Training Monotony and Strain Calculator (Foster)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
Monotony is calculated with average daily load ÷ standard deviation of daily loads over the week; strain = total weekly load × monotony. A monotony above 2 signals an increased fatigue risk.
Explanation
The training load of a single session, measured for example with the session-RPE method (duration in minutes × perceived effort out of 10) or by our TRIMP training load calculator, says nothing about how that load is spread across the week. It's the lack of variation that's the problem: a week where every day looks like the previous one, with no genuinely light day or genuinely hard day, leaves little room for recovery and increases the risk of chronic fatigue and injury, even at a moderate total load. The Foster method (1998) quantifies this with two indicators. Monotony is the ratio between the average daily load and the standard deviation of daily loads: the more the days resemble each other, the lower the standard deviation and the higher the monotony. Strain multiplies the total weekly load by monotony: it combines volume and uniformity into a single figure, and predicts overtraining better than volume alone. Commonly cited benchmarks place an increased fatigue risk beyond a monotony of 2, and an injury risk beyond 2.5. This indicator complements the acute:chronic workload ratio (ACWR), which compares recent load to baseline load: the two measures describe different risks and are read together, not one instead of the other. To lower monotony, the solution isn't to train less, but to widen the gap between easy days and hard days, and to include at least one full rest day or very low-load day per week.
Example: a week of 200, 600, 300, 700, 100, 500, 400
Inputs
Daily loads: 200, 600, 300, 700, 100, 500, 400.
Calculation
Weekly load = 2800. Average = 2800 ÷ 7 = 400. Standard deviation of the seven values = 200. Monotony = 400 ÷ 200 = 2.0. Strain = 2800 × 2.0 = 5600.
Result
This week shows a monotony of 2.0, right at the increased fatigue risk threshold, despite an apparently good alternation.
Frequently asked questions
What unit should I use for the daily load?
Any load measure that's consistent from one day to the next works: the most common is the session-RPE load (total session duration in minutes multiplied by perceived effort on a 1 to 10 scale), but you can also use a TRIMP score, an intensity-weighted distance, or any other indicator, as long as it's calculated the same way each day. Monotony is a ratio, it doesn't depend on the chosen unit.
Why does a rest day lower monotony?
A day with zero or very low load sharply widens the standard deviation of daily loads, since it deviates a lot from the average. But monotony is the average divided by that standard deviation: a larger standard deviation lowers monotony. This is why planned variation (hard days, easy days, a rest day) protects better than a load smoothed day after day, even at the same total volume.
Is low monotony always desirable?
Low monotony (significant day-to-day variation) is generally favorable for recovery, but the goal isn't to minimize it at all costs: during a training camp or a deliberate overload block, temporarily high monotony can be a conscious choice, provided it's followed by a taper period. It's sustained, unrelieved high monotony that constitutes the risk factor.