This acronym means Rating of Perceived Exertion.

It's often better to use a more subjective training guide which takes into account your physical and emotional state in determining your individual workout intensity, rather than relying on an objective (arbitrary, really) percentage of age-predicted maximum heartrate.

my RPE	Borg RPE Scale	Description
1	6	no exertion at all
	7	extremely light
	8
2	9	very light
	10
3	11	light
4	12
5	13	somewhat hard
6	14
7	15	hard (heavy)
8	16
8.5	17	very hard
9	18
9.5	19	extremely hard
10	20	maximal exertion

For your reference, the American College of Sports Medicine suggests that 4 to 6 RPE equates with "somewhat hard to hard" exertion, and that that correllates with 60% to 85% of Max HR.

Use this scale too to find your appropriate tension guidelines for improving flexibiltiy. A good stretch will be between 3 and 5 on my RPE scale. Always ask you physician if he / she feels there are any limitations or restrictions on your exercising vigorously before you begin or intensify any physical training program.

Borg RPE Scale -- © Gunnar Borg, 1998, Borg's Perceived Exertion and Pain Scales, Human Kinetics, Champaign, IL.