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  • Writer's pictureMelissa Buehler

ACL Injuries - Why such a high rate of tears?

Updated: Mar 7, 2023

By: Melissa Alonzo, PT, DPT, is a private practice owner, Physical Therapist and Strength Coach in Mammoth Lakes.

If you’ve ever had an ACL tear, you know how long and painful the rehabilitation is. And how hard and long it takes to get back to previous levels. Despite all the hard work, the rate of re-injury is high.

1 in 3 athletes who return to sport following an ACL-R, sustain a second ACL injury. Nearly half of those injuries occurring within 2 months of returning to sport. Women have almost 6 times of a higher chance to injure their contralateral ACL when compared to men (26% to 5% respectively).

Younger athletes are more likely to return to preinjury levels. But, athletes younger than 20 year old have 6 times increased chance of a graft rupture. And are at a 3 times increased risk of a contralateral ACL tear, when compared to athletes older than 20.

Following ACL reconstruction (ACL-R), a person will find themselves in rehab for an extended amount of time. This can be anywhere from 4 months to 6 months. Many find that they still lack the strength and control to return to previous levels. Despite the long rehab, many discover the hard work is not finished yet.

A second ACL injury tends to occur within the first 6 months to 2 years following return to sport. Research (Losciale 2019) has shown that there are deficits in strength, landing mechanics, proprioception, psychological readiness that persists for 2 years and may continue up to 20 years after surgery.

Athletes, specifically kids and young adults, get stuck in this vicious cycle of injury à surgery à rehab à injury; this keeps repeating itself until they are forced to leave their sports. Lindsey Vonn just had her fifth surgery on her right knee this year and left the sport of skiing in 2019.

Despite the surgery, long rehab and return to sport testing, the rate of re-rupture continues to be high. Why is that?

Muscle Strength

Many experts agree that the high rate of reinjury is due to persistent muscle weakness.

Following ACLR, muscles weakness is persistent and associated with abnormal biomechanics, poor knee function, new knee injury and development of early onset osteoarthritis. A study found that 6 months after ACLR, up to 40% of people have side to side differences in quad strength. Strength difference have been found up to 2 years out from surgery. (Volk 2022)

Muscle atrophy following an ACL injury is complex. It is thought to be due to a complex cascade of neurological and inflammatory events following injury or surgery. Many people experience a phenomenon that is called atherogenic muscle inhibition (referred to as muscle inhibition).

Muscle Inhibition is described as a reflexive, ongoing neural inhibition of the muscle around an injured joint, even though the muscle itself is not injured.

Muscle inhibition is thought to be a protective mechanism that restricts range of motion and prevents further damage of the joint following injury. Inhibition impedes voluntary muscle activation and can be long lasting, contributing to the significant loss of muscle strength.

Icing and electrical stimulation have been found to be successful in treating inhibition. This is treated early in the rehab. So why does quad weakness persist?

Mitochondria Health

A group of researchers are looking into disruptions at the cellular level due to injury. Davi, et al. (2022) found that there are long-lasting impairments in mitochondrial health following ACL injury.

Mitochondria are the energy centers of cells. They play a central role in metabolism, regulate energy supply, reactive oxygen species production, calcium homeostasis and apoptosis. Derangement in their function is a common feature of many atrophying conditions (bed rest, disuse and aging).

Davi et al (2022) completed an experiment on rats with ACL injuries (they inflicted the injury). They found clear evidence that there are long lasting impairments in mitochondrial health. It starts 7 days after the ACL injury and continued through day 56. The muscle is unable to process locally available oxygen, leading to widespread energy deficits, disruption in protein synthesis and muscle atrophy.

Disruption to the mitochondria may explain why quadricep muscle size and composition is severely compromised after ACL injury. But these findings show the disruption is short lived (up to 56 days), why are there strength deficits up to 2 years?

Muscle Hypertrophy

In the typical rehab center, bands and body weight exercises are used for exercise prescription. There have been findings that low load can produce hypertrophy in a muscle. But, low intensity strength training after surgery leads to lower muscle power response (Bieler 2014).

While low intensity training could be appropriate for some populations, this is not appropriate for the athletic population. Once an athlete has been cleared by the surgeon and has clean functional movements, they need to load heavy and at a high intensity. Research shows that high intensity strength training at 75-90% 1RM with high volume, is necessary in order to overcome muscle atrophy and inhibition (Bieler 2014).

The another overlooked aspect of building muscle for rehab is diet. An athlete needs to meet their body’s protein requirements, which will be higher following injury. If they don’t consume enough carbohydrates and proteins, there won’t be enough building supplies (and energy for building) available.


Neuroplastic changes following ACL injury is likely due to altered sensory feedback from the injury. There is a loss in mechanoreceptors from the original ACL and persistent muscle inhibition and weakness. The sensory feedback loop that is used for joint positioning and force production, is disrupted.

Due to these disruptions, neuroplastic changes occur by increasing input from other sensory systems. Dustin Grooms and his associates (2017) found increases in activity in the brain in the frontal and parietal regions in those with ACL-R. This is demonstrated by an increase in reliance on visual feedback and cognitive effort.

These changes in the sensory feedback loop can disrupt an athletes response time when passing through gates or the joint position when cutting on the soccer field. In a sport like skiing, an athlete must be reactive and precise in their movements. Any sort of hesitancy could mean the difference between winning and losing, or catching an edge and getting hurt.

Strength deficits, nutrition and neuroplasticity are all proving to be suspects in the ACL crisis. A person’s readiness to return to sport and confidence is also a factor in the full spectrum of rehabilitation.

The most amazing part of being a human is the body’s ability to adapt to a training load and the positive neuroplastic changes that occur with practice. Practice makes perfect, strength creates resiliency and confidence. A gradual return to sport prevents injuries. With the correct training and motivation, I believe a person can come back stronger, no matter how many times they’ve injured their ACL.



Vlok, Arnold, et al. "Exercise descriptors that determine muscle strength gains are missing from reported anterior cruciate ligament reconstruction rehabilitation programs: A scoping review of 117 exercises in 41 studies." Journal of Orthopaedic & Sports Physical Therapy 52.2 (2022): 100-112.

Losciale, Justin M., et al. "The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis." journal of orthopaedic & sports physical therapy 49.2 (2019): 43-54.

Bieler, Theresa, et al. "The effects of high-intensity versus low-intensity resistance training on leg extensor power and recovery of knee function after ACL-reconstruction." BioMed research international 2014 (2014).

Davi, Steven M., et al. "Long-Lasting Impairments in Quadriceps Mitochondrial Health, Muscle Size, and Phenotypic Composition Are Present After Non-invasive Anterior Cruciate Ligament Injury." Frontiers in Physiology (2022): 5.

Grooms, Dustin R., et al. "Neuroplasticity associated with anterior cruciate ligament reconstruction." journal of orthopaedic & sports physical therapy 47.3 (2017): 180-189.

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