Does load management using the acute:chronic workload ratio prevent health problems? A cluster randomised trial of 482 elite youth footballers of both sexes

Key Points

1. The acute:chronic workload ratio (ACWR) is a popular load monitoring and management tool, yet until now its effectiveness has not been tested in an experimental study.
2. In this cluster randomized controlled trial, elite youth footballers whose workloads were being managed with the ACWR were equally likely to report injury or illness as footballers whose workloads were not being managed.
3. These findings should be interpreted with caution due to major methodological limitations (high dropout rates, low questionnaire response rates, and poor adherence to the intervention), resulting in high risk of bias.

BACKGROUND & OBJECTIVE

The acute:chronic workload ratio (ACWR) is a popular load monitoring and management technique. The ACWR compares and quantifies an athlete’s current training load relative to the training load they’re accustomed to (1). High ACWRs are thought to increase athletes’ risk of injury, although the consistency and methodological rigor of these findings have recently been disputed (2).

Nevertheless, up to this point, all research on the ACWR has been observational. To support using the ACWR in practice for injury prevention, experimental research was needed. The purpose of this cluster randomized controlled trial was to determine if load management using the ACWR could reduce the prevalence of injury and illness in elite youth footballers.

A simple way to calculate ACWR is by dividing workload over the last week by average weekly workload over the last four weeks.

Load management remains just as much an art as a science.

METHODS

Approximately 680 elite youth footballers of both sexes were cluster randomized to the intervention group (18 teams) or control group (16 teams). Coaches in the intervention group used the ACWR to plan individual athletes’ workloads. Workloads were calculated from athletes’ self-reported rating of perceived exertion (RPE) after each training session multiplied by the duration of the session (in minutes). For example, a training session with a 7/10 RPE x 90 minutes = 630 arbitrary units.

Coaches were charged with maintaining ACWRs in the 0.8-1.5 range, which has been theorized to be the “sweet spot” for reducing injury risk (1). The control group trained normally, without load monitoring. Once per month over the 10-month season, athletes self-reported all health problems (injury or illness) over the previous week, including whether the problem was “substantial” (i.e. impacted participation or performance) (3). Between-group differences and relative risks for the prevalence of (1) all health problems and (2) substantial health problems were calculated.

RESULTS

Immediately following randomization, 9 teams withdrew (7 intervention, 2 control) from the study, leaving 482 participants. During the study, 24% of the intervention group and 13% of the control group failed to respond to the health questionnaires and were excluded from analysis, leaving 394 participants (164 girls, 230 boys; age = 17±1).

There was no significant difference in prevalence of health problems [1.8% (-4.1%-7.7%), p=0.55] (see Figure 1) or substantial health problems [-4.1% (-9.9%-1.6%), p=0.15] between the intervention and control groups. Nor was there a significant decrease in the likelihood of reporting a health problem [1.01 (95% CI: 0.91, 1.12), p=0.84] or substantial health problem [0.88 (95% CI: 0.72, 1.06), p=0.17] in the intervention group compared to the control group.

On average, the intervention group reported training loads for 74% of their sessions and responded to 62% of health questionnaires. The control group responded to 76% of health questionnaires. Only 45% of coaches in the intervention group reported that they complied with using ACWR to plan their athletes’ training every week.

LIMITATIONS

The high withdrawal rate following randomization (~25%), modest response rates to training load and health questionnaires (74% and 69%, respectively), high exclusion rate from analysis due to non-response to questionnaires (18%), and limited or unknown adherence to the intervention (55%) resulted in a high risk of bias. Therefore, the findings from this study should be interpreted with caution.

CLINICAL IMPLICATIONS

In this cluster randomized controlled trial, 177 elite youth footballers whose workloads were being managed with the ACWR were equally likely to report any injury or illness as 217 footballers whose workloads were not being managed (65.7% vs. 63.8%). The findings were similar when considering only “substantial” health problems that impacted participation or performance (31.1% for the intervention group, 35.3% for the control group).

There are several possible explanations for the null findings in this study:

1. Adherence to the intervention was poor. It’s possible that health problems could in fact be reduced with better adherence.
2. The intervention was based on maintaining ACWRs in the supposed “sweet spot” range (0.8-1.5). However, the evidence on which this sweet spot for injury risk reduction was based has recently come into question (2), suggesting the intervention may have been ill-founded.
3. The majority of previous research on the ACWR has focused on its association with injury. Yet this study included illness in addition to injury in its definition of a health problem. This may have masked the effect of the intervention on injury.
4. The final sample size was only 52% of the originally intended sample (n=760). This resulted in wide confidence intervals surrounding the relative risks, especially for substantial health problems. As is often the case, larger studies are needed to increase confidence in the precision of the outcomes.

This study does not support using the ACWR to reduce the prevalence of health problems. However, this finding doesn’t imply load management is futile. The intervention in this study was by methodological necessity a one-size-fits-all, single-factor approach. In the real world, practitioners can act on a case-by-case basis and incorporate other factors into load management and injury risk reduction (e.g., previous injuries, external stressors, strength, and past responses to training loads). As the authors conclude, for now, “load management remains just as much an art as a science.”

+STUDY REFERENCE

Dalen-Lorentsen T, Bjørneboe J, Clarsen B, Vagle M, Fagerland M, & Andersen T (2020) Does load management using the acute:chronic workload ratio prevent health problems? A cluster randomised trial of 482 elite youth footballers of both sexes. British Journal of Sports Medicine. 2020 Oct 9:bjsports-2020-103003. doi: 10.1136/bjsports-2020-103003.

SUPPORTING REFERENCE

1. Gabbett, T. J. (2016). The training—injury prevention paradox: should athletes be training smarterandharder? British Journal of Sports Medicine, 50(5), 273–280. https://doi.org/10.1136/bjsports-2015-095788
2. Impellizzeri, F. M., Tenan, M. S., Kempton, T., Novak, A., & Coutts, A. J. (2020). Acute:Chronic Workload Ratio: Conceptual Issues and Fundamental Pitfalls. International Journal of Sports Physiology and Performance, 15(6), 907–913. https://doi.org/10.1123/ijspp.2019-0864
3. Clarsen, B., Rønsen, O., Myklebust, G., Flørenes, T. W., & Bahr, R. (2013). The Oslo Sports Trauma Research Center questionnaire on health problems: a new approach to prospective monitoring of illness and injury in elite athletes. British Journal of Sports Medicine, 48(9), 754–760. https://doi.org/10.1136/bjsports-2012-092087