Running shoes get the attention. The sports bra does a lot of the work.
A university lab in Portsmouth has spent years measuring how breasts move when women run, and how much different bras help. Its findings are practical: fit matters, some features matter more than others, and more support is not always better.
In a study of 98 sports bras, five characteristics each added to the breast support a bra gave. Pick one to see what it is. The bar shows how many of the five you have covered if you tick them in order.
Based on a University of Portsmouth study of 98 sports bras tested on 77 women. Not a product recommendation.
TWO NUMBERS WORTH KNOWING
Labels help, but not always.
How much bras really reduced movement, and how often a high-support label matched.
36–74%
Range across 98 sports bras
Compared with running bare-breasted, the bras reduced breast movement by between 36% and 74%.
MOVEMENT REDUCTION · PORTSMOUTH
69%
High-support bras that tested as high support
Of the bras marketed as high support, 69% landed in the study's high support group, which reduced movement by more than 63%.
LABELS · PORTSMOUTH
The same lab reports that, in inadequate support, runners' stride length fell by 4 cm and exercise felt harder. Its bra test facility has tested over 700 sports bras on over 8,000 women since 2014.
THE FIVE-STEP FIT CHECK
Search for your fit, not your size.
The Portsmouth group advises checking the fit of any bra rather than relying on a tape measure. These are its five checks, in order.
The underband
Level all the way round and firm, with only a 5 cm gap when pulled away. Most support comes from here.
The cups
Breasts fully enclosed, with no bulging or gaping at the top or sides.
The underwire and centre front
Any wire follows the natural crease, not resting on breast tissue, and the centre front sits flat against the body.
The straps
Comfortable, not digging in. They keep the bra in place but are not the main support, and need not be the same length.
AI-generated image, not a photograph of a real person or place.
CAN SUPPORT GO TOO FAR?
Less bounce, but not zero.
AI-generated image, not a photograph of a real person or place.
A 2025 Portsmouth modelling study, published in the European Journal of Sport Science, used motion capture, force platforms and a 3D scanner with a female-specific body model to look at the spine during running.
It suggested that a theoretical bra removing 100% of breast movement could increase loading on the spine. The researchers call it preliminary work and say the goal is to find the optimal amount of bounce reduction for both comfort and the back.
Questions
Questions people ask
How much do breasts move when running?
The University of Portsmouth researchers say that, because of the breast's limited anatomical support, it can move up to 15 centimetres during an exercise like running without support.
Does a sports bra affect running?
The Portsmouth group recorded that in inadequate breast support exercise felt harder and stride length was 4 cm shorter, which it says could mean running an extra mile over a marathon.
Is a 'high support' label reliable?
Mostly. In the study of 98 bras, 69% of those marketed as high support were in the lab's high support group. The rest tested in the lab's medium or low groups.
How do I know if my bra fits?
Check five things: a firm, level underband with only a 5 cm gap, cups with no bulging or gaping, wire following the breast crease, a flat centre front and comfortable straps.
How common is a poor bra fit?
The Portsmouth group says over 80% of women have been reported to wear the wrong size bra, and it advocates fitting by its 'best fit' checks rather than by tape measure.
