Chairs and seats – Rest for knee – leg – or foot – Connected to chair or seat for relative movement
Reexamination Certificate
1999-02-08
2001-03-06
Barfield, Anthony D. (Department: 3624)
Chairs and seats
Rest for knee, leg, or foot
Connected to chair or seat for relative movement
Reexamination Certificate
active
06196631
ABSTRACT:
BACKGROUND
Seated workers in home, office and industrial environments often experience back pain and other physiological difficulties as a result of ergonomic deficiencies of the various chair designs on the market. Often these difficulties are a result of the absence of appropriate support for the chair user's feet. Due to the problems of discomfort and injury associated with seated work, ergonomic research has been undertaken to identify the causes of these problems and to recommend solutions. In particular, this research has been undertaken to study existing chair designs as well as to develop new chair designs. From this work, six principal chair designs have been identified. Scientific theories have been developed to support claims of ergonomic deficiencies of particular chair designs, as well as to support claims of ergonomic solutions provided by particular chair designs. The chairs embodying these theories are in wide use today.
One such chair design, commonly known as “bench” seating, provides a horizontal seat pan with or without a vertically upright backrest. Users of this chair must exert their back muscles to sit upright while keeping their feet flat on the floor, with their lower legs at a ninety-degree angle to their upper legs. This bench seating or upright posture does allow the sitter to achieve a natural inward curvature of their lower spine without any supportive assistance from the backrest. However, the bench seating posture has been shown by ergonomic researchers to be quite unnatural, as people invariably slouch rearwardly or forwardly into a position in which their back assumed a convex curvature. This tendency to slouch while in bench seating has been shown to occur whether a backrest was provided or not. This is because the bench or upright sitting posture requires a conscious effort to exert the back muscles to hold this position. Recent ergonomic testing has shown that the average person can only maintain this upright bench seating posture for about 3½ minutes.
Because people invariably succumb to forward or rearward slouching, a vertical backrest is typically added to bench style chairs to help the sitter maintain straightness of the back and minimize the extent of the slouch. However, to make supportive contact with the backrest, sitters on a bench style chair have to lean back slightly from the hip/spine juncture. Adjusting the posture of the upper body by leaning back causes the upper back of the sitter to contact the backrest. This, in turn, creates a gap between the sitter's lower back and the lower portion of the backrest. This rearward slouching, when viewed from the side, appears like the letter C, with an outward bowing, or curvature of the spine. The supportive contact points in this posture are the upper back on the backrest, and the butt on the seat pan. The poor posture resulting from slouching rearward to utilize the backrest for support actually increases the strain on the back and causes even greater outward curving of the lower back. This pathological condition is recognized as a cause of many sitting related back injuries.
Kyphosis is the medical term used to describe the previously mentioned pathological condition of the lumbar area of the spine being curved outward. Kyphosis results in a misalignment of the vertebrae that causes uneven pressure on the vertebral discs. Over time this continuous uneven pressure can rupture or wear down the forward edge of the disc. This is due to the concentration of pressure on only a small local area of the disc. The pressure on the forward edge of the disc is caused by the gravitational pull on the upper body of the seated person. As the seated person assumes a slouched, or kyphotic position, the gravitational forces (upper body weight) are leveraged onto this small frontal disc area. This leveraged pressure on the small frontal disc area is extreme and over time can cause misalignment of the vertebrae. Vertebral misalignment causes pinching of the spinal nerves, which results in extreme back pain that disables the worker.
When standing, the vertebral discs of the human spine are set in a curve that acts to evenly distribute the weight of the upper body throughout the entire surface of the discs so that the entire disc area bears the load. This natural inward curvature of the lower back is referred to as “lordosis”. When a person is walking, bending, or lifting, the forces on the discs do concentrate on small specific areas of the discs, but only for brief time periods because the person's motion quickly alternates the pressure from one part of the disc to another. However, when a person is seated, there is little or no muscle movement and therefore no alternation of muscle tension. This static tension registers as pain to the person's nervous system that triggers an instinctual relief mechanism whereby a shift of body position redistributes muscle tension and disc pressure to other locations. This instinctual body shifting is scientifically termed “unconscious mechanataxes”. Commonly it is known as squirming. This squirming constantly shifts stressed pressure points and fatigued muscle areas from one surface location or area to another, by alternation. Accordingly, alternation through squirming may be viewed as a natural defense mechanism that acts to relieve static stress.
A bench seated person who is attempting to maintain equalized disc pressure in the lower back through conscious use of back muscles to maintain lordosis (natural inward curving of the spine when standing) invariably fatigues these muscles. In order to relieve this muscle stress, the sitter slouches in his or her chair thus shifting the stress away form one set of back muscles and the evenly pressured discs over to a different set of muscles and the frontal portions of the vertebral discs. This alternation of posture may relieve some muscle discomfort for a while, but at the expense of causing serious injury to the discs. A lumbar support positioned at the lower portion of the backrest offers no improvement towards achieving lordosis because there is nothing forcing the lumbar support into the lower back, as bench posture does not force the user's lower back into the back cushion. Despite these known problems associated with bench seating, it remains the most widely used chair posture in the world.
A second chair design, often called a “Grandjean” chair provides a horizontal seat pan and a backrest which tilts rearward in relation to the horizontal seat pan at an angle between 104 degrees and 113 degrees. This chair was conceived from the observation of rearward slouching of people on their bench style chairs. Accordingly, the reclined backrest was designed to provide support for the sitter's back in the reclined position while the sitter is on a bench chair seat pan. Though the backrest reclined, the load bearing contact points for the sitters were, as with the bench chair, upper back on backrest cushions and butt on seat pan. With the sitter's lower legs perpendicular to the upper legs and feet flat on the floor as recommended, support for the lower back was still negligible. The gap occurring between the lower back and the lower back cushion on the bench chair design was reduced somewhat by the Grandjean reclined backrest but there was still no significant force on the lower back to prevent kyphosis. Because the Grandjean chair encourages rearward slouching, no conscious exertion of back muscles is required, as sitting up straight is not the posture accommodated by this chair design. So, one problem associated with bench seating, muscle fatigue, was solved by Grandjean by encouraging the second problem occurring with the bench chair, slouching.
A third chair design often called a “Mandal” chair provides a seat pan having a forward, downward tilt with an upright back cushion. A. C. Mandal theorized a chair with a forward tilting seat pan incorporated into the overall chair design. He also observed that this tilted seat pan rotated the sitter's pelvis forward causing lordosis
Barfield Anthony D.
Johnson Jerry
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