After being described as an injury predictor, the Acute:Chronic Workload Ratio (ACWR) has recently come under scrutiny by the sports science community. Dr. Tim Gabbett has been at the forefront of this topic, and his research has been instrumental in shedding light on athlete monitoring and its importance not just for performance practitioners, but for sport coaches as well. He demonstrated the importance of monitoring volumes not only in the weight room but in nearly all sport activities, including games. Gabbett’s research has shown how fluctuations in volumes of sport-related activities may affect injury rates in athletes.
However, Franco Impellizzeri has been adamantly challenging ACWR’s ability to predict injury. He wrote a letter in May 2019 to the BJSM questioning one of the most circulated visuals that describes ACWR (see adapted Image 1 below). Impellizzeri said the figure is unreliable based on the adjustments that were made, and he recently published an article saying:
We suggest ACWR be dismissed as a framework and model, and in line with this, injury frameworks, recommendations, and consensus be updated to reflect the lack of predictive value of and statistical artefacts inherent in ACWR models. (Impellizzeri 2020)
ACWR is a very useful high-performance tool for sport coaches and performance practitioners; it does not predict injury, says @RealBMike. Click To Tweet
I agree with the sentiment behind Dr. Impellizzeri’s statements. Predicting injuries solely on workloads is not possible. Not to mention that the statistical modeling used in Gabbett’s research is skewed toward providing statistically significant data (Impellizzeri 2020). Predicting injuries is tricky business—injuries are multifactorial in nature, meaning not one single variable is the sole cause.
An athlete’s previous injury history, sleep habits, hydration status, nutrition, biomechanics, psychological state, training age, training volumes, injury rates in the sport, environment, aerobic capacity, and many more variables play varying roles in each sustained injury. I agree with Dr. Impellizzeri that ACWR’s predictive value is very limited. However, there is no need to throw out the baby with the bathwater. ACWR is still useful for sport coaches and performance practitioners as a high-performance (HP) tool.
ACWR and Milo of Croton (Progressive Overload)
For those of you who haven’t heard the story of Milo of Croton and his cow, I’ll give you the shortest version that I can:
Milo was an athlete in ancient Greece and wanted to become stronger to improve at his sport of wrestling. He began carrying a baby calf up a mountain every day. As the calf grew, Milo became stronger.
Essentially, all this story describes is progressive overload for strength training. Here is where ACWR comes into play: If Milo had jumped immediately from carrying a calf to carrying a full-grown cow the next week (or if he went up the mountain 15 times a week with the calf compared to seven times), would he have gotten injured due to the large jump in training load and volume? Maybe. But because of the cow’s slow growth and the sustained distance, Milo became stronger via progressive overload while simultaneously avoiding injury.
What is ACWR?
ACWR is a ratio between acute training loads (training volumes during the previous week) and chronic loads (training loads for the last four weeks). The chronic load (CL) values can change (four weeks, six weeks, eight weeks, etc.), but you can lose valuable insight if you choose a longer CL value. With an 8-week average, for instance, it’s harder to see weekly changes compared to a 4-week average. For example:
- Previous week: acute workload of 60 miles
- Previous four-week average of acute workloads: (50 miles + 55 miles + 55 miles + 60 miles) / 4weeks = Chronic workload of 55miles
- ACWR: An acute workload of 60 miles / chronic workload of 55 miles = 1.1 ratio
Going back to Milo of Croton, what was his ACWR? The only thing that changed for Milo was the size of the cow. Because of the slow(ish) growth, he accumulated similar incremental increases of intensity over weeks of training and got stronger while limiting his risk of injury.
The way you choose to measure your training loads depends on how insightful you want your data to be. When I performed some original research for my Master’s thesis on cross country athletes, I wanted to look at the rating of perceived excretion (RPE) and distance run during practice. The most common way of calculating workloads is using an RPE 1-10 and multiplying that by a volume or the duration of training (GPS data). Premier football leagues across Europe use speed and distance to calculate training and game intensities for players, which is the gold standard, in my opinion. The crude calculation of RPE x Duration can provide some insight into how an athlete perceives their training. It also can give great feedback to the coach as to which athletes are performing at the right intensities and which are not.
Why and how is an RPE calculation effective? RPE was invented by Dr. Gunnar Borg as a 6-20 scale to illustrate how the perceived response of exercise correlates with the heart rate response to exercise (see Image 2 below). RPE is a good psychophysical representation of work that’s completed. The psychological aspect takes into account how challenging the exercise is to the athlete based on how they’re feeling at that moment.
