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Fitness

A New Look at The Cause of ACL Injuries

ACL injuries are common injuries in may sports. The knee undergoes many opposing forces during practice and competition. Now, science takes a new look at what causes an ACL injury.

Tom Kelso

Written by Tom Kelso Last updated on Oct 21, 2021

Someone blows out an anterior cruciate ligament (ACL). We have all heard of it, and hopefully not experienced it. The ACL (see where it is located) is an important component of knee joint stability. Tearing an ACL means surgery, recovery, and rehabilitation. Although modern techniques have shortened the time it takes to get back to full functionality, it still takes months (and possibly a lost season) to arrive there.

What exactly do we know about the mechanism(s) of ACL injuries today? It is now thought a combination of factors cause the ACL to rupture. Let’s take a look.

A non-contact (no other person/external force involved) landing from a jump movement can cause an ACL injury. Evidence presented at the Orthopaedic Research Society 2012 Annual Meeting, however, demonstrated the injury mechanism that causes the ACL injury involved a combination of factors as opposed to a single factor, which many have claimed. It is a common belief that a rupture can occur in the ACL via a single plane motion – the tibia (lower leg bone) moving forward due to a significant quadriceps contraction.

According to Timothy E. Hewett, PhD, FACSM, Director of Research, Ohio State University Sports Health and Performance Institute and Cincinnati Children’s Sports Medicine Bio-dynamics Center, ACL injuries can be due to a tri-planar multi-dimensional combination of factors. “Sometimes in science we have a lot of clinical expertise and a lot of engineering expertise but we don’t have much—what I call–‘common sense-pertise’.”1

Hewett continued:

Is it just anterior (forward) translation that strains and tears the ACL? Is it just knee abduction (outward movement) or that inward motion that ruptures the ACL? Is it just internal rotation that tears the ACL? Our study demonstrates that each one of these factors can strain the ACL. But it is the combination of anterior translation, abduction and internal rotation that likely ruptures the ACL.2

Contrary to conventional wisdom, Dr. Hewett’s study demonstrated abduction strained the ACL more than anterior translation. In addition, internal rotation was similar to that of anterior translation.

The researchers also had a unique discovery regarding internal rotation torque. “We showed abduction increased the load on the ACL just as much as anterior translation did. Internal rotation increased load just as much as anterior translation did. But most importantly, when all three are combined, there was an additive effect in all three planes,” Dr. Hewett explained.

To investigate the issue further, Dr. Hewett and his colleagues conducted simulated jump landings on 19 subjects. There were 17 (89.5%) ACL failures using a custom designed drop-stand.

knee injury, acl injury, acl tear, soccer injury, bjj injuryThe subjects were divided into two loading groups: without anterior shear and with anterior shear. They tested the effects of anterior tibial shear, abduction, and internal rotation under dynamic axial loading on ACL biomechanics. They found that single-axis abduction increased average ACL strain from 5.8 to 9.8 percent. In both groups, the addition of abduction or internal rotation increased ACL loading—with abduction loading the ACL more than internal rotation.

Under axial impact, the combination of abduction, internal rotation, and anterior shear increased the average ACL strain significantly. This lead Dr. Hewett to declare, “Data from this study indicates that the most critical dynamic condition that leads to ACL failure is a combination of anterior shear, abduction and internal rotation under axial impact.”

Here is what can be derived from this study: during athletic competition (or any other activity) where a variety of movements/forces are placed on the knee joint, the structural integrity of the joint (the muscles, tendons, and ligaments securing it) can be compromised. In the past, it was believed that one single abnormal deviation was the root cause of an ACL rupture. However, it now looks like multiple deviations will cause a compromise, thus injury.

Can all ACL ruptures be avoided? Obviously there is no way to guarantee that. How can one then minimize their risk of tearing the ACL? Strengthen the knee joint musculature in the weight room and safely practice sports skills. Perform conditioning activities that progressively adapt the knee muscle, tendon, and ligament structures to forces that will be experienced during competition (e.g., position drills, agility drills).

While ACL injury is a frequent occurrence in sports, with these precautions and this new knowledge there is also new hope your knee will not be exposed to a combination of forces that exceed the structural integrity of the joint.

Tom Kelso

About Tom Kelso

Tom Kelso is currently an Exercise Physiologist with the St. Louis Metropolitan Police Department. He also trains clients through Pinnacle Personal & Performance Training in Chesterfield, Missouri.

For 23 years he was in the collegiate strength and conditioning profession, serving as the Head Coach for Strength and Conditioning at Saint Louis University (2004-2008), the University of Illinois at Chicago (2001-2004), Southeast Missouri State University (1991-2001), and the University of Florida (1988-1990). He got his start in the strength and conditioning field as an Assistant Strength Coach at Florida in 1984 where he was also a weight training instructor for the Department of Physical Education from 1985 to 1988.

In 2006, Tom was named Master Strength and Conditioning Coach by the Collegiate Strength and Conditioning Coaches Association for his years of service in the field. In 1999, he was named NSCA Ohio Valley Conference Strength and Conditioning Professional of the year. In 2001, he received an honorary certification from the International Association of Resistance Trainers (I.A.R.T.).

Tom possesses C.S.C.S. and S.C.C.C. certifications with the NSCA and CSCCA, respectively. Additionally, he is certified by the Illinois Law Enforcement Training and Standards Board in basic instructor development and as a specialist instructor by the Missouri Department of Public Safety. In 2012, he became certified by the IBNFC as a Certified Nutrition Coach.

Tom has worked with athletes at the Olympic and professional levels, presented at various clinics/seminars, and worked several athletic-related camps. He is a strong advocate of safe, practical, and time-efficient training and has published a collection of periodical articles, book chapters, complete books, and user-friendly downloads promoting such.

Tom received a Bachelor’s Degree from the University of Iowa in 1981(It's great to be a Hawkeye!) and a Master's Degree in Physical Education from Western Illinois University in 1984. He was a member of the Track and Field team at Iowa and served as a Graduate Assistant Track & Field Coach while at Western Illinois.

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