There are several ways you can perform an interval training workout. For less fit clients on the treadmill, this may mean three minutes of "easy" work at 3 mph and 0% grade, then one minute of "hard" work at 3.5 mph and 0% grade. This might be repeated five times for a total of 20 minutes. This type of training can be utilized all the way up the fitness ladder.
For extremely fit clients, the program may consist of four minutes of running at 9 mph, alternated with one minute of relief at 8 mph, and so on.
Advantages of this type of training are that the total amount of work performed may be more likely to adhere, and it is easy to systematically cause progressive overload.