Some common factors that affect RPE are mood, sleep, feelings toward exercise, etc. The physical aspect is the body’s response to the imposed demands and the environment in which you are training. These include exercise type, training status, hot or cold climates, elevation, and many other variables. Both psychological and physical responses to exercise play a major role in how someone perceives the difficulty of their training. Because of the demonstrated importance of this relationship, RPE and/or reps in reserve (RIR) have become typical additions to strength training feedback.
If two athletes who have the same strength levels perform the same exercise in the same environment with the same weight, and Athlete #1 had an RPE of seven and Athlete #2 had an RPE of 9, is it safe to assume that Athlete #2 experienced more training stress than Athlete #1? Is this feedback valuable?
What Is a High-Performance Model in Athletics?
The HP model is athlete-centered and ensures that the athlete’s development is the top priority for everyone who is in contact with them. This ranges from the player development coaches to ATC to S&C to mental skills. With everyone having the same goal of developing athletes to the best of the group’s ability, the next most important aspect is communicating effectively about what the goal is for each group.
After that comes developing a plan that’s agreed upon for each athlete’s development. It’s critical that each group communicates about what they see in the athlete and then compromise as to the most effective way to develop that athlete. It is essential that each groups’ expertise is understood and agreed upon. While discussion and discourse are encouraged, once a decision has been made by the expert(s) in their field, there should be very little (if any) second-guessing of that decision. If there is, bringing up concerns to that person and not to anyone else is the most effective way to communicate disagreements. This model takes the onus off of the sole leader and emphasizes decentralized leadership within each person’s expertise.
ACWR: High-Performance Tool (S&C Focused)
For an S&C HP model, the primary goal, especially during a season, is to keep athletes as healthy as possible and able to perform to the best of their ability on the field. The secondary goal is to maintain and develop the traits that will help our athletes succeed on the field. This is crucial because there is an inherent risk to training—the only way to ensure no injuries occur in training is not to train at all.
As I stated before, injuries are multifactorial. No one thing is the sole cause of an injury. I agree with Dr. Impellizzeri that there is no shot ACWR can predict that an athlete will be injured; however, proper periodization exists to limit injuries and gain desired physical characteristics in a timely and effective manner. This is where ACWR can be really helpful for coaches who have little background in sports science and creating practice plans. Having large weekly fluctuations in training volume and intensities is not effective for athletic development.
ACWR is effective at illustrating changes in three of the most important aspects of training:
- Stress responses for each athlete
- Adaptations you are expecting
- Consistency in training
1. Stress Responses
Global stress has been the topic of multiple studies (Mann, et al. 2015; Lavell & Flint 1996), and while this topic needs more in-depth research, there are some safe assumptions we can make based on the literature. Mann et al. looked at the stress levels of student-athletes in football throughout a semester. They found that athletes were almost twice as likely to be injured during periods of high academic stress compared to low academic stress. This illustrates the importance of monitoring global stress, which ACWR effectively accomplishes. For example, during finals week in college, a natural deload in training probably occurs based on the anticipation that global stress will increase. If an athlete is still giving an RPE post-training that is higher than what you anticipate, it may be necessary to decrease their training load to help diminish the likelihood of injury.ACWR monitors global stress and gives an excellent picture of how to periodize practice plans for athletes, says @RealBMike. Click To Tweet
Charlie Francis’ High-Low model demonstrates another example of the stress response. The model describes two distinct types of training days: high-stress days emphasizing adaptation and low-stress days prioritizing recovery. Transferring this concept to a high-performance model requires effective communication among coaches. If there’s a lack of communication, daily stress can dramatically increase because there are no days that emphasize recovery (see the top model on Image 3 below). Using ACWR would be a practical way to show when and where you should plan the high-stress days and when to schedule the low-stress recovery days. This gives the coaching staff an excellent picture of how to periodize their specific practice plans for their players within the High-Low model.
Eliciting adaptations with training loads is the basis for improving sports performance. Monitoring training is essential to accomplish this task safely and effectively. Using ACWR and testing athlete adaptations (jump testing, sprint testing, sport performance, etc.) can give you a really clear picture of where you need to make changes in programming. If you have the budget, GPS data is an excellent source for monitoring athletes—not just during sprint mechanics, but also during practices and games, adding a more in-depth look that RPE can’t provide.ACWR along with testing offers a clear view of where you need to make changes in programming, says @RealBMike. Click To Tweet
When attempting to improve maximal power and speed in your sport, you should be training at those levels and testing your athletes on those characteristics. Maximal speed and power adaptations can decrease in as little as three days (Issrusin, 2008). If you aren’t noticing improvements—or, more importantly, are noticing decreases in performance—it’s essential to have the ability to go back and analyze why.ACWR gives invaluable insight on each athletes' psychophysical state during training, says @RealBMike. Click To Tweet
ACWR can give invaluable insight into each athletes’ psychophysical state during training. If an athlete is constantly above your desired RPE, their stress levels may be consistently too high (or they’re pushing too hard and need to be reined in). On the opposite end of the spectrum, if the programmed training intensity is too low, athletes aren’t reaching their minimal effective dose. The inability to produce maximal power due to residual fatigue or lack of stimuli can result in decreased power and speed production. A decrease in power production is the number one adaptation you want to avoid throughout a season.
