Researchers at London's Portland hospital surveyed 1,000 osteopaths, and studied 17 women with a regular menstrual cycle. They found that the risk of injury was linked to fluctuating hormone levels which affect the muscles and ligaments. Both tissues appeared to be vulnerable midway through the menstrual cycle, while the ligaments were at greater risk towards the end.

They found this to be because midway through the cycle, the level of the female sex hormone oestrogen, which gives strength to muscles and ligaments, dropped dramatically, which resulted in sudden weakness. At the end of the cycle levels of another hormone, relaxin, rise to allow the cervix to open so that menstruation can occur, but it also means the ligaments in general are softened.

The researchers therefore concluded that that strains and other injuries were more likely at both these stages. Lead researcher Dr Stephen Sandler, an experienced osteopath, said: "There was a clear link between hormone levels and laxity of joints, making women more vulnerable to injury."

Dr Sandler highlighted this using laxity of the forefinger joint as an example. He found that joint laxity increased throughout the hormone cycle, then reverted back to normal once the period began. Dr Sandler said: "I had noted that, whereas men often came to me with injuries due to sport or over-exertion, women often couldn't explain why simple acts like reaching down to pick something up had caused injury and pain."

Dr Sandler's survey found 21% of oestopaths' female patients reported pain in days 12-14 of their cycle and 17% in days 24-26, with the majority of pain reported mid-cycle being lumbar or pelvic, compared with lumbar or neck pain at the end of the cycle.

Dr Sandler's results also mean that women on the combined pill, who do not experience sudden drops in their oestrogen levels, are less likely to experience injury as the result of loosened joints.

Rebecca Morrison, of the British School of Osteopathy, said: "Studies have shown before that female athletes and those engaged in recreational sport were more prone to injury at certain times in their cycle and now we understand why. This is significant for women everywhere who can plan their schedules around their cycles and avoid potentially painful injuries. It will also aid therapists in the rehabilitation of their patients."