Dynamic Spinal Support
(Based on an independent study by Jo Rainsley)
Posture effects three properties of the back and abdominal muscles: their length, the angles at which they pull on the vertebrae, and their lever arms relative to centres of rotation (Adams et al, 2006). Poor posture and movement can lead to local mechanical stress on the muscles, ligaments and joints, resulting in complaints of the neck, back and other parts of the musculoskeletal system (Dul & Weerdmeester, 1993). However, it has been highlighted through literature that the spine is particularly susceptible to postural stress (Pheasant, 1988; Adams et al, 2006; McGill, 2007; Nachemson & Jonsson, 2007).
Clinical and Biomechanical findings
Occupational epidemiology and ergonomics commonly highlight four risk factors inciting the event of LBP: perceived exertion within the workplace: discomfort or fatigue; occurrence of LBP; and sick leave due to LBP (Muller, 2007). One biomechanical factor contributing to patients with LBP is reduced lordosis; especially in the lower lumbar spine and a particular flat back is a risk factor for future low back pain (Jackson and McManus, 1994; Adams et al, 1999).
The vertebrae form the passive units of the lumbar spine while the intervertebral discs (IVDs) and the posterior ligamentous structures form an active unit (Palastanga et al, 2006). The IVD and cartilage end plate postulate to buffer against gravity and torsion and act as a shock absorber of forces transmitted into the spinal column. IVD compressive force load failure is 3000 Newton’s (N) with a torsional strength of 40kg/cm2. This end point is that similar to steel, however the IVD is able to retain some power after recovery, thereby retaining its properties of elasticity (Adams et al, 2006).
When gravity exerts a vertical force on the spine, depending upon the posture of the spine will depend upon which gravitation force is imposed i.e. in flexion and extension shear loading is higher segmentally in the spine than compressional force loading. The larger the angle in extension and flexion from neutral, the higher the shear force. Intervertebral compressive and shear force components rise when the spine is taken through flexion and extension.
Flexed posture stretches the intervertebral ligaments, and tension in these ligaments increases the compressive force acting on the intervertebral disc. Muscular contractions in excess of normal requirements may have a detrimental effect on the nutrition of the discs, as this is dependent on imbibitions of fluid, which occurs when the compression is reduced (Oliver and Middleditch, 1991). In a seated position with a slump, the pelvis is tilted forwards, the thoracic curvature is increased and the lumbar curvature flattened (Kapandji, 1988). Whilst standing the IVD compressive load of the lumbar spine is 500N (White et al, 1999), sitting with a slouched position IVD pressure is doubled (Nachemson & Jonsson, 2007).
When muscles are lengthened beyond their optimum length, overlap between actin and myosin filaments decreases and cross-bridge formation falls. Consequently, an increasing proportion of the force generated is due to tension in the stretched collegenous tissue sheaths of the muscle. During bending, compression and shear to stimulate flexion of the spine, the intervertebral ligaments provide most the resistance to movement. The tensile strength of the longitudinal ligaments is 200kg/cm2, after a few minutes ligaments creep substantially in whereas disc creep happens over longer distance (McGill and Brown, 1992; McMillan et al 1996) therefore sustained repetitive flexion will have a relatively greater effect on the posterior intervertebral ligaments and as a consequence of this reducing their protective duties of the IVDs. Just 5 minutes of this creep is enough to reduce the ability of the intervertebral ligaments to protect the discs in bending by 40% (Adams and Dolan, 1996). Most disc creep is due to the expulsion of water (Adams & Hutton, 1983; Kreamer et al, 1985; McMillan et al, 1996). Approximately 25% of the creep has attributed to viscoelastic deformation of the annulus (Broberg, 1993) as a result leaving the annulus tissue more elastic (Koeller et al, 1984; Smeathers, 1984). Therefore, if a forward bending task followed a prolonged duration of being sat in a flexed position, the impairment of the spinal stabilisers could be delayed therefore inciting the possibility of injury.
