What’s to Blame for the NFL’s ACL Problem?

Looking at three possible culprits causing the NFL’s annual slew of preseason ACL tears.

A running back hoping to prove his value as a starter after years of being relegated to a “situational role” takes the hand-off in a seven-on-seven drill, and accelerates toward the “B” gap. At the last second, he realizes that his offensive guard hasn’t been able to release off the defensive tackle and get to the second level to close down on the inside linebacker.

“Well, not today bro” — the running back thinks as he instinctively sticks his leg into the ground to change direction and bounce outside…and then crumples to the ground clutching his knee.

He’s carted off the field, taken for an MRI and the worst fears are confirmed: an Anterior Cruciate Ligament (ACL) rupture.

All the hours he put in leading to this moment, finally on the cusp of breaking into a starting role, hitting that lifelong goal, and now he’s faced with the spectre of a nine-month (on average) rehab process with side-to-side deficits potentially lasting two-plus years and increased risk for osteoarthritis in the knee.

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To pour salt on the wound, the team could even drop him with limited ramifications (the NFL is built on non-guaranteed contracts) and move on to business as usual.

This scenario isn’t the latest script for a Lifetime movie—rather it’s what happened to Jerick McKinnon of the San Francisco 49ers, and unfortunately, many more just like him. ACL tears have become a common reality for NFL players and hopefuls as they get back to OTAs and minicamps, known as “phase 3” of the NFL offseason.

Over the past five years, there have been an average of 23 ACL tears prior to Week 1 of the NFL regular season. In 2018, that number stands at 22.

The majority of these ACL tears — nearly 73 percent according to a recent study — are non-contact injuries. Unlike a contact ACL injury where blunt force trauma to the knee or surrounding area stresses the ACL beyond its capacity, a non-contact ACL tear points to some underlying cause(s).

After rummaging through numerous possibilities, three were bouncing around in their seats yelling, “Ooooo, pick me! Pick me!”

1 — Off-Season Training Quantity and Intensity

2 — Off-Season Training type

3 —Playing surface

Let’s go through each in a little more detail:

1 — Training Quantity and Intensity

In the off-season, NFL players and hopeful NFL players aren’t allowed to work out under team supervision. Rather, they’re working with trainers or on their own to get ready for OTAs and minicamp.

This opens up a whole can of possibilities as you have players who have grown accustomed to a set and segmented scheduled that are now left to their own devices.

As one retired NFL player put it:

“You wake up with nothing to do. You’ve spent months having a schedule, and now you have none.”

Naturally, this can lead to a lot of variability in training — from taking it easy and not training most of the summer to training and working hard every single day. Extensive research shows that a specific ratio of training quantity and intensity optimally prepares the body for current and future activity.

This ratio is known as the “Acute-to-Chronic Workload Ratio.” The terms acute and chronic, in this formula, are defined as days of activity. Acute refers to activity over the past three days and chronic refers to activity over the past 21 days. The original ratio was seven days to the past 28 days, but further research showed the 3:21 ratio as being more reliable.

This ratio dissects whether the activity level in those past 21 days was appropriate in preparing for the current three days of training. Every activity on each of those days is given an arbitrary unit (AU) based on the rate of perceived exertion, aka RPE. This is a scale of 1–10 with 1 being “watching Netflix on a waterbed” and 10 being “I might die” multiplied by the number of minutes. Those units are then totalled up for that day.

So if you had a lifting session this morning for 90 minutes at an RPE of 6 and then cardiovascular training in the afternoon for 45 minutes at an RPE of 8, that would equal (90 x 6) + (45 x 8) or 900 AU for that day. These units are averaged for the acute (last three days) period and chronic (last 21 days) periods and then divided.

Taking that one step further, let’s say player “X” got a new contract last season and goes into the summer thinking “I’m set, I’m just gonna take it easy with my own workouts and then get into shape during OTAs and minicamp.” His workload ratio could look something like this:


Ideally, that number on the bottom corner (the acute to chronic workload ratio) should be between .8 and 1.3. If that ratio is lower than .8 or higher than 1.5, studies have found the risk of soft tissue injury goes up by over five times.