Returning to Image 3 (above), during the sprint training days, which athletes would most likely be able to produce the most power?
- Those following the first weekly plan that’s consistently in the moderate to high stress, or
- Those doing the second plan that places fewer stressors on them on their low days before strength and power training?
Consistency of training is something I think all strength coaches and performance practitioners recognize as critical. As I illustrated in the Adaptations section, strength and power characteristics can decrease extremely fast. Consistently stressing the body in a periodized manner can create gains that occur throughout an athlete’s career, and having a consistent daily schedule is critical for athletes, especially in-season.
Athletes who travel constantly need to have an anchor point during a season. This is easiest to plan during homestands in which athletes aren’t stressed with travel and don’t have major changes to daily their routine. An ideal schedule may vary from athlete to athlete, and it’s difficult for studies to illustrate exactly why consistency is important.The more consistent the schedule, the fewer unnecessary stressors placed on the athletes. Limit changes to the weekly schedule to avoid this stressor. Click To Tweet
Intuitively, the more consistent the schedule, the fewer the unnecessary stressors placed on the athletes. Limiting the instances of “Hey, change of plans, we are going to practice today at 1 pm instead of 3 pm” is really important to accomplish this. There are plenty of stressors that athletes—especially “minor league grinders” and college athletes—face during the year. Travel, sleep debt, food security, schoolwork, and family issues, just to name a few. Not knowing the weekly schedule should not be one of them. I totally understand that changes are going to happen during the year due to unforeseen circumstances. However, unnecessary schedule changes should not be a stressor on the long list that athletes already face.
As I’ve noted, ACWR is not an effective tool that predicts predisposition to injury, as Dr. Gabbett has described in his research. It can, however, be an extremely effective tool for performance coaches monitoring their athletes’ workloads, especially when the performance coach does not prescribe a large portion of the workload. Additionally, ACWR can be an extremely effective tool for a strength coach or performance practitioner to demonstrate to the player development staff where and when stressors are placed on athletes, and how and why to consolidate those stressors to emphasize recovery days and higher intensity days better.
“In preparing for battle I have always found that plans are useless, but planning is indispensable.”—Dwight D. EisenhowerBlindly sticking to a plan when ACWR calls for change will lead to decreased performance and possibly injury, says @RealBMike. Click To Tweet
Everyone who has been on a coaching staff with a sport team knows that plans, practices, and player moods can all change at a moment’s notice. Adaptability is the greatest ability that S&C coaches and performance practitioners can have. Having a plan is essential for success, but blindly sticking to a program when ACWR is calling for change will lead to a decrease in performance and possibly injury. Modify your plan to fit the current situation and execute that plan!
“A good plan, violently executed now, is better than a perfect plan next week.”—George Patton
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1. Gabbett, T. J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? Br J Sports Med, 50(5), 273-280.
2. Gabbett, T. J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? Br J Sports Med, bjsports-2015.
3. Impellizzeri, F., et al. (2019). The acute-chronic workload ratio-injury figure and its “sweet spot” are flawed. SportRxiv (preprint).
4.Impellizzeri, F., et al. (2020). Acute to random workload ratio is “as” associated with injury as acute to actual chronic workload ratio: time to dismiss ACWR and its components.SportRxiv (preprint).
5. Issurin, V. (2008). Block periodization versus traditional training theory: a review. Journal of sports medicine and physical fitness, 48(1), 65-75.
6. Lavallée, L., & Flint, F. (1996). The relationship of stress, competitive anxiety, mood state, and social support to athletic injury. Journal of athletic training, 31(4), 296-299.
7. Malone, S., et al. (2017). The acute: chonic workload ratio in relation to injury risk in professional soccer. Journal of science and medicine in sport, 20(6), 561-565.
8. Mann, J. B., et al. (2016). Effect of physical and academic stress on illness and injury in division 1 college football players. The Journal of Strength & Conditioning Research, 30(1), 20-25.