# Time-trial (TT) demands

**Load this doc when:** planning for an individual time trial, prologue, or any sustained
solo effort against the clock (road TT, triathlon bike leg, "the race of truth").

**Data/knowledge split:** MCP gives you power, critical-power/FTP, and the ride streams. This doc
explains the *demand profile* — what actually limits a TT and how to pace and prepare for it.

**Contrast with the other demand docs:** a TT is the near-opposite of `mtb-xco-demands.md` and
`road-racing-demands.md`. Those are intermittent, stochastic, and tactical; a TT is **steady,
solo, and self-paced** — a quasi-time-invariant effort where physiology, aerodynamics, and pacing
discipline decide the result, not positioning or surges.

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## What limits a TT: sustainable power vs aerodynamic drag

TT performance is a tug-of-war between the **power you can sustain** and the **power the air steals
back**. Both matter, and the second is bigger than most riders assume.

- **Aerodynamic drag is the dominant resistance at TT speeds.** At racing speeds the rider's body
  is roughly **~80% of total system drag**, and overcoming air resistance consumes the large
  majority of power output (Crouch et al. 2017, *Sports Eng* 20(2):81–110;
  DOI 10.1007/s12283-017-0234-1). The classic validated power model that decomposes road cycling
  power into aerodynamic, rolling, gravitational, bearing, and inertial terms — and shows aero
  dominates on the flat — is Martin et al. 1998 (*J Appl Biomech* 14(3):276–291;
  DOI 10.1123/jab.14.3.276; **no PMID — cite by DOI**).
  - *(The often-quoted "~90% of power goes to aero at 40 km/h" figure traces to Kyle & Burke 1984,
    a trade-magazine article not indexed in PubMed — treat the exact percentage as approximate;
    the verifiable anchors are Crouch 2017 and Martin 1998. See `citations.md`.)*

- **Reducing aerodynamic drag area (CdA) is the single largest performance lever.** Wind-tunnel-
  guided position changes reduced professional cyclists' drag by **~14%**, with corresponding TT
  time savings (García-López et al. 2008, *J Sports Sci* 26(3):277–286; PMID 17943597;
  DOI 10.1080/02640410701501697). Position first, then equipment. A watt saved on the body is often
  cheaper than a watt gained in the legs.

## The physiological determinant: critical power / threshold

A TT is run at or near the highest power sustainable for its duration, so the key predictor is
**critical power / functional threshold**, not peak or anaerobic power.

- Critical power correlated with 17-km and 40-km TT performance at **r = −0.77 to −0.91** — more
  strongly than ventilatory threshold or VO2max (Smith, Dangelmaier & Hill 1999, *Int J Sports Med*
  20(6):374–378; PMID 10496116; DOI 10.1055/s-2007-971147). *(Negative r: higher power = faster
  time.)*
- **Training implication:** the engine that matters is threshold/critical power and the fraction of
  VO2max you can hold for the event duration. Build it with threshold and VO2max work
  (`vo2max-intervals.md`) on a high aerobic base (`periodization.md`).

## Pacing: even power on the flat, variable on terrain/wind

- On a **flat, windless** course, **even / constant power is near-optimal** — resist the urge to
  surge (Atkinson, Peacock & Passfield 2007, *J Sports Sci* 25(9):1001–1009; PMID 17497402;
  DOI 10.1080/02640410600944709).
- On **hilly or windy** courses, **vary power with the terrain**: push *above* target into climbs
  and headwinds, ease *below* on descents and tailwinds. Because time is lost disproportionately
  where you're slow, spending extra effort where it buys the most speed nets a faster overall time
  than constant power (Swain 1997, *Med Sci Sports Exerc* 29(8):1104–1108; PMID 9268969;
  DOI 10.1097/00005768-199708000-00017; refined by Atkinson et al. 2007 above).
- **Prescribe pacing from power**, not feel — the whole point of a TT is holding a target the
  cardiovascular system can't self-regulate under adrenaline.

## Durability matters for long TTs

For 40 km and up (and for TTs late in a stage race), **fatigue resistance** decides whether your
threshold holds to the line. Durability is the resistance to deterioration of threshold/economy/
power over prolonged exercise (Maunder et al. 2021, *Sports Med* 51(8):1619–1628; PMID 33886100;
DOI 10.1007/s40279-021-01459-0). Train it via `durability.md`; for a long TT, your *fatigued*
threshold is the one that counts.

## HR in a TT: more usable than MTB, still secondary

Unlike the intermittent efforts in `mtb-xco-demands.md`, a TT is steady-state, so HR tracks effort
more faithfully and **aerobic decoupling is a valid durability signal** on a TT-pace effort. But:
- HR still **lags the first minutes** (don't chase it off the start ramp) and **drifts upward**
  from cardiac drift, heat, and dehydration over a long TT.
- **Pace by power; use HR as a secondary check**, and screen the usual confounds
  (`data-confounds.md`) before trusting a decoupling or drift number.

## Practical checklist (pull data from MCP, reason here)

- [ ] **CdA** addressed — position dialed before equipment (García-López 2008)
- [ ] **Target power** set from current critical power / FTP for the event duration (Smith 1999)
- [ ] **Pacing plan** matched to course: even on flat, terrain-variable on hills/wind (Swain 1997;
      Atkinson 2007) — expressed in power, not HR
- [ ] **Durability** trained if the TT is long or late in a stage race (`durability.md`)
- [ ] **Warm-up** appropriate to duration (short TT = fuller prime; long TT = lighter)