A hamstring injury might seem like something an athlete can recover from quickly. But this common injury can sideline you for several weeks and become a nagging injury throughout your career.

More athletes seem to be suffering from hamstring injuries—especially in the NFL. In a recent STACK article, we looked at the factors that might be contributing to the rash of hamstring injuries.

According to Ryan Flaherty, CEO of Prolific Athletes (Carlsbad, California) and trainer to elite NFL athletes such as Russell Wilson, Marcus Mariota and Carson Palmer, more than 150 players have suffered a hamstring injury during the 2015 NFL pre-season. That is simply absurd and unacceptable.

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When asked about the cause, Flaherty said it all comes down to one thing: NFL athletes not sprinting enough during their off-season training.

The NFL season takes a toll on the body. Body contact is the most obvious culprit, but constantly sprinting, cutting and changing direction causes serious wear and tear over 16-plus weeks of practices and games.

Football players train hard in the off-season. However, they also make a concerted effort to allow their bodies to recover. To facilitate recovery, many of them don’t sprint at high speeds.

“Most of these guys spend six weeks in the off-season doing some agility, speed and strength work, but for the most part they just want to protect their bodies and get ready for training camp, so they’re not doing max-velocity sprints,” Flaherty says.

The thought process behind this decision is completely logical. Hamstring injuries most frequently occur when an athlete is sprinting near top speed. During the stride, the hamstrings act to decelerate the lower leg as it swings from back to front. This causes the hamstrings to maximally contract, and if the muscle is not prepared, an injury will occur.

That’s why max-velocity sprinting is often thought to cause hamstring injuries, not prevent them. But according to Flaherty, this is a myth. He says, “In the United States, we don’t do max-velocity sprinting. We just want to develop the hamstrings in the weight room and find a better exercise for it. There’s no way in a weight room setting with any strength exercise to get a maximum voluntary contraction from the hamstring.”

It’s a sure bet that almost every NFL athlete is training his hamstrings with some type of exercise in the weight room, using moves like RDLs and Physioball Hamstring Curls. These exercises aren’t necessarily bad, but they’re not enough. “The players get into camp and they have to go for a deep route and track a ball down, and they haven’t sprinted in six weeks, and a hamstring goes,” Flaherty explains. “The reason they’re all pulling hamstrings is because none of them are doing max-velocity speed work.”

Flaherty points to international sprinters to illustrate that the hamstrings can in fact be trained with speed work. “I could show you track sprinters from all over the world, and you would see the most impressive hamstrings in your entire life and not one of them has trained them in the weight room,” he says. “That’s because they understand that hamstrings are developed with max-velocity sprinting and not in the weight room.”

Flaherty has all of his athletes—even his quarterbacks—do max-velocity sprint work to train their hamstrings. And the results speak volumes. Not one of his athletes sustained a hamstring injury during the pre-season.

Below is the Flaherty-designed workout we watched Marcus Mariota perform as he was preparing for his NFL debut.

• Falling Starts — 2-3×30 yards
• Resisted Sprints — 4×25 yards
• Full Sprints — 2-3×25 yards
• Resisted Lateral Shuffle — 2-3×20 yards each direction
• Resisted Backpedal — 2-3×20 yards

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