Ligaments help to provide stability to joints. They prevent the joint from moving more than a normal range (though what is ‘normal’ varies from one individual to another). Some people have lax ligaments that allow more than ‘normal’ movement: often spoken of as ‘double jointed’ or hyper-mobile. In the spine there is a complex arrangement of ligaments, both between each vertebral segment and between the spine and pelvis, which allows flexibility in some directions and produces restraint in others.

Sometimes ligaments can be overstretched, or even torn (as in a sprained ankle). The ligament may then not control the joint adequately – thus leading to ‘instability’ which may put abnormal stresses on the joints and discs in the spine. The instability is compounded by breakdown in the muscle stabilising system through muscle weakness, fatigue and poor coordination.

In women, the pelvic joints need to be supple for child bearing, and so the ligaments soften and stretch more readily. Sometimes they do not tighten up after childbirth and therefore allow too much movement: hence ‘sacroiliac instability’.