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"Biomechanics utilizes the laws of Physics and the concepts of Engineering to describe the motion undergone by various body segments and the forces acting on these body parts during normal daily activities". Biomechanics was defined by Frankel and Nordin in 1980(cited by Chaffin and Andresson, 1984). As Biomechanics relates to the muscles skeletal system, it can be applied to improve the work station of the occupants (occupational biomechanics). Anthropometry is the empirical science which attempts to define reliable physical measures of a person's size or body dimension. Work station designs and almost human appliances are largely base on this data.
Figure 1: Sitting anthropometric dimensions
Definitions of sitting anthropometric dimensions displayed in above figure. (Chaffin and Andersson, 1984)
Sitting height: The vertical distance from the floor to the horizontal midsection of the back of the thigh of a subject sitting with the thigh in contact with the seat.
Elbow height: The vertical distance from the floor on the posterior tip of the olecranon when the arm is flex to 90o at the elbow and the shoulder is the 0 position. Can be measured from sitting height.
Thigh height: The vertical height from the floor to the highest part of the thigh. Can be measured from sitting height.
Patella height: The vertical distance from the floor to the superior tip of patella.
Orbital height (eye height): The vertical distance from the floor to orbit when sitting with the spine straight. Can be measured from sitting height.
Shoulder height: The vertical distance from the floor to the superior aspect of the acromion. Can be measured from sitting height.
Abdominal depth (buttock-abdomen): The sagittal distance from the anterior abdominal wall to the posterior part of the buttock.
External sitting depth (buttock-patella): The sagittal distance from the anterior aspect of the knee to the posterior part of the buttock.
Internal sitting depth (buttock-patella): The sagittal distance from the anterior aspect of the fold to the posterior aspect of the buttock.
Table 1: Vertical anthropometic measurement in sitting postures (Chaffin and Andersson, 1984)
When study about biomechanics not only need to know about human body but also need to know about the basic concepts in mechanics that should be known are Newton's laws, force, buckling, moment and static equilibrium.
A force can move an object, prevent an object from moving and can deform its shape. Force has both magnitude and direction and it can be compressive, tensile or shear, depending on the direction of application.
Compressive force Tensile force Shear force
Figure 2: Examples of forces
Stable or balance seating is achieved if there is a balance of force and moment in all planes. For generally users, if they feel not comfortable with their seats they will stabilise themselves. In addition, the sitting postures are varied in different works such as work with desk, drive or work with computer.
According to the location of the centre of mass of the body, Schoberth (1962) characterised the sitting posture as three types: anterior, middle and posterior sitting posture (cited by Chaffin and Andresson, 1984). These postures differ with the location of the center of body mass, which affects the distribution of body weight placed on the spine and feet. Each posture can also be defined according to the shape of the lumbar spine.
Figure 3: Lateral view of the pelvis and cocyx (Foye, 2009). http:/emedicine.medscape.com/article/309486-media
Figure 4: Sitting posture
Figure 5: Rotation of pelvis and lumbar spine
Table 1: Characteristics of sitting posture (Zacharkow, 1988).
- Angle between the thigh and body is less than a right angle.
- CG in front of the ischial tuberosities.
- Forward rotation of the pelvis with lumbar spine straight or slight kyphosis
- Angle between the thigh and body is 90 degree.
- CG directly above the ischial tuberosities.
- The lumbar spine straight or slight lordosis
- Angle between the thigh and body is an obtuse angle or more than a right angle.
- CG behind the ischial tuberosities.
- Backward rotation of the pelvis with kyphosis or lordosis lumbar spine
As to the angle of the backrest of the chair a lot of studies have reported the decreasing of myoelectric activity in the thoracic and lumbar regions of the back and decrease disc pressure in the lumbar spine when the backrest is inclined 95 - 130 o. The backrest angle of 90 o or less than that are not recommended (Zacharkow, 1984). An optimum sitting position is upright and it is symmetrical with the pelvis anteriorly tilted. The iliac crests are in alignment and they level in the lateral plane. Hip, knees and ankles are all placed at 90 degrees of flexion (Ham et al.1998). In this position the pelvis provides a stable base forming a foundation which supports the rest of the body.
