Bras and knees. You might not think they have much to do with each other.

But it turns out that wearing the right bra might actually reduce a woman’s risk of a serious knee injury.

Women are far more likely to tear their anterior cruciate ligament (ACL) than men and the results can be catastrophic.

The ACL, which runs diagonally through the middle of the knee and helps stabilise the joint, can take between nine and 12 months to recover from reconstruction surgery.

To work out why the risk is so much greater for females, researchers have looked at biological factors and training routines. But until recently, no-one had considered the role of the bra.

An international team of experts in biomechanics and sports medicine decided to take the task on and studied 35 female recreational athletes.

They found when women wore highly supportive bras while completing jumping and landing tasks, many of the common risk factors associated with ACL injuries decreased.

Knee flexion angles were smaller, which means the knee didn’t have to bend as much on landing, and there was also a reduction in dynamic knee valgus, which is when the knee moves inwards from the foot.

Lead researcher Douglas Powell, director of the Breast Biomechanics Research Center at the University of Memphis, found the results surprising.

“We’re talking about a meaningful level of [stress] on the ACL.”

The study found women tended to use an ankle-dominant strategy for landing when they were bare-breasted or wearing minimal breast support. But this strategy can lead to knee injuries because it puts more strain on smaller muscles.

But when they wore a supportive bra, the women used a more hip-dominant strategy, which puts less stress on the ACL.

Dr Powell says the switch to hip-dominant landings occurs when breast support is increased as the trunk moves less, and in turn, the pelvis is easier to control.

“So we see changes in movement patterns at the trunk, pelvis and lower extremities since it’s all connected,” he says.

“This [finding] is useful as we could go on to teach women to use a more hip-based strategy.”

The study, published in the Journal of Applied Biometrics, included women with cup sizes ranging from B to D and the findings suggest the larger the cup size, the higher the risk of traumatic knee injury.

“But in saying that, there’s so many differences in movement patterns from person to person I don’t think we can definitively say breast size drives [injury risk] … it’s a contributing factor,” Dr Powell says.

Nichola Renwick, a senior research associate in breast biomechanics at the University of Portsmouth, says the study “scratches the surface” of an area that needs further investigation.

“I think it still requires a lot more insight … there’s a lot of different considerations to take into account, particularly around the impact [on the knees] from different activities and cup size.”

Dr Renwick says the ACL injury prevention angle to this research could encourage more women to get educated about breast health.

The right bra can change how you breathe

Breasts move up and down and side-to-side during exercise. During an hour-long run they bounce in a figure-eight pattern around 10,000 times.

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Because breasts move in a vertical motion that’s out of time with the rest of the trunk, they can forcefully hit the chest if a bra isn’t supportive enough.

The breasts largely depend on the skin for support so not wearing the right bra can stretch the collagen fibres of the skin, leading to stretch marks and breast ptosis, or sagging.

“We’re still investigating it but we think the stretching and breaking of the collagen fibres is what causes exercise-induced breast pain,” Dr Renwick says.