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Hamstring injuries occur so frequently that fans sometimes tend to brush them off as insignificant, but Green Bay Packers defensive back Casey Hayward's is anything but.

According to Tyler Dunne of the Milwaukee Journal Sentinel, Hayward suffered his hamstring pull all the way back in July during Packers training camp. Dunne also mentioned that at the time, Hayward was a "couple weeks away"—presumably from returning.

Regrettably, it didn't turn out that way.

As is frequently the case with hamstring injuries, Hayward's recovery proved difficult to project and maintain. On Wednesday, ESPN Wisconsin's Jason Wilde tweeted the Packers ruled him out for Week 1.

A closer look at the nature of hamstring strains makes it clear why the battle Hayward is fighting is sometimes a difficult one.

Though usually referred to as a singular entity, the hamstring is actually a group of three muscles—the semimembranosus, semitendinosus and biceps femoris. They start at the pelvis, run down the back of the thigh and attach just below the knee. When they contract, the knee bends and the hip extends.

Unfortunately, since NFL athletes use them at maximum intensity on a daily basis to jog, sprint and cut, the hamstrings are very prone to injury.

That risk stems from not only overuse, but also from the very biomechanics of sprinting.

In April 2012, Dr. Anthony Schache and colleagues at the University of Melbourne studied the functional anatomy of the human hamstring muscles in an attempt to discern why injuries occur so frequently.

In the study, investigators measured the force sprinting athletes applied to the ground and combined the data with computer model simulations. By doing so, they discovered that the hamstring stretches to maximum length at the final portion of the forward-leg-swing phase of running. It then starts to contract immediately afterward and just before the foot hits the ground.

In other words, sprinting puts hamstrings through repeated cycles of maximum lengthening followed by forceful contraction. If a hamstring is not in optimum shape—and sometimes even if it is—a strain occurs.

Like all strains, hamstring strains are muscle or tendon tears. A grade-one strain is a mere over-stretch with only microscopic tearing, while grade-two and grade-three strains are partial and complete muscle tears, respectively.

Grade-three strains usually require surgery to reattach the two pieces of torn muscle—such as in the case of Minnesota Vikings linebacker Desmond Bishop.

On the other hand, rest, icing and physical therapy are usually enough to properly treat grade-one or grade-two strains. Fortunately, though medical details are sparse and unavailable to the public, there is no reason to believe Hayward's is any worse than grade two.

However, even though they don't require surgery, grade-one and grade-two strains can linger on and on.

And on.

Why?

Following a strain, a muscle is weaker until it fully heals. It is also less able to resist the types of forces that caused an injury in the first place. Therefore, any strain—even the mildest—predisposes to further, more serious injury.

Additionally, hamstring strains usually manifest themselves as a shorter stride length on the injured side, adding the risk of compensatory injury and other complications of rehab.

Nevertheless, an athlete recovering from a low-grade hamstring strain will eventually feel like him or herself after several weeks of proper treatment.

That said, the last stages of rehab—when a muscle may feel normal but actually needs a bit more recovery time—are prime for re-injury. Players feel like they can push themselves to 100 percent, but the muscle is not yet ready and tears once again.

It doesn't end there, either.

According to a study by Dr. G. Verrall and colleagues, previously injured hamstrings are at a 4.9-time higher risk of future strains when compared to those without prior injury.

What does all of this mean for Hayward?

Simple.

He likely has a textbook case of the hamstrings.

Eventually, the healing process will catch up to the rehab demands Hayward places on his ailing limb—even as early as Week 2.

Yet as mentioned above, it could also take much longer—the true definition of "week to week."

Unfortunately, the Packers have little time to wait. The 49ers, Redskins and Bengals—all 2012 playoff teams—are looming.

Dr. Dave is a resident physician at the University of Washington who later plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine. Except where otherwise cited, medical information discussed above is based on his own knowledge and clinical experience.

Follow @DaveMSiebert

