When the tattoo is healing, which is primarily the first 7-10 days, that's when it is the most vulnerable. It takes that long for the skin surface to really close. Things can infect it from the outside, hence the washing/wetting caution, plus it can be infected from things like sweat, which tend to come more internally.
It'd give the heavy running a rest for a week while the skin does a lot of healing. Sweat and water can also make the scab come off too soon, which can cause patches to "heal out" meaning the color comes out, leaving paler spots that need to then be recolored.
