During the nine days, people do not wear freshly laundered clothing as a sign of mourning for the destruction of the Temples. This applies to shirts, pants, coats etc (all examples of regular clothing), but not to underclothes1 (Laws of the Three Weeks, Rabbi Simcha Bunim Cohen, page 85).
What are not mentioned in either group (unless I missed it) are clothes meant for exercise, for sweating in, such as basketball shorts or sweatpants. What is the status of clothes such as basketball shorts with regard to the laws of the nine days?
Does it make a difference if the clothes are being worn to exercise ("sweat") in, or if they're worn just for relaxation?
1 Rabbi Cohen does write (there) that some are מחמיר even on underclothes, but for the purposes of this question we'll assume that you don't have to.