Fascia
Fibrous connective tissue that covers, supports and separates all muscles and muscle groups. It also unites skin with underlying tissue.

Fast Twitch
Refers to muscle cells that fire quickly and are utilized in anaerobic activities such as sprinting and power lifting.

Fat
Often referred to as lipids, or triglycerides, one of the main food groups, containing nine calories per gram. It serves a variety of functions in the body, however a high percentage of body fat has been proven to be bad for you.

Fibrin
The substance that in combination with blood cells forms a blood clot.

Flex
Bend or decrease angle of a joint; contract a muscle.

Flexibility
(ROM) Range of movement in a joint or group of joints.

Flexion
Bending in contrast to extending, as in leg flexion's.

Flush
Cleanse a muscle by increasing the blood supply to it, removing toxins left in muscle by exertion.

Forced Repetitions
Assistance to perform additional repetitions of an exercise when muscles can no longer complete movement on their own.

Free Style Training
Training all body parts in one workout.

Free-Form Amino Acids
Structurally unlinked individual amino acids.

Free Radicals 
Highly reactive molecules that possess unpaired electrons. Caused by a number of factors, look at Antioxidants for prevention.

Fructose
Often used as a sugar substitute for diabetics, because of its low glycemic index. A healthier option than normal sugar, as fructose comes from fruit.

Full Spectrum Amino Acids
A supplement that contains all of the essential amino acids.