Posture is the position assumed by the body either by means of the integrated action of muscles working to counteract the force of gravity, or when supported during muscular inactivity. Many postures are assumed by an individual during a 24 hour period, and at any given moment posture compromises the positions of all the joints of the body (Kendall et al, 1993). Posture influences the water expulsion from the discs (Adams & Hutton, 1983) as well as load bearing by the zygzpophysial joints. Water loss from IVDs leads to stress concentrations in the disc itself. When a person stands in an upright position, the mass of the head, trunk and arms press vertically on the lumbar spine. Ruff (1950) discovered the compressive force loaded vertically in a human standing in an upright position was approximately 55% of their bodyweight. In a relaxed recumbent sitting position the compressive forces on the lumbar discs approximately double from the forces that are found in a superincumbant bodyweight (Nachemson & Jonsson, 2007). Postures are maintained or adapted as a result of neuromuscular coordination, the appropriate muscles being innervated by means of a complex reflex system. The efferent response is a motor one, the antigravity muscles being the principal effector organs (Oliver and Middleditch, 1991). Individuals, who have impaired proprioceptive or motor function, would be more likely to sustain fatigue damage to spinal tissues during repetitive bending and lifting tasks (Adams and Dolan, 1991).
During static effort the muscle does not change its length but remains in a state of heightened tension, with force exerted over the duration of the effort. This isometric state has a steady consumption of energy while it is supporting a given weight, but does not appear to be doing useful work (Kroemer & Grandjean, 2005). Therefore a muscle that is performing heavy static effort is receiving no fresh blood and no sugar or oxygen and must depend on its own reserves. More importantly no waste products are being removed, therefore the waste products are accumulating and producing the acute pain of muscular fatigue. However when the static effort is less than 20% of the maximum the blood flow should be normal (Kroemer & Grandjean, 2005). Liira et al (1996) suggested that static office workers are more exposed to LBP than those office workers who vary their work position.
Design of the Dynaspine
Seat design over the centuries has implemented backrests in chairs to assist in the maintenance of the lumbar concavity (Singleton, 1982). However, although these designs may assist in lumbar lordosis, therefore reducing the tensile forces on the posterior ligaments of the lumbar spine and IVDs. However, the issue still remains that innervation of spinal stabilisers has not been met. Research indicates power output of static effort restricts the flow of blood to a muscle where a series of physiological condition of muscle fatigue supervenes, whilst frequent shifts of posture are subjectively desirable (Pheasant, 1988; Liira et al, 1996; Kroemer & Grandjean, 2005; McGill, 2007). Kramer (1985) produced evidence that to keep the IVDs well nourished and in good condition, they need to be subjected to frequent changes of pressure, as a kind of pumping mechanism. From a medical point of view, therefore, an occasional change of posture from bent to erect, and vice versa, must be beneficial (Kelsey, 1975; Kroemer & Grandjean, 2005; McGill, 2002).
The design of the Dynaspine is such that it has considered the need to create a lumbar support product to alleviate unnecessary stress on the lower spine. However, the design has also considered the emergent data, from a scientific review, that a dynamic component of lumbar support is also necessary to ensure the integral health of the lumbar spine. Ergonomic evidence; angle of the backrest to optimal reduce pressure on the lumbar spine to be 120 degrees; lumbar pad should offer a height of 100-200mm above the lowest point of the seat surface; lumbar support of 5cm in depth from the front of the lumbar pad and the plane of the back rest to reduced the pressure on the IVD by 200N respectively (Andersson and Ortengren, 1974; Kroemer & Grandjean, 2007) are all contributed to the design of the Dynaspine.
Conclusion
In summary of the COST B13 Working Group on European Guidelines for the Prevention in Low Back Pain (DATE) states that lumbar supports or back belts are not recommended (Level A). However, although the Dynaspine has not had any experimental trials conducted on it, the theory behind the design appears to have met both ergonomic and biomechanical standards for preventing LBP from sitting (Andersson and Ortengren, 1974; Kappler, 1982; Kelsey, 1975; Kroemer & Grandjean, 2007; McGill, 2007)
Callagham and McGill (2001) suggested that no single, ideal sitting posture exists; rather a variable posture is recommended as a strategy to minimise the risk of tissue overload. Dynamically and statically, an efficient posture is one that is; stable, minimises stress and strain on the tissues and minimises energy cost (Oliver and Middleditch, 2005).
Dynaspine allows for a dynamic, ergonomically sound seated position, which reduces pressure on the spinal ligaments and discs.
