Work chairs are used ubiquitously by various workers in a variety of work places such as factory floors, offices, hospitals, etc. The nature of the work that a worker performs often requires the worker to sit on the work chair with a certain posture so as to get the job done efficiently. For example, bank clerks, typists and computer operators are required to sit on the work chairs with a certain posture for a prolonged period of time. As a result, the office workers are relatively vulnerable to various musculoskeletal disorders, some of which are often serious enough to call for an intensive medical attention or even surgical treatment. The medical costs, work lost and workers compensation for such disorders as mentioned above is so staggering that the annual cost can amount to billions of dollars in the United Stated alone.
It is generally believed by scholars and experts that the musculoskeletal disorders of the lower back are caused by demanding physical labor, such as lifting heavy objects. However, the epidemiological research shows that various musculoskeletal disorders of the lower back are often caused by improper sitting posture, and that the incident rate of such posture-related musculoskeletal disorders is by no means lower than that caused by demanding physical labor. According to the statistical data reported by Mogora in 1975, the incident rate of the nonsedentary workers ranges between 6% and 22% as compared with the incident rate of 10-14% of the sedentary workers. A research report, which was prepared and published by Rowe in 1983 on the basis of the diagnostic statistical data of the patients suffering from the musculoskeletal disorders of the lower back, showed that 41% of the patients studied are hard laborers and that 43% of the patients studied are sedentary workers. Another statistical report compiled by Liyd in 1986 showed that the incident rate of the lower back musculoskeletal disorders among miners is 69% as compared with the incident rate of 58% among the sedentary workers. The sum of the two statistical data referred in this research report is not 100% because some of the subjects were involved in both sedentary work and nonsedentary work.
The musculoskeletal disorders caused by the sitting posture include a pain in the lower back and a stiffness in the neck or shoulders. When a person is in a standing posture, his/her trunk-to-thigh included angle is 180 degrees while his/her spinal column bends in such a way that cervical vertebrae are lordotic (bending forward), thoracic vertebrae kyphotic (bending rearwards), lumbar vertebrae lordotic and sacral vertebrae kyphotic, as illustrated in FIG. 1. In other word, the best standing posture is formed by such spinal curvatures as described above. On the other hand, in a sitting posture, his/her trunk and thigh from an angle of 90 degrees, with the legs swiveling 90 degrees in relation to pelvis so as to bring about the extension of gluteus muscles and hamstring muscles. As a result, pelvis is caused to rotate rearwards by the muscular tension which is brought about by the extension of gluteus muscles and hamstring muscles, thereby causing lumbar vertebrae to straighten, as shown in FIG. 2. The straightening of lumbar vertebrae can bring about an asymmetrical pressure exerting on the intervertebral disc, a stretch of posterior ligaments of lumbar vertebrae, a muscular tension of erector spinae, and a strain on the central nervous system. For further illustration, please refer to FIGS. 3a-3b.
The straightening of lumbar vertebrae can bring about a pressure exerting asymmetrically on the nucleus of the intervertebral disc, which has a relatively thin rear edge and is therefore vulnerable to deformation and crack. In addition, the nucleus of the intervertebral disc is pushed by the asymmetrical pressure to squeeze the central nervous system, thereby resulting in a nerve pain or unconsciousness.
The tension of erector spinae can undermine its contractibility. As a result, a greater amount of energy is needed to avert the deformation of lumbar vertebrae. It is a well-known biological phenomenon that an excessive expenditure of body energy can cause a person to suffer from bodily fatigue.
When a person is in a standing posture, the posterior ligaments of his/her lumbar vertebrae are relaxed. However, the posterior ligaments are stretched, thereby increasing tension in these ligaments in a sitting posture. In order to help the person remain in the sitting posture, the tension may over-stretch and traumatize these ligaments and tear the attachments to the spinal processes.
When a person is seated, the tension on the lumbar spinal nerves increases due to a substantial increase in the length of the spinal canal. Accordingly, the nerves may be over-stretched and squeezed by the protuberances which may exist in the spinal canal. In addition, the peripheral nervous systems are also agitated by such stresses as described above such that the bodily fatigue is aggravated.
According to the study by Kapandji in 1974, the lumbar vertebrae are capable of bending forward to form an angle of 60 degrees in relation to pelvis in a standing posture. However, when a person is in a sitting posture, his/her pelvis must rotate backwards. In order to keep the upper portion of his/her body in an upright position while seated, his/her lumbar vertebrae must make a flexion of 35 degrees in relation to pelvis. As a result, the lumbar vertebrae are allowed to bend within the angular range of 25 degrees. For this reason, the scope of his/her activities is limited. In addition, a substantial amount of body energy is needed to sustain the contraction of muscles for remaining in the sitting posture. Under such circumstances as described above, the musculoskeletal disorders are easily developed or aggravated.
