This article is part of our Injury Analysis series.
The injury grabbing all the headlines in NFL training camps is hamstring strains. The ailment is far too common but not surprising given the makeup of today's NFL player and the demands of the sport.
The hamstring is a dynamic muscle group. It's a two-joint muscle working to bend the knee (knee flexion) and extend the hip. The hamstring works in the opposite direction of the quadriceps muscle group. This antagonistic relationship allows the two muscle groups to accelerate and decelerate the knee, like when a running back is attempting to hit a hole or a receiver explodes off the line of scrimmage. The relationship between the hamstring and quadriceps can play a role in injury if there is an imbalance in strength between the two opposing muscle groups. If the athlete's hamstring is considerably weaker than his quadriceps, it is unable to equally counteract the strong contraction of the quad leaving the hamstring vulnerable to strains. Once a strain occurs, hamstring injuries can take considerable time to heal. The healing process is divided into multiple phases, but we will focus on the final few parts.
After the body's initial reaction to the injury, the damaged tissue begins to be repaired and restored. Scar tissue, compromised of collagen and elastin, develops at the injury site. This phase, known as the proliferative phase, generally takes four to six weeks depending on the severity of the strain. Occasionally, an injured player will return following the proliferative phase, though he often