The principle of biomechanics used for normal sitting and for the disabled sitting is almost the same. The special seating designed for children with disabilities considers the requirements like stage of development, disabilities and other disorders. The mobility and posture problems that disabled people face might be due to muscle weaknesses, muscle imbalances and spasm or poor sensation. There are many reasons for their disabilities like birth defects (children who have movement disorders), accidents (an injury to legs or the spine, which controls leg movement), debility diseases and old age.
First and the foremost for disabled sitting is the stable sitting because the unstable sitting makes upper body difficult to balance. Stable or balance seating is an important thing to consider when disabled are using it. Seating position should balance weight and movement in all planes (Letts, 1991). Therefore a special seat base with a back support can be used which would assists the patient who is being positioned for physical therapy. The supports are made from materials such as plywood, foam and polymer. These materials provide postural support and relieve the pressure for patients who have unbalanced bodies.
A wheelchair is one of the most common devices used to assist users who have limited abilities to walk or move. Wheelchair user population is the portion of people who are benefitted by using wheelchairs. This includes children, adults, and elderly persons.
Wheel and caster
Figure 6: Components of wheelchair
In 1994, Barnaby and team had a survey of wheelchair using by interviewed over 3,000 wheelchair users at Dundee Limb Fitting Centre.
Figure 7: The number of wheelchair users by age (Barnaby et al. 1994).
Figure 8: The number of wheelchair users by category (Barnaby et al. 1994).
The above bar chart shows that 50% of total users are 30-60 years old. Approximately 10% are children under the age of 20. The pie chart shows users by category. The largest diagnostic categories are sclerosis 26%, spina bifida 10%, rheumatoid arthritis 9% and cerebral palsy 8%.
Classified by source of energy, there are two types of wheelchairs, namely manual and power wheelchairs. The most injury to musculoskeletal in manual wheelchair users is the shoulder. This is because patients usually have overload on their upper bodies to turn the wheels of the wheelchairs. In 2008, Desroches et al. published a paper in which they studied a relationship between the effectiveness of the resultant force at the handrims and the mechanical load by the net shoulder moments. The results in their studies showed more force requirements are more at risk of shoulder injuries on the ground that overuse. Furthermore, elbows, wrists and hands are also injured because they are not designed to resist the repetitive contact with friction and heat on wheelchair pushrims for a long time. This constant contact is a one of the possible causes of the incidence of carpal tunnel. All of these physical sufferings have been found in long term use of manual wheelchairs.
For children the Institute of Child Health Wolfson Centre report that providing power wheelchairs to 3 year old children aided in their general development and also boosted every individual's confidence. Provisions should be provided for children aged 2-3years possessing osteogenesis imperfecta or arthrogryposis. Children with these conditions are cognitively normal and have normal balance and motor control. However, it is inappropriate to apply it to children with cerebral palsy who are less likely able to manage the controls at such an early age. The report suggested that children with neuromotor disorder should be assessed when they are 6 or 7years old when they have developed improved functional skills for handling control (Ham et al, 1998).
Cerebral palsy seating
Cerebral palsy is a group of central nervous system disorders which affects the body's ability to control movement and posture. Based upon the form of motor impairment, cerebral palsy can be divided into four types: spastic cerebral palsy, athetoid cerebral palsy, ataxic cerebral palsy and mixture of these.
Spastic cerebral palsy is the most common type of cerebral palsy. Patients make stiff and jerky movements due to a malfunction in their upper motor neuron (the cerebral). This malfunction causes a lack of inhibition of the spinal reflex results in stopping the functioning of the motor neuron afferent unit.Â Spasticity is characterised by increased tendon jerks with an increase in muscle tension and deep tendon reflexes, particularly on flexors in the arms and on the extensor in the legs. Increased muscle tone due to an imbalance between the agonist and antagonist muscle group can increase the risk of scoliosis and rotation of spinal segment (Ham et al.1998). Spastic persons often have a hard time moving from one position to the other and have a hard time holding and releasing the objects. Some people with severe cerebral palsy are completely disabled and they require lifelong care.
