It increases your perceived exertion

“Perceived exertion is essentially the feeling that you are working harder than you actually are when exercising,” explains Charlotte Gill, founder of sports bra brand PeBe.

“For women, this often occurs due to a problem with their sports bra. A poor quality, ill-fitted or low-support sports bra worn during high-impact exercise can lead you to feel like you’re working out harder, especially if you’re using it for high-intensity exercises such as running or HIIT. Your posture will be impacted negatively because you don’t have the right support to cushion the excessive breast movement and this all adds to the sensation of exertion.”

There is also evidence that when breasts are not properly supported, women experience increased upper body muscle activity, meaning you might feel fatigued more quickly – and more tempted to stop running – which has nothing to do with fitness levels or ability.

How wearing the wrong sports bra impacts your running, from reducing pace to increasing perceived exertion