Training Strain
Also known as: Foster Strain, Weekly Strain Index, Monotony × Load
A composite weekly load metric that multiplies weekly AU by training monotony, producing a number that rewards productive volume and penalises uniform distribution. Strain is the second half of Foster's monitoring pair (monotony × weekly load = strain). Where weekly AU alone says 'how much did you do', strain says 'how much did you do, weighted by how blurred together it was'.
Formula
strain = weeklyAU × monotony
Where weeklyAU = sum of dailyAU across 7 days, and monotony = mean(dailyAU) ÷ SD(dailyAU). Useful interpretive bands from Foster's team-sport work:
strain < 4,000, sustainable training week
strain 4,000-6,000, productive accumulation, monitor for drift
strain > 6,000, Foster's original threshold for elevated overreaching / illness risk
A week of 2,450 AU at monotony 1.43 ≈ 3,500 strain. The same 2,450 AU at monotony 2.4 ≈ 5,900 strain, the same volume reads as nearly twice the physiological cost because the body never gets a clean recovery day inside the week.Example
Two athletes complete identical 2,800 AU weeks. Athlete A: 4 hard sessions (700 AU each) + 3 rest days. SD ≈ 360, monotony ≈ 1.11, strain ≈ 3,100. Sustainable. Athlete B: 7 days at 400 AU each, no rest day. SD ≈ 0 (in practice ~50), monotony ≈ 8, strain ≈ 22,000+. The math screams; the body screams a week later. Even if you allow that B's reality has some inter-day variance, monotony lands above 2.5 and strain pushes past 7,000, well over Foster's threshold.
How Afitpilot Uses This
Afitpilot's primary weekly load readout is weekly AU split by modality (strength / endurance / mixed), with the chronic / acute load chart providing the trend view. Strain is on the same roadmap candidate as monotony, both are cheap to compute from existing sRPE inputs and would surface a useful distinction the weekly AU alone misses: same volume, different recovery cost. When surfaced, strain would not auto-modify the plan, but a sustained 2+ week drift above Foster's threshold band would be flagged in the coach drawer alongside Hooper drift and ACWR, the same multi-signal pattern that flags the early overreaching continuum.
Strain in practice
| Who / Context | Value | Note |
|---|---|---|
| Foster's elevated-risk threshold | strain > 6,000 | Derived from team-sport athletes; use as a starting reference |
| Recreational productive week | strain 2,000-4,000 | Typical for adult athletes with at least one rest day |
| Marathon build week | strain 5,000-7,500 | High weekly volume × moderate monotony, sustainable for 4-8 weeks then deload |
| Polarized-training week | strain 1,500-3,500 at the same volume | Strong hard/easy contrast keeps monotony low, strain low |
| Strain after returning from a planned rest week | Often spikes 50-100% | Expected transient, read the 2-3 week trend, not the spike |
| Sport with highest baseline strain in Foster cohorts | Endurance running and cross-country skiing | Multi-day uniform-load weeks are the modality, not a flaw |
Known Limitations
- •Strain inherits every limitation of monotony, modality-blind, threshold-dependent, undefined at zero variance, and stacks them on top of weekly AU's own limitations as a load metric for strength work.
- •The 6,000 threshold from Foster's team-sport data is widely cited and rarely re-validated. Endurance athletes in deliberate high-volume base blocks routinely exceed it without injury or illness; strength athletes rarely reach it. Treat the threshold as a heuristic, not a stop sign.
- •Strain spikes hard when athletes complete a planned rest week as actual full rest (low load, low monotony) followed by a return-to-training week (moderate load, high monotony from contrast with the rest week). The metric reads the transition as risky when it's structurally productive.
- •The multiplicative structure means small monotony increases produce large strain increases at high weekly volumes. An athlete at 5,000 weekly AU with monotony rising 1.5 → 1.8 sees strain go 7,500 → 9,000, a 20% jump from a small distribution change.
- •Strain is a population-level monitoring tool, not an individual predictor. Foster himself emphasises trend over absolute value: rising strain over 2-3 weeks is more actionable than any single week's number.
Science Context
Strain emerged from Foster's 1998 framework as the actionable composite of monotony and weekly AU, the metric athletes and coaches actually look at when monotony is the underlying input. The original validation was on team-sport athletes (soccer, basketball, swimming) over multi-week observation windows, with strain above ~6,000 associated with elevated reports of upper-respiratory infection and overreaching symptoms. Subsequent work (Foster et al. 2001; later replications in cycling, rugby, and rowing) supports the qualitative pattern, rising strain tracks subjective complaints, without strongly validating any single numerical threshold. Modern training-monitoring reviews (Halson 2014; Bourdon et al. 2017 consensus statement) include strain alongside ACWR, monotony, and Hooper as one of the cheap-to-compute scalars worth tracking, with the standard caveat that no single number diagnoses overreaching and the multi-signal trend across weeks is the actual signal.