When consistency becomes risk.

monotony = mean(daily_TSS) / stdev(daily_TSS)

A week of 50 TSS every day: mean = 50, stdev ≈ 0 → monotony approaches infinity. A week with one 200-TSS long ride, one 100-TSS tempo, and five rest/easy days has low monotony — large variation.

Foster’s research on Olympic speed skaters in the 1990s showed monotony above 2.0, combined with high strain (total TSS × monotony), predicted elevated illness risk the following week.

Below 1.5 — Healthy variety

Hard-easy rhythm. Big differences between days. The goal for most training blocks.

1.5 to 2.0 — Watch

Training load is spreading too evenly. Could be a week of slightly-hard runs with no rest, or consistent moderate riding without sharpening.

Above 2.0 — High risk

Our rule engine triggers HIGH_MONOTONYabove this threshold. Combined with any negative TSB, the risk of illness or soft-tissue injury is real. We’ll cap the next block’s readiness score at 55 and recommend easier days.

The fastest fix is adding variation — both a true rest day and a slightly harder session than you normally do:

• Take a real rest day.Not “easy 40-minute run”, actual zero TSS. This drops your stdev denominator and pulls monotony down fast.
• Add one intensity session. A 15-minute tempo block or 5 × 3-minute intervals. Raises your peak day TSS, bumps stdev up.
• Extend your long day.Push the weekly long ride/run 20 min longer. The fattening of the tail of your distribution reduces monotony more than you’d expect.

Paradoxically, adding training can lower monotony — because you’re adding it where the gap is biggest, not where you’re already loaded.

Triathletes often look fine on per-sport monotony — easy swim, easy ride, easy run — but combined monotony is dangerous. A week with 40 TSS of swim, 60 of ride, 50 of run stacked evenly across days has combined monotony near 2.5 even though no single sport looks busy.

Our model computes monotony on combined daily TSS across all sports — so triathletes get warned when per-sport tools wouldn’t. That’s core to why multi-sport coaching matters.

Two amateur runners, both holding CTL around 60. Same weekly total TSS, same weekly hours. One is fine, the other is 10 days from a stress fracture. The difference is monotony.

Runner A — varied week

Daily TSS: 0, 75, 30, 95, 0, 140, 60

• Total: 400
• Mean: 57
• Stdev: 52
• Monotony: 57 / 52 = 1.10
• Strain: 400 × 1.10 = 440

Two rest days, a long run, a quality session, two easy days. Classical hard-easy rhythm. The legs get real recovery twice per week. Nothing in the rule engine triggers. Injury risk low.

Runner B — “consistent” week

Daily TSS: 55, 60, 55, 60, 55, 60, 55

• Total: 400
• Mean: 57
• Stdev: 2.7
• Monotony: 57 / 2.7 = 21.1
• Strain: 400 × 21.1 = 8,440

Same weekly TSS. On paper Runner B looks disciplined and consistent. The body sees seven moderate bouts with zero recovery. This pattern is the classical signature of the “grey zone” trap — too hard to recover from, too easy to drive adaptation. Illness risk over the next 7–10 days is measurably elevated in Foster’s studies. Our rule engine flags HIGH_MONOTONYand the next block’s readiness caps at 55 until variation returns.

The fix, in numbers

Runner B pulls Wednesday to zero and pushes Saturday to 140. New week: 55, 60, 0, 60, 55, 140, 55.

• Total: 425
• Mean: 61
• Stdev: 44
• Monotony: 61 / 44 = 1.39

Slightly more total stress, dramatically less monotony, much healthier week. The counter-intuitive lesson: adding a hard day can make the week safer because it pulls stdev up faster than it pulls the mean.

• Treating “easy” as zero.A 30-minute easy run is not a rest day. It’s 20–30 TSS that pollutes your stdev and quietly lifts monotony. If you need a rest day, take a real rest day.
• Adding intensity to fix monotony without fixing fatigue. Throwing a sprint session on top of already negative TSB reduces monotony on paper but produces strain spikes. Fix monotony with distribution (one more rest day, one more hard day), not with additive volume on top of an exhausted system.
• Only tracking per-sport monotony. For triathletes and duathletes, combined daily TSS across all sports is the number that matters. Three sports each with low internal monotony can stack into a combined monotony above 2.0 invisibly.
• Confusing monotony with fatigue. ATL measures fatigue. Monotony measures distribution of stress. You can have fresh legs (low ATL) and bad monotony — typically right before a back-half breakdown — because the body was loaded evenly enough to prevent recovery adaptation without ever tipping into overt fatigue.
• Panicking at monotony 1.8 during a volume block. Deliberate high-volume blocks can run 1.7–1.9 for short periods if total load is moderate. The problem is sustained high monotony with high strain. Context always wins over a single number.

Strain

Foster’s companion metric: total weekly TSS × monotony. Strain is what actually correlated with illness risk in the original speed-skater study — monotony alone wasn’t enough. A week at 300 TSS and monotony 2.2 has strain 660, manageable. A week at 700 TSS and monotony 2.2 has strain 1,540 and is the kind of combination that shows up in illness logs the next week.

TSB and ATL

Monotony is a 7-day window. ATL is a 7-day EWMA. They overlap in scope but measure different things — ATL is your cumulative fatigue at the end of the week, monotony is how evenly the stress was distributed. Use both. See CTL, ATL, TSB.

Training readiness

Monotony feeds directly into our readiness score as a penalty. Above 1.3 starts subtracting; above 2.0 penalizes heavily; high monotony flagged by the rule engine applies a hard cap at 55. Training readiness →