FACTORS OF DEVELOPMENT OF THE MUSCULOSKELETAL DISORDERS AS SOURCES OF EXPOSURE RELATED TO THE TASKS PERFORMED IN THE WORKPLACE

ERGONOMICS

The scope of work in ergonomic activities includes the relationship between the tasks performed by the worker in the workplace, the means used to perform this work and the working environment. In terms of practical activities, two directions of ergonomics are distinguished: preventive (design) ergonomics and corrective ergonomics. Preventive ergonomics is responsible for optimisation of the human-technician-environment system at the design stage. Corrective ergonomics analyses the existing workplace conditions by evaluating the compatibility of these conditions with the existing requirements. In order for both preventive and corrective ergonomics to meet the requirements of appropriate selection of criteria to ensure safe work, the knowledge of the following fields is necessary:

• physiology of work – in terms of changes in the body during work;
• psychology (engineering) – in terms of the psychological specificity of cognitive processes: reception, processing of information, decision making, mental fatigue processes;
• anthropometry – in terms of measurement characteristics of limb dimensions and ranges of the human body in the workplace;
• biomechanics – in terms of development of forces, body posture and their optimal values during the performance of work.

In the area of ergonomics, the biomechanics of work plays a significant role in determining the requirements to be met under operating conditions in order to ensure such load on the musculoskeletal system that prevents the development of MSDs. Biomechanics is an interdisciplinary field of science combining the knowledge of anatomy, mathematics and physics. The main subject-matter of the biomechanics study is to study the structure of movements of living organisms, in particular humans. One of the many topics in the field of biomechanics are those related to work. The subject of work biomechanics is the examination of the causes and effects of external and internal forces resulting from the work process on the worker.

The biomechanical load is influenced by typical tasks leading to the development of MSDs, such as heavy physical work, e.g. lifting and handling heavy loads, pushing, pulling. However, also office work or work on a production line, packaging of small objects, etc., i.e. work which mainly involves the upper limbs in repetitive sequences of movement and force with a static load of the back causes biomechanical load which may result in musculoskeletal disorders.

The worker’s workload associated with the performed work is closely related to such biomechanical factors as body posture, exerted external force (type, direction of action and force value) and time factor. The time factor can be considered as the holding time of a specific body posture and exerted force, or as the time of repeated sequences of work tasks. These three basic biomechanical factors in combination determine the load of the musculoskeletal system in the biomechanical aspect. The exposure to the environmental factor, such as vibration, also increases the likelihood of developing MSDs.

Mental strain may also cause excessive musculoskeletal load, and thus lead to the development of MSDs. Mental strain is linked to the feeling of stress caused by different psychosocial factors. The impact of different factors on stress – and thus on health – is explained by different models. Among the theoretical models that help to understand and quantify the work strain in relation to the working environment, the Karasek ‘demand – control’ model, later extended to support, is the best tested and validated (Karasek et al., 1998). The psychosocial external factors include: labour monotony, time pressure, concentration, accountability, workload, break opportunities, transparency, control, autonomy and support from colleagues, superiors or family members.

The interaction between these three factors (biomechanical, vibration and mental) is also important. Uncomfortable positions with additional effect of overall body vibration increase the risk of spine pain by up to four times compared to the sitting position. However, the combination of the worker’s exposure to biomechanical and mental loads increases the risk of musculoskeletal injuries.

The multi-factor nature of the development of MSDs, together with the dependencies between the work-related external factors and the worker-specific internal factors, are shown in the Interaction Model. A combination of parameters that describe individual external factors creates an exposure. The model divides the internal structure of the human body into the mental system and musculoskeletal system (Fig. 1). The concept of this model is based on the assumption that biomechanical factors and vibration – and the combination of these factors – directly affect the musculoskeletal system. Psychosocial, biomechanical and vibration factors and their combination affect the psychological system.

Fig. 1. Link between external factors and their impact on the musculoskeletal system and on the mental system. (Based on Roman-Liu, 2013). Notes: E_musc- exposure to the musculoskeletal system; E_ment- exposure to the mental system.

The biomechanical load is closely related to the assumed body posture, the exerted force (type, direction of action and force value) and duration. The relation between the body posture and the exerted force is defined as the unit load. However, the time factor not only refers to the load holding time, the frequency of repetition of the specified tasks, but also to the maximum holding time of a static load or to the maximum time of repetitive work. In order to properly assess the load, it is necessary to consider these three factors together, and it is necessary to assess their cumulative impact.

