cycling-training/references/tt-demands.md

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)