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Goldilocks approach to training—not too little, not too much
Clinicians or strength and conditioning professionals who prescribe training workloads aim for workloads that are high enough to improve fitness (ie, performance), but not so high as to risk injury. At the extremes, no training results in an unprepared athlete, whereas an overuse injury is, by definition, an error in training prescription.1 Banister et al2 first described an athlete's training state as the difference between positive (ie, ‘fitness’) and negative (ie, ‘fatigue’) influences. To quantify this concept, ‘fitness’ was represented as the workload (an arbitrary ‘training impulse’) of the athlete over a 3–6 weeks period and ‘fatigue’ was represented by the workload performed over a shorter time frame of 1 week. We recommend the terms ‘chronic workload’ for the longer window of training (ie, Banister's ‘fitness’) and ‘acute workload’ for the immediate window of training (ie, Banister's ‘fatigue’) (figure 1). High chronic workloads (ie, intense training), combined with reductions in acute workloads before important competition (ie, taper), would be expected to improve sporting performance.2
Acute:chronic workload ratio is the important metric
‘Training-stress balance’ was a performance concept Andrew Coggan introduced to capture the positive and negative effects of training. Although we used ‘training-stress balance’ in previous publications,3 we prefer and recommend the concept of ‘acute:chronic workload ratio’. This ratio describes the acute training workload (eg, most recent week's training load) to the chronic workload (4-week rolling average of acute workload). If chronic workload is high (ie, the athlete has developed ‘fitness’) and the acute workload is low (ie, the athlete is experiencing minimal ‘fatigue’), then the athlete is considered well prepared. Conversely, if acute workload exceeds the chronic workload (ie, the athlete has performed inadequate training to develop ‘fitness’ or workloads have been rapidly increased resulting in ‘fatigue’), then the athlete is considered underprepared and likely at an increased risk of injury (see figure 1). As such, the acute:chronic workload ratio indicates both the athlete's risk of injury and preparedness to perform.
Athlete workloads can predict injury
Individual sport athletes had largely been the focus of research and application of the acute:chronic workload ratio. The measures of training load were used for periodisation plans and tapering for performance.2 Several studies have applied this workload model to predict injuries in team sport athletes. In studies of rugby league4 players, high chronic workloads were associated with a reduced risk of injury, while large ‘spikes’ in acute workloads relative to chronic workloads were associated with increased risk of injury. A consistent theme throughout this paper was the importance of progressively and systematically increasing workloads to minimise the risk of injury. Importantly, the predictive ability of this approach is very high; we reported a positive likelihood as high as 70 times.5
Returning safely from injury requires consideration of the workload completed
While various functional tests are commonly used to assess the progress of an injured athlete, there is little evidence that they can predict safe return to play. It is often noted that the best predictor of a musculoskeletal injury is previous injury history.6 This could be due, at least in part, to the reduced fitness (chronic workload) caused by the recent injury-induced lay-off. Specifically, the importance of the amount of training performed in the current week (ie, acute workload) relative to training the athlete has been prepared for over the preceding 4 weeks (ie, chronic workload) is crucial.6 Given its association with injury risk, the acute:chronic workload ratio may prove critical when determining return to train, play and ‘compete’ rehabilitation plans for many injuries that have resulted in substantially decreased training load over 2 or more weeks.
Rapid ‘spikes’ in training workload is the problem, while consistent training is the solution
Several studies have demonstrated greater injury rates with higher training workloads.7 However, it should be noted that the benefits of training (ie, well-developed physical qualities and the application of the training workload itself) may provide resilience to athletes, resulting in protection from injuries.7 Potentially, a carefully staged programme that culminates in high workloads makes for durable athletes, while attempting to cut corners and ‘spike’ to high workloads results in high injury risk. Indeed, weekly increases in workloads of >10% are associated with increased injury risk, while smaller increases in workloads result in much lower risk of injury.7
It's not the destination, it's the (workload) journey that matters
To optimally prepare for competition demands, athletes need to (gradually) increase their workloads so that their fitness (chronic workload) is sufficient to overcome acute fatigue demands. We hope that sport science, coaches, strength and conditioning, and health professionals see the value in the acute:chronic workload ratio and incorporate this form of monitoring into their day-to-day training environment.
Contributors The initial concepts and drafts were formulated by TJG. BTH, PB and RW contributed equally to several drafts of the editorial.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.