Sorry if this offends anyone's sensibilities, but this is a very real issue for me and one that can't be ignored!
I worked at a mental health project over the weekend - a sleep-in shift - and went straight from there to the gym, where I planned to do some speed training.
While getting changed, I realised I had forgotten my sports bra, but didn't think it would matter too much, and went in the one I was wearing.
As soon as I started running it just felt all wrong. Not only was it uncomfortable, but I felt completely off balance. It was so weird! I only managed to keep going for 20 minutes, and kept having to walk! So only did 2 miles when was planning on running about 4. At least I have learnt an important lesson!
I went to a yoga class instead, where the type of bra is not quite such a key issue!