Customer: Can a footrest improve my posture?
C.C. (Colm Campbell): The height of the average desk is around 27 inches. This means a person of average height must sit quite high in order to work with arms gently sloping downwards to the desk or keyboard. This means sitting with knees considerably below the hips. The result of this is a tendency for the lower body to drag the upper body forward into a C position. Using a footrest raises the knees and tilts the person backwards to a better sitting position.
Customer: What height of footrest should I use?
C.C.: You’re average height. I’d recommend a four inch high one. A taller person would need something in the region of two inches. The guiding rule is you should be sitting with your knees in line with your hips or slightly above. There are exceptions of course.
Customer: Like what?
C.C.: Many people have great difficulty in rising from the sitting to the standing position. So sitting height is of the utmost importance. The higher a person sits the easier it is to rise. But this means their knees would be lower than their hips. People with hip problems are a good example.
Customer: Somebody told me you should always sit with your knees way above your hips. Many cars are like this.
C.C.: Sitting with knees too far above the hips reduces the angle between the spine and thighs. Anything below ninety degrees puts tremendous stress on the lower spine and can tweak the sciatic resulting in pain which, in my own personal experience, is even greater than back pain. You mentioned car seats. They are well engineered with all sorts of clever adjustments. But they totally neglect the seat’s main function. To support the spine in good posture. Almost all car seats appear to be designed to make sure drivers sit in the C position. And the whole thing is compounded when using the clutch and brake. Pressing on the pedals transmits large forces up the legs to the lumbar spine.