Why your breasts bounce and how the pattern of the jiggle reveals your age! Extraordinary scientific secrets of bosoms

With each step a woman takes, her breasts move an average of 9 cm - not just up and down, but side to side

Breasts have always been a source of fascination - for both men and women. And as a smart bra launches - which includes skin sensors to measure size, breathing and heart-rate - science is finally shining a light on their charms.

From how breasts move to why you should wear a bra in the swimming pool, here's the surprising science behind your breasts - and how to keep them healthy and well-supported.

BOUNCE FACTOR

With each step a woman takes, her breasts move an average of 9 cm - not just up and down, but also sideways - in a figure of eight motion. When she breaks into a run, they move up to 15 cm with each stride, if unconstrained by a bra.

This means that during a 1 km jog of 1,320 steps, the breasts of an average C-cup sized women will have independently travelled as much as 0.12 of a kilometre.

Furthermore, each breast will have its own movement pattern - and will travel further the older she gets, as elasticity in skin starts to decline.

One study by Portsmouth University's Research Group in Breast Health - which uses sensors and 3D scanning software to analyse breast motion - monitored the bosoms of two sets of women, aged 18 to 25 and 45 to 65, during brisk walks.

They found that while the young women's breasts mainly moved up and down, the older group's chests moved more in all directions.

Professor Joanna Scurr, a world expert in breast bio-mechanics, said: 'In all women, their right and left breasts move differently, depending on the mass and how elastic they are.

'However in older women, there is more movement in both breasts, probably because the supporting structures are not as strong. You will tend to see a more U-shaped swing.'

Bra sizes are notoriously inaccurate and researchers say it's more effective to measure breasts in water

FORGET CUP SIZE

Bra sizes are notoriously inaccurate. In fact, researchers at Australia's University of Wollongong say cup sizes are so unreliable that bra-fitters measuring the same woman get different results depending on whether she is standing or lying down, breathing in or out or has her hands in the air or by her sides.

Instead, a more accurate way to gauge size is volume - with some scientists saying that bras should come in millilitre measurements to estimate precisely how much they need to contain.

When the Australian researchers placed breasts in water to get a better estimate, based on the fluid they displaced, they found the average breast has a volume of around 642 ml - about the same as a large bottle of beer.

80 PER CENT FAT

The shape of a woman's breasts is mainly dictated by the proportions of fat and glandular tissue they contain. Even within cups of the same size, these proportions vary.

While an average pair of breasts weighs about 3 lb each, there is very little natural support to hold them up

Professor Scurr says: 'How much fat the breast contains is genetic and varies substantially from woman to woman. Some women with the same breast size may have as little as 20 per cent fat or as much as 80 per cent.

'If it's more fat, the breast looks plumper, lighter and is squidgier to the touch. A breast that's more made up of glandular tissue is firmer and denser. It's also likely to be heavier, more pendulous and more tear-drop shaped.'

BEWARE GRAVITY!

No other part of the female body is more affected by the forces of gravity than the breasts. While an average pair of breasts weighs about 3 lb each, there is very little natural support to hold them up.

The heavier they are, the more strain there is on the skin and supporting ligaments which run though the breast tissue. These stretch permanently under persistent pressure.

These ligaments, which are spread though the breast like a cobweb, can stretch to as much as 15 cm in late old age. The drooping process, known as ptosis, happens in three stages. Before puberty, the first stage, the nipple is above the line where the base of the breast meets the chest.

Breasts can stretch up to 15cm in old age, with gravity starting to pull in your 30s when the nipple drops 1-2cm

 

By stage two, as gravity starts to pull in your 30s, the nipple drops 1 cm to 2 cm. By stage three, post menopause, the breasts hang more than 3 cm below this line, with the nipple often pointing down.

Professor Scurr says keeping breasts youthful is a matter of prevention because there is no cure.

'It's about protecting the breast through appropriate support as soon as a woman develops them.'

Match your sport

Wearing the same sports bra all the time may not protect your breasts. The best way to preserve your breasts is to match your sports bra type to the activity.

'Movement of the breast varies from sport to sport,' says Professor Scurr. 'In games such as tennis, the upper body rotates so the breasts move a lot from side to side. For that, you need a bra that's got support in the sides, as well as in the middle of the chest, to stop them swinging as you twist.

'However, when a woman runs, half of the breast motion is up and down, a quarter is side to side - and the rest is forwards and backwards. So you'd want a bra which compresses the breasts to stop all those movements.'

Even during yoga, the breasts are exposed to a strong gravitational pull which can affect them because the skin has been found to stretch as much as 39 per cent without proper support.

'When you're in different poses, breast tissue is displaced,' says Professor Scurr. 'So you want a bra that's elasticated which allows upper body movement, but is able to hold the breasts in position if you are upside down.'

... ONE FOR THE POOL

Few women consider wearing anything more than a bathing suit when they swim. But breasts still move as they travel through the water - with front crawl causing the most movement of about 1.5 cm to the sides, according to a study in the Journal of Sports Sciences.

'The buoyant force of water does provide some natural support to the breasts so that's why many larger breasted women find it more comfortable to swim,' says Professor Scurr. 'However we have found that breasts still move quite a lot. That's mainly because, as a woman's arms move through the water, it creates turbulence and pushes them sideways.'

When women put on sports bras, researchers found it 'significantly' reduced the wear and tear on the bust. Try buying a high-neck, supportive swimsuit.

SLEEP IN YOUR BRA

Film star Marilyn Monroe wore a bra in bed, saying it stopped her breasts from sagging. But although her approach has been dismissed over the years, it seems there is some truth in the belief that women with larger breasts can benefit from wearing one while they sleep.

Bigger breasts can drag and pull sideways when a woman lies down, contributing to stretch marks as well as crease marks between the breasts over time.

Indeed 24-hour bras, which come with less rigid boning, are now on sale for women who prefer more support at night.

Professor Scurr says: 'The research suggests that the region of the breast which experiences most pain is on the outside. So for some women, that sideways pull when they lie horizontal can be more uncomfortable than the up-and-down pull when they are standing up.

'There are no negative factors associated with wearing a bra all of the time. So if women find they get relief by wearing a bra at night, there's no reason why they shouldn't.'

LIVE LONGER

How painful breasts can be is still little talked about. Yet as many as half of women say their breasts hurt when they bounce, according to researchers.

Marathon runners are particularly prone to suffering breast pain. One study of 1,285 female runners found a third of women had breast pain - even though they wore sports bras.

As a result, many women stop exercising. Nearly one-in-five women say they their chest stood in the way of them keeping fit.

The knock-on effects on health are so serious that a study published in the Journal of Plastic and Reconstructive Surgery found that a woman with very large breasts can lose five years off her expected lifespan.