Michael phelps swim wear decision
I follow Michael phelps on Instagram and I noticed he trains in swim briefs but wears jammers at meets. Any reason why?
I would most likely say because of drag/resistance.
You may have noticed that media shots of the Olympic warmup pool showed swimmers in briefs. In the actual heats all swimmers that I saw wore tech jammers.
The jammers he's wearing are a tech suit. They're made with a material that has less drag than other suits, which makes the suit "faster." There has been an unsolved question of whether there exists a suit material that is faster than skin; suit companies have probably done some testing and figured out that material on the legs in the jammers is faster than just swim briefs.