Postural load in digital device usage and musculoskeletal disorders
Prolonged use of digital devices has been demonstrated to greatly affect the health of workers who use them during their working day. In fact, 62% of such workers eventually end up with neck and shoulder musculoskeletal disorders (MSDs).
The most frequent signs and symptoms include pain, muscle fatigue, headache, muscle imbalances, a tendency to forward head position (FHP) and augmented spinal loads. The exact symptoms depend on how tense people keep their neck when using video displays (VDTs). For example, using a mobile phone requires more tension, increasing the tendency to FHP and augmenting the cervical load compared to situations which require less tension, such as when using computers. According to international standards, MSDs in the neck are mainly caused by high-frequency (more than two per minute) neck movements and/or the neck being held at a fixed position too long.
There are ergonomic measures that minimise neck MSDs and other types of discomfort related to long-lasting use of digital devices. Traditional strategies based on maintaining spine alignment have proved insufficiently effective in situations where the head is held at an angle smaller than 25º to the neck. Researchers therefore suggest taking breaks to perform small head rotations during long-lasting tasks and/or frequent microbreaks to limit neck and shoulder MSDs in heavy users of digital media.
The benefit of frequent microbreaks and regular small head movements is strongly supported by anatomic knowledge of the cervical spine. Studies have shown that, given the mechanical behaviour of the cervical spine and, more precisely, its intervertebral discs, reasonable physiological loads and small, regular head movements helped maintain healthy and functional intervertebral structures. On the other hand, prolonged static load, as well as non-physiological loads and inadequate movement frequencies, were found to cause cell stress and may induce disc degeneration.
Health impact of visual display terminals
The fast spread of digital devices used for longer periods of time has led to major paradigm shifts in work and leisure. Ergonomic recommendations designed to cope with this traditionally focused on reducing physical loads produced by physical/motor efforts.
We all know that lifting a load by straightening out the knees while keeping it close to the body minimises the risk of lower back damage. However, jobs which mainly revolve around using a computer or some other type of VDT have inevitably led to prolonged sedentary sitting postures (1) and related physical consequences.
In fact, musculoskeletal issues (MSDs) are the most prevalent disorders among workers who regularly use VDTs (2). Neck and shoulder MSDs in particular have a significant prevalence of up 62% among people working with VDTs (3).
Experts in workplace health have conducted studies over many years to analyse this issue in depth and understand & address it in an effective manner. The National Institute of Occupational Safety and Health (NIOSH) in the United States published a valuable review called ‘Musculoskeletal Disorders and Workplace Factors’ (4) which found strong evidence for a causal relationship between neck and shoulder MSDs and prolonged static loads during working. Moreover, it found evidence for links between highly repetitive work and neck & shoulder MSDs. The review has also been used as a solid basis for investigations by other authors, all of which verified the relationship between neck MSDs and prolonged static posture during the use of VDTs (2) (5) (6) (7) (8) (9) (10) (11) (12).
The main neck and shoulder symptoms of a prolonges use of visual digital devices
There are several symptoms affecting the neck and shoulders that are related to the intensive use of digital devices: neck and shoulder pain; muscle fatigue; headache; muscle imbalances; tendency to forward head position (FHP); and augmented spinal load. These symptoms are caused by all video displays (VDTs) without exception. However, the tendency to move the head forward, thus increasing spinal load, mainly depends on neck flexion (8) (13). Figure 1 (12) shows that increases in cervical spine load are related to the increase in neck flexion caused especially by the use of mobile devices such as smartphones.
Figure 1 - Cervical spine loads depending on neck flexion
Mobile phones and other VDTs that tend to be located below the visual horizontal plane for a long time and require neck flexion of 20-25º or more are likely to result in high loads and lead to major cervical problems. Computer displays require less neck flexion and cause considerably lower cervical loads. But they do still lead to neck and shoulder MSDs in computer users – where does this effect come from? As described above, it’s caused by maintaining a static posture for long periods of time. Even if posture is close to being aligned during computer use (with an angle of 25º or less), severe health consequences can still arise.
ISO 1126 and EN 1005-4:2005+A1:2008 (14), (15) are international standards related to the evaluation of movement and static working posture. Figure 2, extracted from EN 1005- 4:2005+A1:2008, describes a qualitative model which shows that health risks increase when a static posture is adopted or frequent movements take place at work. Conversely, low-frequency movements are recommended to reduce health risk.
Figure 2 - Risk related to work posture
The frequency of movement is defined as the number of times a body part moves per minute. According to this standard, more than two movements per minute is a high frequency while fewer than two per minute are a low frequency. A quantitative approach provided by ISO 1126 (Figure 3) determines whether static working postures are acceptable. In general, postures are classified as acceptable or not recommended depending on the angle (measured in degrees) of the different joints and the time during which it is kept static.
Figure 3 - Risk depending on neck posture and maintained time
Where the position of the neck is concerned, holding a static posture over eight minutes (this is called the maximum acceptable holding time) is not recommended regardless of the amplitude of the neck flexion. The acceptability of neck flexion positions over 25º depends on the holding time: the higher the flexion, the lower the acceptable time.
In the case of static postures for computer users, the acceptable range of flexion is between 0 and 25º. All computer workers at any company exceed the holding time of eight minutes during their working day, which means a static working posture is not recommended. Maintaining the neck in this position (0-25º) for less than eight minutes is considered acceptable as long as trunk flexion and head inclination are close to 0º (this should be the case for a well-designed computer workstation).
To summarise, scientific literature and international standards agree that there are harmful effects for the health of computer users in maintaining a static neck posture during long periods of time, even if neck flexion is lower than 25º. This is why neither static nor high-frequency neck movements are recommended.