The principle of seating is maintaining the body posture to balance stably and comfortably, but the spasticity makes the body unbalanced and unstable. This in turn makes the patient uncomfortable and impairs their functional ability. Furthermore, spasticity children need more seating requirements than a wide variety of people with disabilities because of their hyperactive stretch reflexes and abnormal activities. There are 3 key principles which are considered designing the seat for spastic cerebral palsy
Support the limb functions
When the muscles stretch, they apply a strong force between seat back and spastic patient. In 2001, Brown and team studied the characteristics of episodic extensor spasticity in terms of peak seat back force and force-time history. They used an ultra-light force sensing array mat to gather data on the magnitude, location and force-time characteristics by exerting it on the seat back during extensor spasticity episodes. The mean peak force was up to 92.3+ 61.5 lbs. The high forces produced during the extensor, therefore seating material must be strong and durable which resists this force. The materials must be also fatigue resistant to withstand repeated extensor thrusts over a long time. Other than this, the muscle contractions also cause unstable movements of body on the chair. The high shear forces at the interface between body and seat can contribute significantly to pressure sore (Clark, 2006). Especially in a hammock seat, increasing of the shear force on the lateral buttock and posterior aspects of the greater trochanters causes trochanteric pressure sore. So the hammock design is not suitable for the spastic patients who have to sit on wheelchair for a long time. The standard wheelchairs have hammock seats because they are easy to fold and they are not expensive.
In addition, the hip subluxation and dislocation is a problem in seating for children with cerebral palsy (Pountney, 2009). X-rays have shown that the acetabulum of the hip does not develop normally with increasing subluxation and eventual dislocation of the hip (Fulford and Brown, 1976). Therefore an important role of seating in wheelchair is sufficient to produce the required muscle stretch and maintain hip joint integrity.
Support the upper body functions
A suitable trunk space is an important consideration in any seating system. Especially, the seating design for childhood because this time children have continual and rapid change in physiology. Unsuitable posture has effects on the ability to interact with the environment and visual awareness. Nwaobi (1987) studied the effect of body orientation on upper extremity function in children and adolescents with cerebral palsy. In different seating orientations (30o, 15o and 0 o of posterior inclination and 15 o of anterior inclination) show that orientation of the body in space affects upper extremity function and emphasizes the importance of positioning for maximizing upper extremity function. Variable tilt should be adjusted to suit the different functional requirements of the individual.
The proper fitting armrests can decrease the myoelectric activity in the trapezius muscle when sitting and also reduce the disc pressure in the lumbar intervertebral discs. If the armrest is too low, the trunk will end up in a slumped kyphotic position and leaning to one side in wheelchair. If it is too high, it can cause pain in the shoulder, neck and upper back muscle (Zacharkow, 1988).
The comfort of the wheelchairs can encourage users to participate in daily activities. Selecting the most suitable wheelchair is very important as most people that require the aid of the wheelchair spend most of their time. The sitting behaviors in manual wheelchair users were studied by Yang et al., in 2008. In this study they found that manual wheelchair users spent an average of 9.2Â hours per day in their own wheelchairs. They sat for an average of 97Â minutes without displaying any lift-off behavior. A comfortable wheelchair is essential to provide if not, a wheelchair can be bounded person may suffer from other health problems easily.
The sores on patient's bodies are the one problem that often occurs. Too much pressure on the skin for a long time causes a loss of blood flow in the skin. This can make the skin die and lead to pressure sores. The most likely areas of pressure sores occur are spaces of dead skins and flesh, which are compressed between a bone in the patient's body and a hard surface. The pressure sores are found in all regular wheelchair users, especially the permanent wheelchair users who have advance diseases or severe injuries such as paralysis, diabetes and auto immune diseases. This is because they have high sensitivity on their skins as a result of their symptoms. Furthermore, pressure sores or pressure ulcers can be occurred by other risk factors include shear forces, skin temperature and anatomic structure (Polliack and Scheinberg, 2006).
Both during wheelchair propulsion and static seating can provide pressure for users. In 1998, Kernozek and Lewin compared the difference between static and dynamic seat interface pressures. They concluded from their study that peak pressure were greater during dynamic wheelchair locomotion compared with static seating, with the peak varying up to 42%.
In addition, the comfortable to application is importance. When wheelchair people need to use, it is easy to adapt to regular devices such as tables, car seats and flush toilets.