Part 2
Previously, we examined the idea of training load, including how to measure, monitor, and balance it, and what it means for a physical training program in your fire department. Next we’ll dive into the relationship between fitness and fatigue, and additional considerations when it comes to balancing training loads.
Effective training load periodization—a structured approach to managing physical stress—is essential for optimizing performance, reducing injury risk, and enhancing long-term occupational health outcomes. For firefighters, whose unpredictable workloads include heavy lifting, stair climbs, and rapid sprints, a scientific approach to training is vital for maintaining readiness and reducing fatigue-related injuries.
Training stimulates acute fatigue, which drives long-term adaptations if followed by appropriate recovery. Research by Hulin et al. (2014) indicates that sudden spikes in workload without gradual progression significantly increase injury risk—an especially critical concern for new recruit academy participants and firefighters due to their physically demanding occupational tasks. Therefore, implementing strategies to monitor this relationship of training load and fatigue is widely-accepted as best practice.
What Is Acute:Chronic Workload Ratio (ACWR)?
The acute:chronic workload ratio (ACWR) is a concept used to balance training stress by comparing short-term workloads (acute) to long-term workloads (chronic). This ratio is valuable in managing fatigue, reducing injury risk, and ensuring that fitness adaptations occur without overloading the firefighter.
- Acute workload: Typically measured over the last seven days. This reflects the sum of short-term stress imposed on the body during recent training sessions.
- Chronic workload: Calculated as the average workload over the past 28 days. It serves as a rolling measure of an individual’s long-term workload capacity or “fitness” level.
The ratio is obtained by dividing the acute workload by the chronic workload:
This ratio can provide insight into whether an individual is maintaining an appropriate balance between fitness and fatigue. A low ratio may indicate insufficient training stimulus (detraining), while a high ratio may suggest overreaching, which increases the risk of injury. This index data is a valuable tool used to evaluate individual perceptions of training stress and alignment of the efforts to the larger periodized training program (as we’ll discuss in a future article).
Methods for Measuring ACWR
The ACWR can be calculated using different methods depending on the types of workload metrics available. Commonly used methods include:
- Session RPE (sRPE): Combines the individual’s perceived exertion (RPE) with the duration of the training session to calculate a total load. For example, if a firefighter rates their exertion as “7” on a scale of 1-10 during a 60-minute session, the total session load would be 420 (7 x 60). NOTE: If you are participating in multiple training sessions in a single day, you should calculate a sRPE for each training session. The sum of these sessions will be used to calculate daily load.
- Objective measures: Tools like GPS devices, accelerometers, and heart rate monitors can provide objective data on distance, speed, power output, or heart rate. These metrics can be averaged over the acute and chronic periods to calculate the ACWR.
Regardless of the calculation method, it is essential to apply consistent measures across the acute and chronic phases to accurately track workload trends and improve overall validity.
Why Is ACWR Important?
The ACWR is particularly useful for professions such as firefighting, where unpredictable, high-intensity work demands in extreme environments can increase the risk of injury. By keeping workloads within an optimal range, the ACWR helps reduce the likelihood of both overuse injuries and underpreparedness for high-demand situations.
Research highlights a strong relationship between ACWR and injury occurrence. Gabbett (2016) observed that sudden spikes in acute workload exceeding 1.5 times the chronic workload significantly increase the risk of soft tissue injuries. Conversely, maintaining a consistent workload reduces injury risk and fosters adaptation. Hulin et al. (2016) found that athletes with a high chronic workload but low acute spikes had lower injury rates, emphasizing the importance of gradual progression. For firefighters, this model can guide adjustments to training intensity after periods of inactivity, such as recovery from illness or vacation.
| ACWR Value | |
| < 0.8 | the individual is not training enough to maintain fitness, potentially leading to detraining effects |
| 0.8 – 1.3 | considered optimal for building fitness and minimizing injury risk |
| > 1.5 | significantly increases the risk of injury, as it indicates a rapid increase in workload that the body may not be able to handle |
Limitations and Considerations
Although the ACWR is a valuable tool, there are some considerations to be aware of:
- Individual variability: Different firefighters will have different tolerances to workload changes based on their fitness levels, recovery capacity, and injury history.
