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.