Fartlek: A type of interval training program that doesn't use an exact measure of time or distance. You just do your intervals whenever you feel like it. The term Fartlek means "speed play" in Swedish.

fitness walking: A faster and more intense walking technique than casual (lifestyle) walking that burns more calories and helps you lose weight. When you fitness walk, you generally move along at a brisk pace of 3.5 to 4.3 miles an hour, covering a mile in 14 to 17 minutes.

flexibility: The range of motion or distance you can move a joint through. Stretching is the key to maintaining your flexibility.

free weights: Portable weights used in a strength training program.

freestyle: The traditional type of swimming movement that uses the front crawl.