In this case, player “X” is woefully unprepared for OTAs and that creates a lot of risk.

I think most people understand that “doing too much too soon” and overloading/fatiguing the body may lead to injury. Whether they follow that advice is a different story, but not doing enough and under-preparing the body is potentially just as damaging.

That’s why if you’re injured and want to get back to your activity of choice, rest alone won’t cut it. You have to gradually increase your activity intensity and/or quantity or your body will be underprepared.

To further that point, harken back to Jerick McKinnon. He tore his ACL in a non-contact manner during the last play of his first practice after returning from a calf strain that kept him out most of the Niners preseason. Mere coincidence? I highly doubt it.

I’d wager that if we tracked his AU’s and acute to chronic workload ratio, he was in that “under-prepared” zone due to being limited by his calf strain.

The body needs to be prepared and loaded appropriately to be able to handle the rigors of that activity. In the same vein, it can’t be overloaded and fatigued. It needs to be just right — the Goldilocks effect.

If the workload ratio of Los Angeles Chargers’ Hunter Henry, who tore his ACL on the first day of OTAs, was out of whack as he transitioned from his summer programming to Chargers OTAs, that would increase his risk for increased soft tissue injury, which includes ligament injuries, five times.

Additionally, these acute to chronic ratios and risks may be compounded by the shortened amount of time NFL teams are allowed to spend with players. This may seem counterintuitive at first. NFL players having longer offseasons and less practice may mean more injury risk, and there’s a perfectly valid reason why.

By numerous accounts, the shorter amount of time for OTAs and minicamps has led to teams jumping in feet first with their players on day one rather than allowing for a gradual progression of activities.

Here’s how Dr. David Chao, aka ProFootballDoc, put it:

“Because of fewer practices, there are actually no walk-throughs. Everything is full speed, so everything puts the ACL at risk.”

I can understand the rationale from NFL coaches and staffs. They’re trying to install the same amount of information and habits in less time. That lends itself to cramming things in and jumping straight into high-intensity practices.

You try telling this guy to ramp down practice intensity a little bit:


Perhaps Khalil Mack bringing up Acute-to-Chronic Workload Ratios was the final straw.

For a player who isn’t prepared for that intensity or a player who is already fatigued, they’re at a higher risk for ACL rupture, so too little or too much training quantity and intensity is potential reason number one for the NFL’s barrage of non-contact ACL tears. Moving onto number two…

2 — Training Type

There is a multitude of ways for NFL players to train during the offseason: weightlifting, which itself can be divided into different categories such as strength or muscle growth, interval training, like sprints or circuit training, endurance, plyometrics, ballistic, etc., etc.

However, one type of training program has repeatedly been shown in large-scale studies (click hereherehereherehere, or here for more info) to reduce ACL injury risk: neuromuscular training (NMT). The neuromuscular system is an unconscious feedback system through which muscles are timed and activated to optimize joint stability and promote efficient and effective movement.

If we think of the physical components of the body as the “hardware”, then the neuromuscular system is part of the “software”.

The foundation of optimizing this NMT software are exercises that involve unanticipated reactions and movement perturbations. This forces the neuromuscular system to react quickly and maintain joint stability.

Here’s a video with multiple examples:

This maintenance of joint stability by the neuromuscular system is critical for preventing ACL injuries because most non-contact ACL injuries happen during spontaneous cutting and changes in direction in which the knee drops into a valgus (inward) and internally rotated (rotated towards midline) position.

If the knee continues to dive inward and rotate, there comes a point of no return where the ACL is placed under huge stress and ruptures:

Through NMT, the neuromuscular system can recognize that unstable joint position quicker (“Ah, $%&! Red alert!”) and then activate the necessary muscles to bring that joint back into a better position.

For example, let’s say Titans’ safety Jonathan Cyprien, who tore his ACL during practice in early August, had focused more on NMT during his off-season programming. I have no idea whether he did or didn’t nor to what extent, but as he made a reactive cut during practice, and his knee dropped into an inward, high-risk position, his highly trained neuromuscular system would have recognized it quicker, got the knee joint out of danger and not allowed it to reach the point of “no return.”