Posture effects three properties of the back and abdominal muscles: their length, the angles at which they pull on the vertebrae, and their lever arms relative to centres of rotation (Adams et al, 2006). Poor posture and movement can lead to local mechanical stress on the muscles, ligaments and joints, resulting in complaints of the neck, back and other parts of the musculoskeletal system (Dul & Weerdmeester, 1993). However, it has been highlighted through literature that the spine is particularly susceptible to postural stress (Pheasant, 1988; Adams et al, 2006; McGill, 2007; Nachemson & Jonsson, 2007).
Clinical and Biomechanical findings
Occupational epidemiology and ergonomics commonly highlight four risk factors inciting the event of LBP: perceived exertion within the workplace: discomfort or fatigue; occurrence of LBP; and sick leave due to LBP (Muller, 2007). One biomechanical factor contributing to patients with LBP is reduced lordosis; especially in the lower lumbar spine and a particular flat back is a risk factor for future low back pain (Jackson and McManus, 1994; Adams et al, 1999).
The vertebrae form the passive units of the lumbar spine while the intervertebral discs (IVDs) and the posterior ligamentous structures form an active unit (Palastanga et al, 2006). The IVD and cartilage end plate postulate to buffer against gravity and torsion and act as a shock absorber of forces transmitted into the spinal column. IVD compressive force load failure is 3000 Newton’s (N) with a torsional strength of 40kg/cm2. This end point is that similar to steel, however the IVD is able to retain some power after recovery, thereby retaining its properties of elasticity (Adams et al, 2006).
When gravity exerts a vertical force on the spine, depending upon the posture of the spine will depend upon which gravitation force is imposed i.e. in flexion and extension shear loading is higher segmentally in the spine than compressional force loading. The larger the angle in extension and flexion from neutral, the higher the shear force. Intervertebral compressive and shear force components rise when the spine is taken through flexion and extension.
Flexed posture stretches the intervertebral ligaments, and tension in these ligaments increases the compressive force acting on the intervertebral disc. Muscular contractions in excess of normal requirements may have a detrimental effect on the nutrition of the discs, as this is dependent on imbibitions of fluid, which occurs when the compression is reduced (Oliver and Middleditch, 1991). In a seated position with a slump, the pelvis is tilted forwards, the thoracic curvature is increased and the lumbar curvature flattened (Kapandji, 1988). Whilst standing the IVD compressive load of the lumbar spine is 500N (White et al, 1999), sitting with a slouched position IVD pressure is doubled (Nachemson & Jonsson, 2007).
When muscles are lengthened beyond their optimum length, overlap between actin and myosin filaments decreases and cross-bridge formation falls. Consequently, an increasing proportion of the force generated is due to tension in the stretched collegenous tissue sheaths of the muscle. During bending, compression and shear to stimulate flexion of the spine, the intervertebral ligaments provide most the resistance to movement. The tensile strength of the longitudinal ligaments is 200kg/cm2, after a few minutes ligaments creep substantially in whereas disc creep happens over longer distance (McGill and Brown, 1992; McMillan et al 1996) therefore sustained repetitive flexion will have a relatively greater effect on the posterior intervertebral ligaments and as a consequence of this reducing their protective duties of the IVDs. Just 5 minutes of this creep is enough to reduce the ability of the intervertebral ligaments to protect the discs in bending by 40% (Adams and Dolan, 1996). Most disc creep is due to the expulsion of water (Adams & Hutton, 1983; Kreamer et al, 1985; McMillan et al, 1996). Approximately 25% of the creep has attributed to viscoelastic deformation of the annulus (Broberg, 1993) as a result leaving the annulus tissue more elastic (Koeller et al, 1984; Smeathers, 1984). Therefore, if a forward bending task followed a prolonged duration of being sat in a flexed position, the impairment of the spinal stabilisers could be delayed therefore inciting the possibility of injury.
