ive been lifting 3x a week for the last 2 months, doing monday-back&shoulders, wednesday-legs, friday-chest&arms and doing cardio stuff on other days. i was thinking of adding a fourth lifting workout a week on sundays, but that would mean that i would lift 2 days in a row, with the lifts changing each week (because i would still stick with the 3 differnet muscle groups). this means that some weeks i would be doing sunday-chest&arms right into monday-back&shoulders. is that bad? different question: if ive just doing back and ive done pull ups and im moving to lat pull downs, but the machine is busy, how bad is it if i switch to a shoulder exercise until the machine is free? basically, is it important to do all the same muscle group exercises at once or can you mix around as machines become available? thanks a lot!