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3Interval Training is a series of increasingly steeper hills, alternating with valleys, or stints of recovery. The heart rate should rise to the high end of the target zone. Check the heart rate at the end of this phase.

4Cool-downis a low-intensity phase that allows the body to begin removing lactic acid, and other exercise by-products, which build up in muscles and contribute to soreness.

Each column in the chart above represents one interval. The overall duration of the workout determines the length of each interval. Each workout is made up of 20 intervals, so the duration of each interval is equal to the duration of the entire workout divided by 20.

1 to 9 minutes: A workout with a duration of less than 10 minutes is insufficient for the HILL program to complete all four phases adequately. The program, therefore, condenses a workout of this duration at various stages.

10 to 19 minutes: The interval durations initially are set at 30 seconds for a 10-minute workout. For every minute over 10, each interval increases by three seconds. A 15-minute workout consists of 20 intervals at 45 seconds each.

20 to 99 minutes: All intervals last 60 seconds. If the user adds minutes to the pre-set duration while the workout is in progress, the program adds hills and valleys that are identical to the first eight intervals of the Interval Training phase. This pattern repeats until the workout is completed.

The RANDOM program creates a terrain of hills and valleys that varies with each workout. More than one million different patterns are possible.