Would atheletic training affect the DNA of the ofspring?

You have two types of cells in your body: somatic cells (cells comprising all the tissues and organs) and your germ cells (i.e. your sperm or ova). It is ONLY your germline DNA (half of it, to be exact) that gets passed on to your kids, and that DNA is "protected" from the environment--e.g. exercise is NOT going to have any effect on it.

The effects of your own training are limited to your SOMATIC cells, e.g. the cells in your heart, skeletal muscle, etc. It is NOT going to affect your germline DNA, and therefore has nothing to do with what your kids are. You want your kids not to be "wimps," you're going to have to put them on their own exercise programs.

Your DNA is unchanged from the moment you are conceived until the moment you die.

Training will have no effect on it.

Your DNA is your DNA. It is all you've got and will not be changed by exercise. You will pass on your DNA and your partner will pass on their DNA. Your children will have characteristics that combine both. Some genes are more dominant than others so it is hard to predict the outcome.