In other words, NMT increases the margins for error during movement.

Based on some personal knowledge of NFL players’ offseason programs and what is available to the public (I’m the first to admit this is completely anecdotal evidence), NMT doesn’t seem to be that common.

There’s plenty of:



But I have a much harder time finding things like this:

From what I’ve seen and heard, Odell Beckham Jr. is a shining beacon on the hill when it comes to training variation and challenging multiple systems.

If players are in a program that utilizes NMT (neuromuscular training), the evidence shows that it needs to be done at least two times per week with appropriate progressions throughout the regiment to be effective.

It’s a highly skilled level of training, and I’d wager a large number of NFL athletes aren’t aware of its benefits in their offseason training. As these players get back to spontaneous changes of direction during OTAs and minicamp, they’re at a comparatively higher risk for ACL injuries.

These first two underlying causes have focused on training, but the third and final possible reason for the NFL’s ACL problem takes a sudden, sharp left turn. Don’t worry, though. This piece did NMT training. ACLs are safe.

3 — Playing Surface

The more traction a playing surface has, the more likely a player is to suffer an ACL injury (click herehereherehereherehere, and here for more info)

Each time Los Angeles Rams’ defensive Morgan Fox, who tore his ACL during the first week of OTAs, stuck his foot in the ground to make a cut or change direction on the drought-stricken, high-traction grass fields, that foot had a higher chance of getting caught in the ground and torque-ing the knee.

To that point, research has found that less than five percent of ACL injuries in football occurred on a wet (low-traction) surface, whereas dry, hot weather (high-traction) was linked with a higher rate of ACL injuries. Generally, cold weather has been associated with lower rates of ankle and knee injuries, including ACL tears.

Think of this high versus low-traction situation like ski bindings going to work when a skier loses control.

High-traction conditions are akin to crashing and the bindings not freeing your foot (the foot getting stuck in the grass), whereas low-traction conditions are the same situation but with the bindings releasing your foot (the foot skids along the grass), like in the video you just saw. In the latter case, your foot and knee are clear from getting twisted.

NFL OTAs and minicamps take place in high-traction conditions — the dead heat of the summer with hot, dry conditions. Players are cutting and changing directions on high-traction surfaces, which opens them up to the risk of crashing with skis still attached to their feet.

So the next time you see a player slip on a cold, chilly surface or see a rainy-weather forecast as you make your last second fantasy changes— try thanking it instead of screaming expletives.

All in All

When you combine all three factors — an inappropriate training quantity and intensity plus lack of neuromuscular reaction training plus a higher-traction surface, it might explain this trend of non-contact ACL ruptures.

A few of you may be wondering, “Well, what about knee braces? I’ve heard those can help.” There’s no evidence that knee bracing prevents ACL tears. In fact, one two-year study found that ACL injury rates were actually higher amongst players who wore a knee brace compared to those who didn’t.

We’ve come a long way in treating ACL injuries, and they aren’t a death knell by any means, but they still place a significant and possibly preventable physical and mental burden on the player. For those players just trying to make the team, preventing an ACL injury could mean the difference between a livelihood and foundation for their family (the minimum NFL salary is $450,000).

Although we’ll likely never know the definite root cause(s) of these non-contact ACL injuries during NFL OTAs, minicamps and training camps, I hope this piece shed some light on possible causes and ways to reduce ACL rupture rates. After all, the best injury is the one that never happens.

Dr. Rajpal Brar
Dr. Rajpal Brar
Dr. Rajpal Brar, DPT, is a writer for Grandstand Central with a background in coaching, business, and strategy. He’s the founder of his own sports blog/resource TheInjuryInsight and also writes for SilverScreenandRoll, ThisIsAnfield, and TheDoctorWeighsIn. He’s the owner of 3CB Performance, a wellness and athletic development/performance clinic in West Los Angeles & Valencia, CA.


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