BODY POSTURE

The body posture is determined by the tasks performed and the spatial structure of the workplace; the spatial structure of the workplace is created by its structure, shape, size and mutual positioning of its elements. For the purposes of assessing the workload of a worker, the part of the workstation that comes into direct touch contact or visual contact with the user, i.e. through the so-called contact points, is important. The reference planes are significant in defining both the posture of the body parts and the contact points (Fig. 2).

Fig. 2. Basic reference planes for defining body posture.

The sagittal plane is a symmetry plane dividing the human body into two parts, one of which is the mirror reflection of the other. This plane divides the body into the right and left parts; it can be determined that this plane passes through the nose. Movement of body parts in the sagittal plane are known as flexion and extension, or dorsiflexion and plantarflexion in the case of feet. The coronal plane is a plane perpendicular to the sagittal plane; for illustrative purposes, it can be determined that this plane passes through the forehead of the human body. The movements in the coronal plane are called abduction (from the centre line of the body) and adduction (towards the centre line of the body). The transverse plane is a plane perpendicular to the sagittal and coronal planes, and parallel to the ground. Movement in this plane is called rotation. However, in the case of the limbs, this movement is referred to as pronation (rotating inward around the longitudinal axes) and supination (rotating outwards around the longitudinal axes), and in the case of the torso and head – as rotation.

Posture is defined by the values of angles in the joints between the body segments involved in the performance of tasks (Fig. 3). The posture of the upper limbs is described by the values of seven angles relating to flexion/extension of the arm (q₁), abduction/adduction of the arm (q₂), rotation of the arm (q₃), flexion at the elbow (q₄), pronation/supination (q₅), abduction/adduction of the wrist (q₆), and flexion/extension of the wrist (q₇) (Roman-Liu, 2015).

It was assumed that the posture of the limb described by the values of angles q₁, q₂, q₃, q₄ affects the load on the back. The posture of the lower limbs is determined only with regard to the sagittal plane, and the angle of the hip joint (Ω) and the angle of the knee joint (Ɵ) are taken into account (Fig. 1). The posture of the lumbar part of the back is defined by the values of angles in the sagittal (α), coronal (β) and transverse (γ) plane, and the posture of the cervical part of the back is defined in a similar manner. The head posture is defined in the same way and with the same parameters in both the sitting and standing positions of the body, i.e. the angle of inclination (τ), flexion (φ) and twist (χ) of the neck. The posture of the lower limbs shall be determined with reference to the sagittal plane. The angle of the hip joint (Ω) and the angle of the knee joint (Θ) are taken into account.

Fig. 3. Graphic illustration of angles to define the posture of: the upper limb (q1 – angle of adduction/abduction of the arm in the transverse plane; q2 – angle of flexion/extension of the arm in the sagittal plane; q3 – angle of rotation of the arm around the axis; q4 – angle of flexion at the elbow; q5 – angle of rotation around the axis of the arm; q6 – angle of abduction/adduction of the wrist; q7 – angle of flexion/extension of the wrist); neck (τ – angle of inclination; φ – angle of flexion; χ – angle of twist); back (α – flexion, β – extension ; γ – rotation); lower limb (Ω – angle of the hip joint; Θ – angle of the knee joint) (Based on Roman-Liu (2015)).

The postureof the body in a given spatial structure of the workplace also depends on the size of the user’s body. For optimal body postures, it is required to adjust the workplace dimensions to the body dimensions of the group of users. It is therefore important that the position of the contact points be adapted to the dimensional characteristics of the target users, often with different body sizes.

The optimal, least burdensome postureof the body is its natural position, i.e. the standing position with the spine straight and the upper limbs lowered along the body. In this position, all angles between individual body parts are equal to zero. The higher the deviation of the body from the natural position, the higher the musculoskeletal load.

EXTERNAL FORCE

In addition to the body postureduring individual work tasks, the forces related to the work performed are an important element of the biomechanics. The forces occurring in the process of human physical activity may be divided into external forces and internal forces relative to the musculoskeletal system. According to this division, external forces are those exerted on the work object, and internal forces cover muscle strength. External forces can be measured experimentally with a number of different instruments, e.g. a dynamometer.

