The following definitions will be helpful in understanding the studies dealing with muscle injury responses to strength training. Delayed Onset Muscle Soreness: Severe muscular discomfort occurring 1-3 days after performing exercise. DEXA: Dual-Energy X-ray Absorptiometry - a scientific test using x-ray technology to determine body composition. MRI: Magnetic Resonance Imaging - a scientific test that uses magnetic fields and radio waves to produce colorful images which differentiate between muscle and fat. Miometric Muscle Action: Refers to shortening of the muscles, often referred to as concentric contractions; also called positive work. Muscle Hypertrophy: An increase in muscle size. Pliometric Muscle Action: Refers to lengthening of the muscles, often referred to as eccentric contractions; also called negative work.