What is the X in X-mas?
athletic training which alternates between two different activities, such as walking and jogging, or between two different rates of speed
method of competitive training in which rest and exercise intervals of controlled duration are alternated. Rest intervals allow time for the athlete's pulse rate to return to near normal before beginning the next exercise period. During exercise intervals, the athlete performs at a specified level of performance, slightly less than his best effort. Interval training is frequently used by track and competitive-swimming coaches