You may do it unconsciously when you sit for long periods: hunch your shoulders and bend your back. Over time, this posture can shorten muscles in your back, causing painful stiffness and giving your spine an excessive forward curve. Known medically as kyphosis, at an advanced stage the condition may pinch nerves running through your spine, resulting in tingling or numbness in your arms and legs, says orthopaedist Dr David Kubosch. There are ways to prevent kyphosis, and Kubosch recommends two exercises familiar to yoga practitioners that can help. They’re particularly useful for people who pu…