Risk factors - Background information
Frequency and repetition of movements
Work is repetitive when it requires the same muscle groups to be used over and over again during the working day or when it requires frequent movements to be performed for prolonged periods.
Rapid or prolonged repetition may not allow sufficient time for recovery. This can cause muscle fatigue due to depletion of energy and a build up of metabolic waste materials. Repeated loading of soft tissues is also associated with inflammation, degeneration and microscopic changes.
Force can be applied to the muscles, tendons, nerves and joints of the upper limb by:
- handling heavy objects when performing tasks, i.e. an external load;
- fast movement or excessive force generated by the muscles of the body. These are often generated to be transmitted to an external load, such as when trying to undo a stiff bolt;
- local compression and stress from items that come into contact with parts of the upper limb, such as the handle of a pair of pliers digging into the palm of the hand;
- impact or shock, such as when hammering or using the hand as a tool.
Use of excessive force can lead to fatigue and if sustained, to injury, either through a single-event strain injury or through the cumulative effect of the repeated use of such force. Local compression and stress can also cause direct pressure on the nerves and/or blood vessels and increase the risk of discomfort and injury.
The need to grip materials, products or tools is a potential risk factor if excessive force is used.
The amount of force required to grip is influenced by the properties of the objects being gripped. For example;
- The materials where the object is gripped (e.g. a screwdriver handle with a flexible grip requires less force when being used than one with a harder handle;
- The size of the object (e.g. pliers with a handle span that is too wide or too narrow will be more difficult to grip).
Muscle force is greatest when a power grip is used (e.g. gripping a handle in the palm with fingers and thumb). This allows a large surface area of the hand to be used.
When assessing body posture, it is important to understand the concept of a "neutral position".
A neutral position is where the trunk and head are upright, the arms are by the side of the body, forearms are hanging straight or at a right angle to the upper arm, and the hand is in the handshake position.
For example, when a person's arm is hanging straight down with the elbow by the side of the body, the shoulder is in a neutral position. Working postures can increase the risk of injury when they are awkward and/or held for prolonged periods in a static or fixed position.
An awkward posture is where a part of the body is used well beyond its neutral position.
For example, when employees are performing overhead work, their shoulders and arms are far from the neutral position.
When awkward postures are adopted, additional muscular effort is needed to maintain body positions, as muscles are less efficient at the extremes of the joint range. Resulting friction and compression of soft tissue structures can also lead to injury.
Static postures occur when a part of the body is held in a particular position for extended periods of time without the soft tissues being allowed to relax. When holding a box, for example, it is likely that the hands and arms are in a static posture.
Static loading restricts blood flow to the muscles and tendons resulting in less opportunity for recovery and metabolic waste removal. Muscles held in static postures fatigue very quickly.
Duration of exposure
Duration refers to the amount of time that a task is performed. It includes:
- The amount of time that the task is undertaken continuously without a break (e.g. two to three hours)
- The amount of time that the task is undertaken in each shift (e.g. four hours per day)
- The number of working days the task is performed (e.g. five days per week)
The duration of exposure is an important concept in assessing the level of risk. It is generally accepted that many types of ULDs are cumulative in nature. Therefore,when the duration of a person's exposure is increased the risk of injury is increased. This is because when parts of the body undertake work for periods without rest, there may be insufficient time for recovery. Consequently, the amount of time available for the body to recover from a specific task or tasks is important.
Short exposures are unlikely to create a significant risk of injury, except where the task is exceptionally demanding and/or the worker has not been allowed to build up to its demands over a period of time. This can occur after return to work from holidays or with an increase in work pace.
Working environment refers to aspects of the physical work environment that can increase the risk of ULDs. This includes several factors:
Hand-arm vibration results from the use of hand-held power tools and equipment, as well as the use of fixed machinery such as bench grinders where the work piece is held by the worker.
Vibration can increase the risk of ULDs and is known to cause vibration white finger and carpal tunnel syndrome, loss of sense of touch or temperature, painful joints and loss of grip strength.
ART does not specifically cover the assessment of hand-arm vibration, as this is addressed as a separate health topic.
Working in cold temperatures, handling cold products or having cold air blowing on parts of the body can place additional demands on the body.
Exposure to cold can result in decreased blood flow to the hands and upper limbs, decreased sensation and dexterity, decreased maximum grip strength and increased muscle activity (which is part of the body's natural response to being cold).
Exposure to cold may also require the use of personal protective equipment such as thick gloves, which can make handling more difficult, and compound the risk by requiring additional force to grip items.
The visual demands of the task are an important consideration, since a worker's posture can be largely dictated by what they need to see. Dim light, shadow, glare or flickering light can encourage workers to adopt a bent neck and back, and poor shoulder postures in order to see their work.
Further information on lighting can be found in the HSE document "Lighting at Work" (HSG38).
All individuals are different and, for biological reasons, some people will be more or less likely to develop an ULD. Individual factors also play a significant role in how ULD symptoms are reported and managed. Some individual factors that may increase the risk of developing symptoms are below. These should be considered in the system for risk management.
- Differences in competence and skills (e.g. new workers may need time to acquire work skills and pace of work)
- Workers with varying body sizes, which can result in awkward postures at shared workstations
- Age and health status, including a prior history of symptoms
- Individual attitudes which may affect compliance with safe working practices
- Vulnerable groups, such as new and expectant mothers
A worker's psychological response to work and workplace conditions (that is, work as experienced by workers) has an important influence on health and the reporting of symptoms.
These are referred to as psychosocial factors. They include the design, organisation and management of work and the overall social environment in general (the context of work) and also the specific impact of job factors (the content of work). Some examples are:
- Workers have little control over their work and work methods (including shift patterns);
- Workers are not, as a rule, involved in making decisions that affect them;
- Work is machine or system paced (and not monitored appropriately);
- Work is monotonous, and/or workers are not able to make full use of their skills;
- Work demands are perceived to be excessive;
- Work systems limit opportunities for social interaction;
- Payment and incentive systems encourage working too quickly or skipping breaks; and
- High levels of effort are not balanced by sufficient reward (resources, status, self-esteem, remuneration).
Many of the effects of psychosocial factors occur via stress-related processes which include direct biochemical and physiological changes. Also included are instances where individuals try to cope with stressful demands with behaviours that, in the long term, may be detrimental to health. An example is where an individual, because of high workload or deadlines, foregoes the rest breaks to which they are entitled.
It is very likely that physical and psychosocial risk factors combine and that the greatest benefit will be achieved when both are identified and controlled.
Combinations of risk factors
Risk factors commonly interact with each other to create an overall risk of ULDs.
For example, a repetitive task which involves gripping a heavy power tool for six hours would result in an awkward forceful gripping posture and exposure to vibration over a prolonged duration. Repetition, force, working postures, duration, and working environment are all risk factors for injury in this task. In contrast, if this task was only done for a short period of time in each shift (e.g. 10 minutes), then the risk of injury may not be high, even though the other risk factors are still present.
Generally there is an increased risk of injury when there are a number of risk factors acting in combination. However, one risk factor acting alone can create an unacceptable risk of injury if it is sufficiently great in magnitude, frequency or duration.
ART will help identify repetitive tasks that involve significant risks and where to focus risk reduction measures.
Even if you have a good approach to risk assessment, you cannot prevent all upper limb disorders. So, the use of ART, on its own, will not provide the whole solution.
You also need to have suitable systems in place for reporting ULD symptoms and managing any cases of ULDs that may still occur.
HSE guidance document Upper limb disorders in the workplace (HSG60) suggests an approach based on 7 stages in a management cycle.