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As Jo Wakefield-Scurr discovered in her test lab, when a woman exercises, her breasts can move considerably.

Breast tissue sits on muscle but isn’t supported by any; this means that “when we move, the breasts move,” Jo explains. During running they follow a figure of eight pattern, with the overall amount of movement adding up to as much as 21cm!

This movement can lead to a lot of pain, therefore “one of the key considerations for appropriate breast support is that it helps to reduce breast pain,” says Bra Professor Jo.

How much support you need is still open to debate

The best supporting bra Jo tested in the lab reduces movement by 82%, but there’s “this real range of support performance across the sports bra market,” she says. The lowest level of support they’ve found in a bra is 36% reduced movement.

What they don’t know is how much support is enough support. If a bra feels comfortable to you, it may be offering all the support you need. “A really important performance variable for a sports bra is that its comfortable,” says Jo.

Breasts can experience the same g-force during running as a Formula 1 driver

It’s not just about the amount of movement, says Jo, it’s also about the accelerations that the breast experiences. If not supported properly during running, breasts can experience the same g-force as a Formula 1 driver gets hit with.

“We’ve measured the amount of acceleration of the breast and we’ve seen that it equates to about 5G during running, for an average breast size,” says Jo.

Sports bras can help with sweat management

Another side effect of exercise is, of course, sweat. Unlike an everyday bra, a sports bra will ideally be made of wicking material, “able to cope with an increased amount of sweating and being able to move that away from the body,” says Jo.

Sports bras can help you perform better

As Jo found out under test conditions, when running bare-breasted, women will actually change the way they run. “They actually run much more cautiously, we see less rotation of the torso, we even see a shortening of the stride length,” says Jo. In her research, she’s seen women reduce their stride by up to 4cm. “That accumulates to the loss of a mile over a marathon distance.”

In turn, better breast support means improved mechanics, which reduces pain and anxiety, and “all in all that makes us more efficient,” says the expert in biomechanics.

Size matters when choosing a sports bra

Sports bras can be divided into two different ‘concepts’ to reduce movement: with one, the breast tissue is compressed towards the chest wall; the other lifts and holds the breasts, encasing the whole of the breast tissue. The latter is known as an ‘encapsulation’ sports bra, and they tend to look more like everyday bras.

Size matters. “We tend to find that compression are better for smaller breasted women and encapsulation bras tend to be better for larger breasted women,” explains Jo. But to confuse things further, there’s now a hybrid style which incorporates the two, with more of a crop top aesthetic.

The type of sporting activity affects which bra you should choose

You might see ‘low’, ‘medium’ and ‘high intensity’ bras – this simply indicates the amount of support you’re getting, from light to high.

Research done in Jo’s lab suggests that the breast moves differently during different sporting activities, and therefore the bra should reflect that. With activities like yoga, you might want lighter support from your bra; when running you’re likely to want high support.

“Fit is really important,” says Jo. “The sports bra can’t function optimally if it doesn’t fit appropriately.” But it’s not easy finding a good fit, especially as sizing isn’t consistent across the board. Some manufacturers sell by cup size, others by generic sizes like small, medium and large – and these can vary even within the same brand.

There is not a direct, linear relationship between performance and cost.

Jo Wakefield-Scurr, The Bra Professor

The Bra Professor advises using these five fit criteria:

1. Check the underband doesn’t ride up and you can’t pull it out further than 5cm at the side.
2. Check the shoulder straps can’t be pulled up by more than 5cm.
3. Check the cups: if too tight, you’ll see bulging of breast tissue; if too loose, you’ll see gaping or wrinkling.
4. Make sure underwiring isn’t sitting on any breast tissue, especially under the arm.
5. Check between your breasts. If it’s an encapsulating sports bra then the centre part should sit flat on the chest wall.

Washing your bra will shorten its life span

Some sports bras contain lots of bonding and lamination, which essentially means glue. When washed, the glue gets hot and can start to degrade. How often you wash your bra, the temperature, your detergent, whether you tumble dry it or stick it on a radiator could all effect the bra’s lifespan more than the mechanical stress of wearing it.

If your bra is at the more expensive end, it might be wise to consider washing it less. “Do look at the manufacturer’s guidelines for washing and try to follow those,” advises Joanna.

Expensive doesn't necessarily mean better

Some sports bras have multiple components, many of which will have been through long research and development journeys and claim to add their own innovative benefit. This all ramps up the price. But does it make them better?

Jo’s team are currently doing research into the relation between performance – how much support a bra gives you – and cost. “What I can tell you is that there is not a direct, linear relationship between those two variables,” she says.

To hear Greg's findings in full and find out if Teresa is going to swap her current sports bra for an expensive alternative, listen to the full episode here.

The information contained in this article was correct at the time of broadcast on 16 November, 2023.