I did the same thing when leaving an employer -- with more than a certain amount, I think it was $5K, I had the option of leaving money in that plan or moving it anywhere I wanted.
So if you leave an employer/complete training or whatever, you'd just roll the money over at that time to the fund that you choose.
In the meantime, if the options stink at your employer, you likely have an option to move the money into a bond fund or money market fund or some other secure type of investment rather than putting it into an equity fund that you don't trust.
If you're feeling creative, many 401Ks will allow you to borrow from the fund; mine allowed this up to 50% of total funds in the 401K. You could then take the "borrowed" funds (that you've borrowed from yourself) and invest them in the market if there's another fund not offered by your 401K that you really like; planning to pay back the borrowed funds out of the sign-on bonus from your new employer when done with medical training.