Consider These Facts:

• The average length of an NFL career is about 3.5 years. (NFLPA)
• When a muscle is damaged by overuse, it can easily lose up to 40 to 50% of its normal strength, a decrement, which can impact performance in a profoundly negative way (“Muscle Function and Protein Metabolism Following Initiation of Eccentric Contraction-Induced Injury,” Journal of Applied Physiology, Vol. 79, pp. 1260-1270, 1995).
• We evaluated 332 elite collegiate US football players at the 2005 National Football League Combine. All players underwent radiographic examinations, including plain x-rays and/or magnetic resonance imaging when necessary. All knee pathologic conditions and surgical procedures were recorded. Data were analyzed by player position to detect any trends. Fifty-four percent (179) of the 332 players had a history of knee injury. (“PubMed” Incidence and variance of kne injuries in elite college football players. Bradley J, Honkamp NJ, Jost P, West R, Norwig J, Kaplan LD.)
• Approximately 50-55% of power comes from the lower extremity
• An additional 25-30% of power comes from elastic energy
• Antagonist inhibition occurs most readily during rhythmic activity associated with motor learning. The hamstrings can rupture if they contract to oppose the quadriceps while sprinting. (“Supertraining” Siff, C Mel)