According to the study by Keegan (1953) who was an orthopedic surgeon, the extent to which the lumbar vertebrae straighten or bend rearwards is less serious when the trunk-thigh angle is changed from 180 degrees to 135 degrees, as illustrated in FIGS. 4b-4c. Keegan found that when the trunk-thigh angle is maintained at 135 degrees, the lumbar spine is in a neutral configuration with minimal musculoskeletal stresses. This finding was confirmed by studies conducted under zero-gravity conditions in space. The relaxed posture referred to above is the posture in which the trunk and the thighs form a 128-degree angle. In other words, the relaxed posture is similar in definition to the resting or normal posture in human anatomy. It is therefore suggested that the trunk-thigh included angle is an important factor capable of minimizing musculoskeletal stresses, and that the ideal angle is about 135 degrees. This implies that a good chair is one which is capable of preventing the pelvis of a person sitting thereon from swiveling rearwards so that a preferred spinal curvature is maintained. This is exemplified by the adjustable platform stool and the sit-stand stool, which are shown respectively in FIGS. 6a and 6b. Other examples include Mandal's high chair, Balan's chair designed jointly by Hog and Westonofa, Congleton's neutral chair, Opswik's saddle chair, and Palmgren's chair similar in shape to the bicycle seat. Such chairs as mentioned above are suitable for use by a teacher or bank teller by virtue of the fact that they allow a person sitting thereon to remain in a standing posture, and minimize the need of a sustaining force for keeping the person in the standing posture. It is readily conceivable that such chairs can not be used by a worker, such as a sewer, who has to use his/her leg to operate the machine. There are certain chairs which can cause a person sitting thereon to slide forward, thereby bringing about an unbearable shear force exerting on the hips of the person. There are also certain chairs cause the spine of a person sitting thereon to curve forward excessively, thereby producing a hollow in the back of the person. There are still certain chairs having a seat profile or a backrest profile which are so poorly designed that a person sitting thereon is not allowed to change sitting posture occasionally, thereby making the person very uncomfortable because of the poor ventilation effect.
For the purpose of better understanding of the present invention, some of the accompanying drawings are further expounded hereinafter.
FIG. 1 shows a schematic view of the spinal column of a person in a standing posture. The spinal column comprises cervical vertebrae 1, thoracic vertebrae 4, lumbar vertebrae 5, and sacral vertebrae 7. The pelvis is denoted by the reference numeral of 8. The reference numeral of 2 denotes that the cervical vertebrae 1 are lordotic while the reference numeral of 3 denotes that thoracic vertebrae 5 is kyphotic. In addition, the lumbar vertebrae 5 is shown to be lordotic, as denoted by the reference numeral of 6.
FIG. 2 shows a schematic view of the spinal column of a person in a sitting posture. The thighs are caused to rotate such that the hamstring muscles and the gluteus muscles are stretched to bring about tension, which causes pelvis to swivel rearwards and the spinal column to flex.
FIG. 3a is a schematic view illustrating that the lumbar vertebrae of a person are straightened when the person is in a sitting posture. The portion indicated by a circle 10 is enlarged, as shown in FIG. 3b in which an arrow 11 is intended to show that the lumbar vertebrae are straightened. In addition, arrows 12 and 13 are used to denote respectively that a force is exerted on the intervertebral disc asymmetrically, and that the posterior ligaments, the back muscles and the central nerve system are all stretched after the lumbar vertebrae are straightened.
FIGS. 4a-4e are schematic views showing respectively that angles of 200 degrees, 180 degrees, 135 degrees, 90 degrees and 50 degrees are formed by the trunks and the thighs. The curvatures of lumbar vertebrae are relatively small when the trunk-thigh angles are respectively 200, 180 and 135 degrees. However, the curvatures of lumbar vertebrae are substantially greater when the trunk-thigh angles are 90 and 50 degrees.
FIG. 5 is a schematic view of a normal or resting posture under zero-gravity conditions.
FIG. 6a is a schematic view of a high sitting posture, with the gluteal fold 10 being located right on the front edge of the seat, and with the ischial tvberosity 11 being located about 3-4 centimeters behind the front edge of the seat. The high sitting posture can cause the lower limbs to become numb because of the concentration of pressure on the gluteal fold 10. The pelvis, the hip joint and the femur are denoted respectively by the reference numerals of 8, 9 and 12. If the chair seat surface is extended forward and the extended portion is slanted downwards, as shown in FIG. 6b, the thighs will be located on the slanted extended portion and at the inclination of 20 degrees so that the pressure exerting on the vicinity of the gluteal fold can be minimized.
FIG. 7 shows a schematic view of the erect sitting posture and the slumped sitting posture which are denoted respectively by the reference numerals of 20 and 30. The ischial tuberosity 11 is used as a reference point in the illustration. In the erect sitting posture, the curve line of the posterior edge of the lumbar vertebrae is located near the ischial tuberosity. On the other hand, in the slumped sitting posture, the curve line of the posterior edge of the lumbar vertebrae is located farther from the ischial tuberosity. The lumbar support should be located between the two curve lines.
