Training load and athletic injuries have long been synonymous, thus the number of studies aiming to quantify their precise relation has spiked in recent decades. One of the most popular metrics, and often considered the “gold standard”, over time has been what is known as the acute: chronic workload ratio (ACWR) (5). While some hold the position that ACWR is the north star of athlete load monitoring, others do not (4), making it a contentious topic and leaving coaches/sports scientists alike to debate its efficacy.
What Is ACWR?
ACWR is a metric that attempts to monitor athlete load by dividing a ‘fatigue’ component by a ‘fitness’ component. Load is either considered external, such as total distance covered while running, or internal, such as rate of perceived exertion (RPE) while running. While the external load or output between two individuals can be identical, the internal load may be far different, demonstrating that the overall load will vary depending on one’s level of fitness. ACWR theorizes that utilizing both measures can aid in determining whether one is in a state of readiness and ability to tolerate high loads or in a state of fatigue and potentially risking injury or severely decreased levels of performance. The ACWR is analyzed by dividing acute load (AL), typically load over the last week, by chronic load, which is typically a rolling average of the previous 3-6 weeks. If the acute workload is demonstrated to be higher than chronic workload, then this risk for injury is also considered to be greater too. Some sports scientists contend that anything over a ratio of 1 is considered to be a risk for injury while others contend that anything between 0.8 and 1.3 is safe, however, the exact ratio is highly debatable and lacks clear scientific evidence.
Is It Effective?
Intuitively, one can infer that too much of anything is not a good thing, and athlete training load is no different. The question that remains is how much change in load is too much? Research has demonstrated that an ACWR of 1.77 and greater (2, 3) may increase the risk for injury in athletic populations, however, an exact ratio has never been established and remains highly variable in the academic literature. This is not to say that athlete load monitoring should not be done or can’t be done, but when significant time and resources are used to collect such data, it is wise to examine its efficacy.
The idea of an ACWR greater than ~1.3 representing the threshold for injury risk sounds nice. It’s precise and allows one a marker or hard line to ensure they do not cross for the overall safety of the athlete. While the intention is likely pure, athletics are messy, and so too are the methods with which training load is conducted and analyzed. The ACWR methodology has several fundamental flaws that cannot be overlooked, making it difficult to justify as a reliable method of load monitoring. Below are a few of the most notable.
– Variations in data collection methods
o The use of internal data collection methods such as rate RPE presents issues of data reliability because they are highly subjective. Assigning the difficulty of a task to a number on a scale (1-10) can be highly variable to one’s experience, the current level of fitness, the task being conducted, current well-being, and overall disposition among many other factors.
– Arbitrary Units
o The ACWR commonly uses arbitrary units (AU) to quantify the overall load of an activity. For example, if a long-distance runner does a 10-mile run in 70 minutes and has an RPE of 6, then they would be assigned 420AU for that run (70min * RPE 6 = 420AU). This does not provide any information about the average heart rate, average stride length, steps per minute, or any other objective data that may provide greater insight into the actual efficiency or difficulty of the activity for that individual. Where this issue stands out is in the time portion of the measurement. Take the same example of the long-distance runner who was assigned 420AU for their 10-mile run and compare that to an MMA fighter who does a 3-round fight consisting of 3, 5-minute rounds. They would rate that activity as an RPE of 10, however, they would only be assigned 150AU (15min * RPE 10 = 150AU) making the long-distance run count towards a great AL. This presents issues when examining the AL versus the CL without any granularity on the nature of the activities.
– Progressive Overload
o At times athletes must partake in what is known as ‘functional overreaching’ to improve their overall levels of fitness (1) (assuming that adequate recovery is granted) however this will depend on their current level of fitness and ability to withstand the load. For some, a week of training, possibly two or three in a row, may necessitate a sharp spike in training volume/intensity to induce the stimulus required for adaptation. This in turn will then cause the ACWR to spike and suggest the athlete is at risk for injury. Assuming that this is what was intended to happen, then the coach and athlete are likely aware of this too, however, individuals attempting to adhere to a rule of never climbing over an ACWR of ~1.3 may end up contradicting this training.
While the use of the ACWR for athlete load management may be of merit to some and in fact be better than no monitoring at all, it is difficult to justify its use as an efficacious method at large. Several fundamental flaws exist within its modality and the parameters with which it is defined remain relatively obscure. The intention of using the ACWR to reduce athletic injury is a noble one, however, blindly accepting its use and implementing it in any high-performance setting without considering the aforementioned discussion points appears to be rather irresponsible.
1. Aubry A, Hausswirth C, Louis J, Coutts AJ, and Le Meur Y. Functional overreaching: the key to peak performance during the taper. Med Sci Sports Exerc 46: 1769-1777, 2014.
2. Bowen L, Gross AS, Gimpel M, and Li F-X. Accumulated workloads and the acute: chronic workload ratio relate to injury risk in elite youth football players. British journal of sports medicine 51: 452-459, 2017.
3. Carey DL, Blanch P, Ong K-L, Crossley KM, Crow J, and Morris ME. Training loads and injury risk in Australian football—differing acute: chronic workload ratios influence match injury risk. British journal of sports medicine 51: 1215-1220, 2017.
4. Impellizzeri FM, Woodcock S, Coutts A, Fanchini M, McCall A, and Vigotsky A. What role do chronic workloads play in the acute-to-chronic workload ratio? Time to dismiss ACWR and its underlying theory. Sports Medicine 51: 581-592, 2021.
5. Maupin D, Schram B, Canetti E, and Orr R. The relationship between acute: chronic workload ratios and injury risk in sports: a systematic review. Open access journal of sports medicine 11: 51, 2020.