Posture is the position assumed by the body either by means of the integrated action of muscles working to counteract the force of gravity, or when supported during muscular inactivity. Many postures are assumed by an individual during a 24 hour period, and at any given moment posture compromises the positions of all the joints of the body (Kendall et al, 1993). Posture influences the water expulsion from the discs (Adams & Hutton, 1983) as well as load bearing by the zygzpophysial joints. Water loss from IVDs leads to stress concentrations in the disc itself. When a person stands in an upright position, the mass of the head, trunk and arms press vertically on the lumbar spine. Ruff (1950) discovered the compressive force loaded vertically in a human standing in an upright position was approximately 55% of their bodyweight. In a relaxed recumbent sitting position the compressive forces on the lumbar discs approximately double from the forces that are found in a superincumbant bodyweight (Nachemson & Jonsson, 2007). Postures are maintained or adapted as a result of neuromuscular coordination, the appropriate muscles being innervated by means of a complex reflex system. The efferent response is a motor one, the antigravity muscles being the principal effector organs (Oliver and Middleditch, 1991). Individuals, who have impaired proprioceptive or motor function, would be more likely to sustain fatigue damage to spinal tissues during repetitive bending and lifting tasks (Adams and Dolan, 1991).
During static effort the muscle does not change its length but remains in a state of heightened tension, with force exerted over the duration of the effort. This isometric state has a steady consumption of energy while it is supporting a given weight, but does not appear to be doing useful work (Kroemer & Grandjean, 2005). Therefore a muscle that is performing heavy static effort is receiving no fresh blood and no sugar or oxygen and must depend on its own reserves. More importantly no waste products are being removed, therefore the waste products are accumulating and producing the acute pain of muscular fatigue. However when the static effort is less than 20% of the maximum the blood flow should be normal (Kroemer & Grandjean, 2005). Liira et al (1996) suggested that static office workers are more exposed to LBP than those office workers who vary their work position.
Design of the Dynaspine
Seat design over the centuries has implemented backrests in chairs to assist in the maintenance of the lumbar concavity (Singleton, 1982). However, although these designs may assist in lumbar lordosis, therefore reducing the tensile forces on the posterior ligaments of the lumbar spine and IVDs. However, the issue still remains that innervation of spinal stabilisers has not been met. Research indicates power output of static effort restricts the flow of blood to a muscle where a series of physiological condition of muscle fatigue supervenes, whilst frequent shifts of posture are subjectively desirable (Pheasant, 1988; Liira et al, 1996; Kroemer & Grandjean, 2005; McGill, 2007). Kramer (1985) produced evidence that to keep the IVDs well nourished and in good condition, they need to be subjected to frequent changes of pressure, as a kind of pumping mechanism. From a medical point of view, therefore, an occasional change of posture from bent to erect, and vice versa, must be beneficial (Kelsey, 1975; Kroemer & Grandjean, 2005; McGill, 2002).
The design of the Dynaspine is such that it has considered the need to create a lumbar support product to alleviate unnecessary stress on the lower spine. However, the design has also considered the emergent data, from a scientific review, that a dynamic component of lumbar support is also necessary to ensure the integral health of the lumbar spine. Ergonomic evidence; angle of the backrest to optimal reduce pressure on the lumbar spine to be 120 degrees; lumbar pad should offer a height of 100-200mm above the lowest point of the seat surface; lumbar support of 5cm in depth from the front of the lumbar pad and the plane of the back rest to reduced the pressure on the IVD by 200N respectively (Andersson and Ortengren, 1974; Kroemer & Grandjean, 2007) are all contributed to the design of the Dynaspine.
Conclusion
In summary of the COST B13 Working Group on European Guidelines for the Prevention in Low Back Pain (DATE) states that lumbar supports or back belts are not recommended (Level A). However, although the Dynaspine has not had any experimental trials conducted on it, the theory behind the design appears to have met both ergonomic and biomechanical standards for preventing LBP from sitting (Andersson and Ortengren, 1974; Kappler, 1982; Kelsey, 1975; Kroemer & Grandjean, 2007; McGill, 2007)
Callagham and McGill (2001) suggested that no single, ideal sitting posture exists; rather a variable posture is recommended as a strategy to minimise the risk of tissue overload. Dynamically and statically, an efficient posture is one that is; stable, minimises stress and strain on the tissues and minimises energy cost (Oliver and Middleditch, 2005).
Dynaspine allows for a dynamic, ergonomically sound seated position, which reduces pressure on the spinal ligaments and discs.
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