FIG. 8 shows a schematic view of the thoracic support. The erect sitting posture and the fully extended sitting posture are denoted respectively by the reference numerals of 81 and 82 while the posterior curve lines of the thorax of the erect sitting posture 81 and the fully extended sitting posture 82 are denoted respectively by the reference numerals of 811 and 821. An semi-extended sitting posture 83 is shown by dotted lines, with the posterior curve line of the thorax of the semi-extended sitting posture 83 being designated by the reference numeral of 831. The posterior curve line 831 is the ideal location at which the thoracic support should be located.
It is believed by other researchers and the inventor of this application that the trunk-thigh angle should be changed from 135 degrees to 110 degrees when the person remains in a high sitting posture under the influence of earth gravity. Being in such high sitting posture, the upper portion of the person's body can flex freely so as to perform work in a satisfactory manner. In addition, the incident rate of the musculoskeletal disorders is reduced. When a person is seated, the body weight is transmitted through the spine and the pelvis via the ischial tuberosity onto the seat. It is believed that the body weight can be supported effectively by the ischial tuberosity in a sitting posture. It has been shown by Swearington that a total area of 98 cm2 (49 cm 2 each side) around the ischial tuberosities is capable of supporting 50 percent of the weight of the total body.
The ischial tuberosities are the most prominent anatomic landmark in the seated posture, and, therefore, are proposed as reference points for seat-design. Because they are the weight-bearing points in a seated posture, their position on the seat can be easily identified. Furthermore, they act as the stationary pivot axis for the pelvic rotation which occurs during posture changes, such as when moving from a slumped sitting posture to an erect sitting posture.
The seat-pan for a sitting posture should consist of two contour support surfaces, a pelvic support and a thigh support to accommodate the geometry of the pelvis and the femur. The pelvic support should be small because it only supports the area of the buttocks posterior to the gluteal fold. This pelvic support provides a horizontal platform for the ischial tuberosities to support the majority of the body weight in an upright direction. Although the pelvic support can be used alone, there may be excessive pressure on the gluteal folds since the thighs are not supported. Therefore, it is proposed that an extension of the pelvis support be provided to support the thighs at the appropriate angle and to distribute the pressure over a large area. This thigh support should not be so deep (i.e. long) and may be at the inclination of 20 degrees.
In addition to the seat-pan of the chair, the natural curvature of human spinal column should be taken into consideration in the process of designing a work chair. It is imperative that stress exerting on the spinal column should be reduced or eliminated, and that a good backrest should be provided so as to enable lumbar vertebrae to flex properly without resorting to the contraction force of the erector spinae.
As shown in FIG. 6a, the ischial tuberosity is generally located about 3-4 cm away from the front edge of the seat-pan. While sitting in the proposed posture, a person can change from a slumped sitting posture to an erect sitting posture by pivoting the pelvis on the ischial tuberosity. The position of ischial tuberosity is fixed so that it can be used as a reference point for designing the backrest of a work chair. The horizontal distance between the lumbar vertebrae and the ischial tuberosity is largest in the slumped sitting posture. On the other hand, the horizontal distance between the lumbar vertebrae and the ischial tuberosity is smallest in the erect sitting posture, as shown in FIG. 7. It is therefore possible that a good lumbar support can be designed on the basis of the space and the movement range of these two spinal curvatures and the ischial tuberosity. A lumbar support is located at the middle line of these two curvatures, with the movement range serving as a horizontal adjustment distance of the lumbar support. The shape, the horizontal distance, vertical height and curvature radius of the lumbar support are dependent on the data of the measured curve lines.
The lumbar support can be extended upwards so as to provide the thoracic vertebrae with a support. However, the upper edge of the thoracic support should be at the level of the seventh thoracic vertebra (T7). If this edge is to low, it does not provide enough support; in contrast, if it is higher than T7 vertebra, it will contact the inferior angles of the scapular and cause discomfort. Therefore, the location of T7 should be the upper edge of the thoracic support. When the lumbar support is ideally located, the worker seated on the chair can change postures from an erect sitting posture to a fully extended sitting posture by leaning backwards against the support. A thoracic support should be placed between these two extreme curves. If a thoracic support is located along the erect spinal curve, it will interfere too much with required torso motions. If a thoracic support is located along the fully extended curve, it will not provide sufficient support during normal backward extension. Between these two boundary curves, a semi-extended curve can be traced and used as a reference for the thoracic support. It must be noted that the lumbar support is the primary structure which supports the lumbar spine during task performance to prevent backward rotation of the pelvis and to preserve the lumbar lordosis; the thoracic support is a secondary structure which supports the upper back during periodic backward leaning.
The seat-pan of a work chair must be provided with two functional units, a pelvic support and an thigh support, so as to enable a worker sitting on the Work chair to remain in a high sitting posture. Similarly, the backrest of a work chair must be provided with two functional units comprising a lumbar support and a thoracic support.