- Load spikes: Sudden, unexpected workload increases due to emergency calls or shifts can affect the ACWR. It is important to account for these unplanned increases and adjust training accordingly to prevent overload.
- Cognitive and emotional load: While ACWR focuses on physical load, it is essential to consider how cognitive fatigue and mental stress may impact an individual’s overall workload and recovery needs. This can be further impacted by shift work and sleep dysregulation.
By incorporating ACWR and sRPE into the planning and monitoring of training, firefighters can build resilience while reducing the risk of both physical and mental burnout, allowing for sustained performance and safety over time.
Practical Applications for Firefighters
- Workload monitoring: Use ACWR to prevent unsafe workload spikes (Gabbett, 2016).
- Load balancing: Use recovery-focused activities to accommodate unplanned load spikes. (Gabbett, 2016)
- Deloading weeks: Implement regular low-intensity weeks to facilitate recovery (Bompa & Buzzichelli, 2019) and maintain overall cardiorespiratory capacity with longer duration low-threshold training.
- Training variance: Incorporate strength, endurance, mobility, and agility exercises for well-rounded conditioning (Bannister, 1991).
Applying evidence-focused periodization strategies empowers firefighters to achieve peak performance while minimizing injury risk. Regular workload monitoring, recovery integration, and training variability are essential to sustaining physical readiness and career longevity.
In part 3 of this series, we’ll examine cognitive fatigue and its impact on training load.
References
Bannister, E. W. (1991). Modeling elite athletic performance. In H. Green et al. (Eds.), Physiological Testing of Elite Athletes.
Bompa, T., & Buzzichelli, C. (2019). Periodization: Theory and methodology of training (6th ed.). Human Kinetics.
Eckard, T.G., Padua, D.A., Hearn, D.W. et al. The Relationship Between Training Load and Injury in Athletes: A Systematic Review. Sports Med 48, 1929–1961 (2018). https://doi.org/10.1007/s40279-018-0951-z
Frank, B. S., Hackney, A. C., Battaglini, C. L., Blackburn, T., Marshall, S. W., Clark, M., Padua, D. A. (2019). Movement profile influences systemic stress and biomechanical resilience to high training load exposure. Journal of Science and Medicine in Sport 22, (1):35-41. https://doi.org/10.1016/j.jsams.2018.05.017
Gabbett, T. J. (2016). The training-injury prevention paradox: Should athletes be training smarter and harder? British Journal of Sports Medicine, 50(5), 273-280.
Gabbett, T. J., Hulin, B. T., Blanch, P., Chapman, P., Bailey, D., & Orchard, J. W. (2016). High training workloads alone do not cause sports injuries: How you get there is the real issue. British Journal of Sports Medicine, 50(8), 444-445. https://doi.org/10.1136/bjsports-2015-095567
Griffin, A., Kenny, I.C., Comyns, T.M. et al. (2020). The association between the Acute:Chronic Workload Ratio and injury and its application in team sports: A systematic review. Sports Medicine 50, 561–580. https://doi.org/10.1007/s40279-019-01218-2
Hulin, B. T., Gabbett, T. J., Lawson, D. W., Caputi, P., & Sampson, J. A. (2016). The acute:chronic workload ratio predicts injury: High chronic workload may decrease injury risk in elite rugby league players. British Journal of Sports Medicine, 50(4), 231-236. https://doi.org/10.1136/bjsports-2015-094817
Mason, M. R., Heebner, N. R., Abt, J. P., Bergstrom, H. C., Shapiro, R., Langford, E. L., & Abel, M. G. (2023). The acute effect of high-intensity resistance training on subsequent firefighter performance. The Journal of Strength & Conditioning Research, 37(7), 1507-1514.
Alan Russell, MS, LAT, ATC, PES, CES, a resident of Little Elm, Texas, is a sports medicine clinician with more than 30 years of experience working with elite, collegiate, youth, recreational, and tactical athletes. As the education coordinator for the Public Safety Athletic Trainers’ Society, he is an internationally invited presenter, educator, and author. The spouse of a firefighter, Alan is driven by a deep commitment to providing first responders with the same level of care given to elite athletes, understanding that their critical work leaves no opportunity for an off-season