Current employment situation and neck MSDs
Technological advancements in today’s workplaces mean that employees essentially spend their working day in front of a computer. Almost every task carried out in a company can be executed by a computer without the human operator needing to stand up. It is hard to imagine an office job which doesn’t involve a computer. This fact, together with the increase in remote work, means that work tasks are now faster, more efficient and more profitable than ever before (16). And since this type of technology is here to stay for the foreseeable future, we need to pay close attention to how to optimise computer workstation designs (17).
As previously stated, prolonged static postures have damaging consequences for the neck and shoulders of computer workers. Ergonomic actions and measurements to improve workstation design features and minimise MSDs and discomfort have been favourable to reducing low and middle back pain and lower limb discomfort. However, workstation redesign has appeared to be ineffective in completely eliminating neck and upper limb MSDs (18). Strategies focused on reducing the intensity of the load on the spine are based on achieving spine alignment (this is the reason why traditional ergonomic strategies have been beneficial for the lower and middle back and lower extremities).
Where the neck is concerned, the circumstances change: when people use computers, their necks are close to being aligned (i.e. held at an angle of less than 25º); thus, the loads in that region are not remarkable (as shown in the previous paragraph) and not responsible for neck MSDs. To obtain effective strategies for avoiding neck and shoulder MSDs, researchers instead suggest changes in the temporal pattern of work tasks, such as promoting task rotations or performing additional and frequent microbreaks (2) (12) (19).
Changes in static posture are recommended to reduce soft tissue neck loads and relief muscle fatigue. Moreover, these movements also stimulate blood flow to cervical intervertebral discs (20) (21). Understanding the effects of a long-lasting static posture on the soft tissues of the neck requires in-depth knowledge about cervical anatomy.
How is anatomy related to neck MSDs in computer users?
The spine is the structural and functional central axis of the body (Figure 4). It is composed of 33 vertebrae divided into 4 segments: cervical (7 vertebrae), thoracic (12 vertebrae), lumbar (5 vertebrae), sacral (5 vertebrae fused into the sacrum bone) and coccygeal (4 vertebrae fused into the coccyx bone).
The spine as a mechanical structure
Mechanically, the spine is a well-designed structure. The bones are complemented by a variety of soft tissues (intervertebral discs (IVDs), ligaments and muscles) that give it the stability and stiffness necessary to support the mass of the body and protect the spinal cord while enabling the flexibility required for movements in three dimensions.
Intervertebral discs as load absorber
The IVDs are composed mainly of connective tissue and water and located between all vertebrae except the first and second cervical. IVDs play an essential mechanical role within the spine, including load cushioning, reducing mechanical stress caused by impact (acting as shock absorbers), allowing for the movement of individual vertebrae, weight dispersion, enabling the passage of nutrients and fluids to the spine and spinal cord, and the elimination of waste products.
The thickness of individual IVDs depends on the spine segment. Lumbar IVDs are around 9 mm thick, while thoracic IVDs are 5 mm and cervical IVDs 3 mm. The proportion between the thickness of IVDs and the height of the vertebrae gives an idea of the mobility of the spine segments that are not fused (cervical, thoracic and lumbar): the larger the proportion, the more mobile the segment. The cervical segment has the shortest vertebrae, so this proportion is the highest in the whole spine. From an anatomical point of view, the cervical segment is the most mobile part of the spine, followed by the lumbar and thoracic spine respectively (22).
Importance of blood flow to the IVDs
IVDs are the largest avascular elements of the human body. In the early stages of skeletal development, blood and lymph vessels are part of the very structure of the IVDs, but through the maturation process they tend to decrease and migrate to the outer part of the IVDs. The flow of nutrients and waste products within IVDs therefore takes place via diffusion and convection (23).
Due to the unavoidable effect of gravity, IVDs are continuously subjected to a load in the vertical plane (if in a standing or sitting position). Recent studies suggest that a dynamic mechanical load is essential to maintain the balance of fluids inside the IVDs. This load acts as a stimulus to regulate the physiological function of the discs because of the lack of blood vessels inside them.
However, it is well-known that a prolonged load on the spine promotes the degeneration of IVDs. In vitro experiments in mechanobiology affirm that a dynamic load from 0.2 to 0.6 MPa (through the day, the pressure on the IVDs gradually increases due to the upright position and reduction in horizontal posture) at a physiological frequency of 0.2 to 1 Hz is essential to maintaining the health and function of IVDs because it facilitates diffusion. Prolonged static loading, on the other hand, as well as non-physiological loads and inadequate head movement frequencies, cause cell stress that may lead to disc degeneration (24).
In conclusion, the cervical spine is the most mobile segment within the spine, so maintaining long-lasting static postures goes against its anatomical nature, as it were. Physiological loads and small movements are also required to keep IVDs in good conditions. Taking breaks during periods of static posture and performing small movements at a low frequency are therefore important to the health of IVDs over time. Various studies have demonstrated the relationship between the time of exposure to computer work and pain & discomfort reported by workers. In most of them, workers who take limited breaks showed greater symptoms of MSDs (18). De Vera et al. (2007) (25) confirmed without doubt that micro-breaks and regular head movements significantly improved work comfort during long-lasting tasks on digital devices.
- Intensive use of mobile digital devices may lead to musculoskeletal disorders induced by the increase of spinal load.
- Even with a comfortable head position, maintaining an overly static posture for long periods of time gives rise to neck and shoulder strains.
- Long-lasting static postures act against the anatomical nature of the cervical spine.
- Dynamic mechanical load is essential to manage the balance of fluids inside the intervertebral discs.
- Regular small head movements and microbreaks are mandatory to preserve healthy intervertebral discs and postural comfort during long-lasting use of digital devices.