Women are more likely to injure themselves at specific times in their menstrual cycle, research suggests.
Back pain is particularly common
London's Portland Hospital surveyed 1,000 osteopaths, and studied 17 women with a regular menstrual cycle.
The study suggests the risk of injury is linked to fluctuating hormone levels which affect the muscles and ligaments.
Both tissues appear to be vulnerable midway through the menstrual cycle, while the ligaments are at greater risk at the end.
Midway through the cycle, the level of the female sex hormone oestrogen, which gives strength to muscles and ligaments, drops dramatically, resulting in sudden weakness.
At the end of the cycle levels of another hormone, relaxin, rise.
This is to allow the cervix to open so that menstruation can occur, but it also means the ligaments in general are softened.
The researchers found 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 compared hormone levels in the blood with the laxity of the forefinger joint.
He found joint laxity increased throughout the hormone cycle, then reverted back to normal once the period began.
His 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.
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.
The majority of pain reported mid-cycle was lumbar or pelvic, compared with lumbar or neck pain at the end of the cycle.
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."
Rona Call, 35, a patient of Dr Sandler at The Portland Hospital, said she found she was always putting her back out at the same time every month.
She said: "I take good care of my back and regularly go to yoga and Pilates but would hurt my back just by bending down to tie my shoelaces up.
"This research will empower women to help protect themselves and prevent injury."
Last year Swedish researchers found sportswomen were more prone to injuries at certain times during their menstrual cycle.