adding load to the arm will therefore hasten the onset, and increase the rate of, muscle fatigue.
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The data from the head and arm load studies showed that adopting postures, which enable wearable computer interaction, resulted in increased muscular activity in the muscles responsible for maintaining that posture. When a load was then added to the body the muscle activity further increased. Increased and sustained muscular contraction increases the rate of localised muscle fatigue, tendon tension, and impedes blood perfusion. If the working muscle is not adequately perfused, concentrations of noxious by-products increase, energy stores become depleted and acidity levels increase. These can all have serious long-term detrimental effects to the muscle. One of the main characteristics of MSDs is that they develop over a long period of time [4]. This temporal component suggests that in the short-term acute exposure to wearable computers may not cause problems. In the long-term though, prolonged or repetitive wearable computer usage might have a cumulative affect and lead to the development of MSDs. We propose the term OB ALDs (On-Body Attached Load Disorders) to classify MSDs whose aetiology is due to wearable computers. Prospective epidemiological study methods provide the best information for determining causal relations between proposed risk factors and new (incident) cases of disease [5].It is proposed that these techniques should be incorporated into fields where wearable computers are used. By doing this, it may be possible to determine if wearable computers generate OBALDs, and if so, determine what aspects of their usage are primarily responsible (e.g. posture, weight, repetitive movement).
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4. Implications for wearable computers
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Figure 2: Arm EMG due to added forearm load. During the arm load testing perceptions of discomfort and pain for areas around the arm and shoulder (neck, forearm, elbow, upper arm (U.A), front of the shoulder (S.F), side of the shoulder (S.S) and back of the shoulder (S.B)) were measured using the Borg CR-10 scale [3].
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5. References [ l ] Winkel, J., & Mathiassen, S. E. Assessment of physical work load in epidemiological studies: concepts, issues and operational considerations. Ergonomics, 1994, 37(6) 979-988. [2] Hamilton, N. Source document position as it effects head position and neck muscle tension. Ergonomics, 1996, 39(4) 593610. [3] Noble, B.J., & Robertson, R.J. Perceived exertion. Human Kinetics. Champaign, Illinois. 1996. [4] Grieco, A. Molteni, G., De Vito, G., & Sias, N. Epidemiology of musculoskeletal disorders due to biomechanical overload. Ergonomics, 1998,41(9) 1253-1260. [5] Sorock, G.S. and Courtney,T. K. Epidemiological concerns for ergonomists: illustrations from the musculoskeletal disorder literature. Ergonomics, 1996,39(4)562-578.
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Figure 3: Pain perceptions in the arm and shoulder Due to the complex structure of the arm and shoulder the Borg CR-10 data (figure 3 ) shows that adding weight to the arm did not just affect the main muscles involved in
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