In the workplace, tasks requiring exerting force shall be designed in such a way that they are performed in an optimum manner, taking into account the positions at work, the direction of action and the value of the force, the frequency of its occurrence and the duration of the action. The physical force must be exerted by those muscular groups which are capable of overcoming the value of the external force, which also involves an appropriate body postureduring the performance of a given work task. Account should also be taken of the fact that during work, different muscles and muscle groups should be activated alternately so as not to cause static overloads and muscle fatigue. Therefore, the values of forces exerted by a worker during work should be kept at an acceptable level, which in the case of manual work depends on:

• the factors associated with the means of work (weight, shape, size and location of the object);
• the duration and frequency of repeating the force being exerted;
• the postureof the worker’s body;
• the subjective factors associated with the worker (technique of work, sex, age, health status and training).

The optimal approach to the issue of force in connection with the workplace is to express the force as a relative value, which is dependent on the worker’s capabilities. In this case, the force exerted during the specified task is related to the maximum force of the same type exerted by the worker. Therefore, when considering, for example, the maximum force of the upper limb, account should be taken of the type of force exerted (pulling, pushing, squeezing by hand, etc.) and the posture.

WORK RHYTHM

Even when work is generally considered to be light and causing a low load on the motor system, excessive musculoskeletal load due to improper working technique may occur. In addition to the force exerted and the body postureduring work, the cause for the musculoskeletal disorders is also the work rhythm, i.e. the duration determined by the change in muscle tension.

The static work of muscles is understood as the isometric muscle contraction in which the length of a particular muscle does not change. The constant muscle length can be interpreted as a situation where the postureof the relevant body segments remains constant. From the ergonomic point of view, the static muscle tension is characterised by the duration of that tension for a specified level of force. In practice, there are situations where static work is not performed continuously. An example is work involving short breaks in the activity of the muscle(s) carrying out static work, the so-called micro-breaks. These sequences are defined as intermittent static work, as opposed to continuous static work, when these breaks do not appear or appear relatively rarely. Figure 4 shows the relationship of holding time for a certain level of static and intermittent force.

Fig. 4. Maximum holding time of force depending on the muscle tension level (muscle contraction in %MVC) for continuous static work (stat) and intermittent work (7s of muscle tension and 3s of muscle relaxation)

During the working day, a series of tasks comprising the total load and defined as the work cycle is performed – each task corresponds to the next phase of the cycle.

One work cycle can:

• last the entire work shift, when sequences of tasks are not repeated,
• last a part of the work shift, when sequences of tasks are repeated.

If the work cycle is short, i.e. the specified work task sequence is repeated multiple times, the work is defined as repetitive. The top limbs are particularly exposed to repetitive loads. Repetitive work may be understood as continuous, repetitive loading of the same tissues. The concept of repetitive work in the interpretation given by Kilbom (1994) means that the work is repetitive if it requires repeated execution of ‘similar’ work task cycles. The similarity of the cycles is understood here in terms of time sequences, developed muscle strength and spatial movement characteristics. The repetitiveness is described by the frequency of repetition of specific tasks which are differentiated by the postureof the body parts and the exerted force.

Silverstein et al. (1986) made a division of repetitive work into work with high repetitiveness cycles and wok with low repetitiveness cycles. Cycles of less than 30 seconds, or where during more than 50% of the cycle duration the same basic cycle is performed, were considered as high repetitiveness cycles. A basic cycle is a cycle that consists of a series of repetitive steps during a work cycle. A low repetitiveness cycle is a cycle with a duration of more than 30 seconds, in which less than 50% of the cycle time is devoted to the same sequence of movements.

The body posturetogether with the exerted force at a given moment consists of a biomechanical unit action, i.e. a biomechanical external load E at a given moment t. Changing the postureand/or force also changes the size of this load. Continuous load occurring during work can be considered as quasi static, expressed as a discrete function of time (Fig. 5).

Fig. 5. Schematic representation of the load as a continuous function of time and as quasi static, where the corresponding portion of time is assigned to specific load levels

The performance of each work task requires the involvement of the whole body, provided that different body parts are involved in different ways. Normally, when working in a standing position, there is a physical involvement of the upper limbs, lower limbs and back. However, there are tasks during which only the upper limbs are involved, while the lower limbs and back are statically loaded. There are also tasks that involve only certain parts of the upper limb, such as hand with a fixed arm and forearm. This causes, on the one hand, the loading of certain parts of the body with repetitive work, and on the other hand, the static load of other parts of the body involved in holding the position. Such a situation is referred to as static load or static work.

The holding time of the body postureduring work depends on the postureitself. In their research Miedema et al. (1997) determined the maximum holding time of various body postures (Fig. 10).

Fig. 6. Results of psychophysical examinations providing data on the maximum holding time of selected body parts (Miedema et al. (1997))