Target heart rate - The ideal intensity level at which your heart is being exercised but not overworked. Determined by finding your maximum heart rate and taking a percentage (60% to 85%, depending on fitness level) of it.

Tendon - A flexible, non-elastic tissue that connects muscle to bone. The Achilles tendon is the large connector from the heel bone into the calf muscle.

Tether - Attached to a belt and then to a ladder or some other fixed point at poolside, a tether helps you turn a too-small pool into a swimmer's treadmill. For example, you can have a great workout in a hotel pool or any other pool that's too small for laps.

Threshold - The heart rate at which lactic acid begins to build up faster than you can break it down. You should do the bulk of your training at just below that level.

Trapezius or "traps" - The triangular muscles stretching across your back from the spine to the shoulder blades and collarbone. They work with the deltoids to lift your arms and shoulders. Good to have strong ones for carrying a backpack!

Triceps - The muscles on the back of the upper arms that straighten your elbows and allow you to push your arms forward. You use them when you're fly-fishing or pushing a running stroller.

U

Underpronation - Another term for supination, or the excessive outward-rolling motion of your feet. The opposite of pronation, or inward movement. Learn how to choose running shoes.

V

Vastus intermedius, lateralis and medialis - 3 of the 4 muscles of the thigh that make up the quadriceps. Strong quads help protect your knees. The 4th muscle is the rectus femoris. Cycling, skiing, running and hiking downhill work these muscles!

VO2 max - The largest volume of oxygen your body can take in and assimilate. This figure is very high in trained endurance athletes.

W

Warm-up - Gentle, slow exercise at the beginning of a workout to prepare muscles, heart rate, blood pressure and body temperature for the activity.

Water dumbbells/water barbells - Flotation devices shaped like paddles that provide extra resistance to your arm muscles when used underwater. They can also be used to hold you at the surface for back and leg exercises.

Weight-bearing exercise - Exercise in which you support your weight or lift weight. Lifting weights or doing weight-bearing exercise (such as running, skiing or walking) can help slow down the rate of bone loss and osteoporosis, and therefore reduce fractures.

Wicking - Moisture movement by capillary action. It usually refers to technical fabrics that move sweat away from the skin to the outer surface of the fabric, where it evaporates. Learn how to choose fitness clothing.

Y

Yoga - A system of exercises for attaining bodily or mental control and well-being. Various forms of yoga include poses (or asanas) for building strength and flexibility, breathing exercises for cleansing, and/or meditation for relaxation and stress reduction.