Why you need a sports bra

Can I tone up my breasts?

'Your breasts are chiefly composed of fatty and fibrous tissue,' says personal trainer Stuart Ennis from Virtual Fitness. 'That means you can't tone up the breasts themselves - but if you shed pounds through regular aerobic exercise, such as jogging or keep-fit classes, it may lead to a slightly reduced bra size. Sadly, you can't pick and choose where you lose weight from!'

If you're worried about the dreaded droop, however, you can do some work on the underlying pectoral muscles - which, in turn, may give your breasts an extra lift. 'Swimming breast-stroke is great for this,' says Lucy Wyndham-Read of LWR Fitness.

'Or for an easy exercise you can do anywhere, simply hold your arms straight out in front of you, at shoulder level. Bring your hands in slowly towards your shoulders, clenching them as though you're pulling something strenuously. Repeat five times.'

Do I really need a sports bra?

It's really important to ensure your breasts are well-supported during any kind of exercise - and that means buying a sports bra. But wearing the wrong type of bra can lead to fragile ligaments becoming irreparably stretched - so it's vital to ensure you choose the bra that's right for you.

So what's the choice? 'If you're an A or B cup, you can wear a stretchy crop top, which works by compressing the breasts to stop them from moving around,' explains Lucy. 'But women who are a C cup or larger need a firmer, more supportive bra that holds each breast individually.'

And the key is all in the fit - so make sure you try the bra on. 'Stand in front of the mirror and move your arms around then jog on the spot,' Stuart advises. 'You may feel silly, but it will let you know if there's too much movement during exercise. If your breasts jiggle, you're wearing the wrong size.'

Of course, the level of support you need can vary depending on the type of exercise you plan to do. If you're jogging or playing tennis, you'll need a very firm hold: if you're cycling or power-walking, your breasts won't move around quite so much.

Good ranges to try include Shock Absorber and Bravissimo's range of D to G cup sports bras for larger breasted women.

And finally, don't forget to get measured regularly: just because a bra fitted you two years ago, it doesn't mean it will today. Your breasts can change shape and size over time - especially after breastfeeding and the menopause. Ideally, you should replace your sports bra every six months: the fibres can stretch over time, which means they offer less support.

To see for yourself what happens to breasts during exercise when you wear no bra, a normal bra and a well-fitting sports bra, click here to watch Shock Absorber's animation