How to Calculate Acute Training Load for Running

Acute training load (ATL) represents the cumulative stress your body has absorbed from training over a recent short-term window, typically the last 7 days. It captures not just how far you ran, but how hard. A week with 40km of easy running and a week with 40km including two tempo sessions and a race produce very different physiological stress, and acute training load reflects that difference.

ATL is one half of the ACWR (Acute-to-Chronic Workload Ratio) equation. The other half, chronic training load (CTL), represents your longer-term average over approximately 28 days. By comparing what you have done recently (ATL) to what you are accustomed to (CTL), ACWR tells you whether your current training is within your body's adaptation capacity or exceeding it.

Raw metrics like distance or duration do not adequately capture training load for injury risk purposes. Running 10km at recovery pace and running 10km at threshold pace produce vastly different mechanical and metabolic stress. You need a unit that accounts for both duration and intensity.

Training Stress Score (TSS) fills this role. Originally developed for cycling (by Andrew Coggan as part of the TrainingPeaks methodology), TSS can be adapted for running using pace as the intensity measure.

The running TSS formula:

TSS = (duration in seconds x intensity factor^2) / 3600 x 100

Where intensity factor (IF) = your normalised pace / your functional threshold pace (FTP). FTP is the pace you could sustain for approximately one hour in an all-out effort, roughly your recent 10K race pace for most trained runners.

Practical examples: A 60-minute easy run at 80% of your threshold pace produces a TSS of roughly 48. A 60-minute tempo run at 95% of threshold produces a TSS of roughly 90. A 30-minute interval session with repeats at 110% of threshold can produce a TSS of 55 despite being half the duration.

The takeaway: TSS captures what distance alone cannot. Two 50km weeks can have vastly different TSS totals depending on the composition of sessions within them.

There are two common methods for calculating acute and chronic training load, and they produce different results.

Simple rolling average. Sum the TSS from the last 7 days for acute load. Average the weekly TSS over the last 4 weeks for chronic load. Divide acute by chronic. This is the original method used in Gabbett's early ACWR work. It is easy to compute manually and easy to understand.

Exponentially Weighted Moving Average (EWMA). Instead of giving equal weight to every day in the window, EWMA gives more weight to recent days and less to older ones. A hard session yesterday affects your acute load more than a hard session 6 days ago. The decay constant determines how quickly older sessions lose influence.

Menaspa (2017) demonstrated that the EWMA method is more sensitive to day-to-day fluctuations and produces fewer false negatives (situations where the metric says you are safe but you are actually in a high-risk state). The practical difference matters most around spikes: if you do a very hard session on day 1 of the week and rest for the next 6 days, the simple rolling average still shows high acute load on day 7. EWMA correctly shows that the spike has faded.

injury.vision uses the EWMA method for both acute and chronic load calculations, with a 7-day time constant for ATL and a 28-day time constant for CTL.

If you want to compute ATL manually, here is the process:

Step 1: Determine your functional threshold pace (FTP). This is the pace you could sustain for approximately one hour at maximum effort. If you have a recent 10K race, that pace is a reasonable estimate. If you have a recent 5K, add roughly 15 to 20 seconds per kilometre.

Step 2: Calculate TSS for each session. For each run, compute the intensity factor (normalised pace divided by FTP), then apply the formula: TSS = (seconds x IF^2) / 36.

Step 3: Sum the last 7 days of TSS. This is your simple rolling acute load.

Step 4: Compare to chronic load. Average your weekly TSS over the last 4 weeks. Divide acute by chronic. That is your ACWR.

In practice, doing this manually for every run is tedious and error-prone. This is precisely why tools like injury.vision exist. When you connect Strava or Garmin, TSS is computed automatically for each imported activity, and ACWR is recalculated daily using the more accurate EWMA method. You see your current ACWR on your dashboard without doing any arithmetic.

Mistake 1: Using distance instead of TSS. Two runners at 50km per week can have wildly different training loads depending on intensity distribution. A runner doing 50km all easy has lower acute load than a runner doing 40km easy plus 10km of intervals. Distance-based ACWR consistently underestimates the risk of intensity spikes.

Mistake 2: Ignoring the denominator. Acute load only has meaning relative to chronic load. A weekly TSS of 300 is moderate for a runner whose 4-week average is 280 (ACWR = 1.07) but dangerous for a runner whose 4-week average is 180 (ACWR = 1.67). Always interpret acute load in context.

Mistake 3: Not accounting for time off. After a week of no running, your acute load drops to zero but your chronic load only decreases by about 25%. The ACWR looks safe (0 / something = 0). But the first week back, even a moderate training load can spike the ratio because the denominator keeps dropping with each passing day. This is why return-from-injury and return-from-rest periods are the highest-risk windows, and why tools that apply a detraining adjustment produce better guidance than raw ACWR alone.

Mistake 4: Calculating weekly instead of daily. ACWR should be computed daily, not just at the end of each week. A Monday long run followed by a Wednesday tempo can spike your midweek ACWR even if the weekly total seems reasonable. Daily tracking catches within-week risk that weekly summaries miss.