Office chair designs ignore key human biomechanical features and contribute to body distortion according to Nigel Corlett, a long time contributor to ergonomics and current scientific adviser for a consulting and research institute at the University of Nottingham. The author claims that the hip joint should be allowed to assume a moderately flexed position as opposed to commonly performed 90 degree thigh/trunk angle in order to obtain an optimum sitting posture.
Corlett supported his opinion through a series of concepts presented as design points.
Design Point 1: Maintaining the lumbar curve allows minimum back muscle activity when the trunk is upright.
When the spine is in a neutral position (no section of the spine flexed, extended, laterally flexed, or rotated), the center of body mass passes through the apex of the lumbar curve and there is a minimal amount of lumbar spinal muscle activity.
Design Point 2: Sitting upright with the thighs horizontal loads the spine even before any work commences.
When one sits on a horizontal surface, the hip joint flexes approximately 70 degrees and the pelvis rotates posteriorly (backward). As the pelvis rotates posteriorly, the lumbar curve flattens and the center of body mass moves forward relative to the lumbar spine. A trunk flexion moment becomes created. To counteract the trunk flexion moment (and avoid having the upper body fall forward), low back muscles contract. Hence, low back muscles are exerting before any work efforts have occurred.
Further, Corlett notes that as the lumbar curve flattens, the anterior (front) margins of the intervertebral discs become slightly pinched.
Design Point 3: A poor seated posture reduces one’s physical capacity for effort and can lead to bodily distortion over the long term.
With low back muscles preloaded, there is less lumbar muscle strength capacity available for performing tasks such as reaching, bending, or twisting. Also, a longer recovery duration is commonly required for static (isometric contraction) loads compared to dynamic (isotonic) loads. Muscle fatigue would likely be more persistent following long-term sitting as opposed to tasks requiring low back movement.
Also, when a postural position is held for long durations, soft tissue adaptive changes occur. If the posture is one of disadvantage, these soft tissue changes contribute to body distortion.
Design Point 4: Having sloping thighs reduces back loading and makes it easier to stand.
When the hip joint is moderately flexed, the pelvis is not forced to rotate posteriorly and the lumbar spine assumes a more neutral position. At most, a modest level of low back muscle contraction occurs.
Furthermore, as the thighs slop downward, the knee joint is closer to 60 degrees of flexion and the foot is in solid contact with the floor. This body posture makes it easier to assume different positions/motion (a desired activity) such as standing and reaching.
Corlett felt a chair would meet the design criteria and be comfortable to the user if characterized by:
- A seatpan curved from front to back such that the thigh was allowed to slop downward while the user’s body weight was transmitted through a horizontal section
- A seatpan that will tilt forward to allow the user’s feet to reach the floor if necessary
Article Title: Sitting On Seats, Working All Day
Publication: Ergonomics In Design 15:1 25-27, 2007
Author: E N Corlett
This article originally appeared in The Ergonomics Report™ on 2007-04-04.