Training Load broadly describes the volume, intensity or combination of volume and intensity that an individual is performing on a daily basis. Arc provides 3 methods of measuring training load:

Training Load is calculated from an athlete's daily distance, captured via GPS integrations such as Catapult, PlayerTek, SPT and many others.

Training Load is calculated from a traditional Training Stress Score. This score is captured from integrations that support TSS, or provide a "Load" metric. TrainingPeaks provides a great article on multiple methods for calculating TSS, which your integration may support: Training Stress Scores Explained.

Training Load is calculated from an athlete's daily RPE. If an athlete completes multiple sessions in one day, the total is added together (e.g. two sessions with an RPE of 8 would equal an RPE load of 16). To provide further control, you may also modify your configured activity types to provide a "multiplier" for RPE. For example, you could create an activity type for Swimming with a multiplier of 1.2. Any RPE reported by an athlete for Swimming sessions will be multiplied by 1.2 - an RPE of 10 would equate to an RPE Load of 12.

Quantifying training load provides insight into whether you are asking too much, or too little of your athletes. Knowing, and understanding how your athlete's training load is tracking can provide insight into injury risks or other areas that may be impacting performance.

Training Load scores on their own are not overly useful - they simply describe the amount of work an athlete has completed on a given day. It is important to understand how Training Load might be affecting an athlete in the short-term and the long-term. Thankfully, we can quantify this with Acute Training Load and Chronic Training Load.

Acute Training Load (ATL) and Chronic Training Load (CTL) measures an individuals training load averages of different periods of time. ATL is used to measure the short-term effect of recent workouts, and is often referred to as a "Fatigue" rating. CTL is used to measure cumulative effects of training, which is often referred to as "Fitness".

Knowing an individual's ATL and CTL can be beneficial in determining if they are over, or under training. To quantify this, we use what is known as "Training Stress Balance".

Training Stress Balance (TSB) is the result of comparing an individuals ATL and CTL. The result of this comparison indicates how well adapted an individual is to their training. If their ATL (Fatigue) is higher than their CTL (Fitness), the individual could be over-reaching. The opposite is also true: if their ATL is much lower than their CTL, they may be finding their training too easy.

There are multiple methods for calculating Training Stress Balance, each with their own pros and cons. The traditional method for calculating TSB is simply

Where a TSB > 0 indicates that the individual is training below their fitness capacity (potentially losing-fitness)

And a TSB < 0 indicates that the individual is training above their fitness capacity (potentially over-training)

The distance from 0 would quantify how much the athlete is either over-training or under-training.

This simple formula falls short in certain scenarios. If for example, you were using this with Distance or RPE Load, and managed a team of athletes with varied levels of fitness, you would find your fitter athletes can sustain TSB's well below zero, while athletes with lower fitness may struggle when slightly below zero. This is because Distance and RPE Load are not "adaptive" scores, like TSS Load, which adapts to an individuals fitness over time.

And so, new formulas were used, in an attempt to standardize these scores (a TSB of 30 for Individual A is more-or-less than same as a TSB of 30 for individual B):

The result of this formula follows the same rules as the above - a positive score indicates under-training and negative score indicates over-training. However by diving by ATL, we are able to understand the individual's in more "relative" numbers. For example, if an athlete is achieving extremely high RPE Loads, diving it by ATL reduces it back down to a number that can be more easily compared with less fit athletes.

This formula is sometimes referred to as the Coupled ACWR (Acute Chronic Workload Ratio) formula. It works very similar to the formula above, however a criticism of it is that the ATL has a weighting effect on the overall TSB, resulting in higher than expected TSB when an individual's ATL is also high. To account for this, we have the uncoupled formula:
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This formula is known as the Uncoupled ACWR formula. This is also a great formula for normalizing TSB for an entire team, however has one flaw: If an individual's ATL approaches very closely to their CTL, you could end up with a very small denominator in the equation. This would result it a huge spike in TSB, when in reality, their TSB should be near zero.

TSB can be a great indicator for understanding how your athlete's are adapting to their training. Try to become familiar with TSS, ATL and CTL to make the most informed decisions.