What is the key to good sitting posture? - supporting the keystone vertebra


The spine is a chain of bones that transmits the weight of the head and ribcage to the pelvis. It changes shape according to their counterbalancing movement in different positions.

Posture is determined by the alignment of these body parts in relation to each other. The more aligned, the better the posture and the lower the load to the joints and muscles.

In standing, the pelvis tilts forwards and the upper body shifts backwards. The spine forms an S-shape around the line of gravity with the head balanced on top.

In unsupported sitting, the pelvis tips backwards and the upper body shifts forwards. The spine drops into a C-shape around the line of gravity with the head falling forward above.

Chairs control the collapse of the spine that occurs in sitting.

The back support of the chair pushes the spine forwards to reposition the upper body over the seat.

If the chair supports the balance point (keystone) of the mobile part of the spine correctly, the ribcage and head will reposition over the pelvis: good posture.

the keystone

The keystone is the vertebra that bears the weight of the upper body and controls the shape of the spine.

When this bone is positioned correctly, it pulls the joints above and below into alignment and centralises body weight.

The level of the keystone differs between individuals and also with the height of the seat.

The ephgrave chair allows for this variation, and can be customised to each user.

conventional vs ephgrave chair

conventional chairs

Using lumbar support alone misses the keystone . The mobile spine collapses above the support unbalancing the body and compressing the lower back and neck. 

The muscles have to work hard to hold the unbalanced body upright against gravity. Over time they get tight and fatigued, causing stiffness and pain.

the ephgrave chair

The moveable back cushion can be positioned to fully support the keystone of each user, lifting it up and forwards to rebalance the body and reduce strain on joints and muscles. Good posture and flexibility are maintained.

The chair does the work for you so your body can relax.