# MTB / XCO demand profile (and why power governs off-road)

**Load this doc when:** planning for mountain-bike racing, deciding MTB-specific workouts, or
questioning whether HR is trustworthy off-road.

**Data/knowledge split:** MCP gives you the power streams, HR, and interval stats from a ride. This
doc explains what the *demand profile* is and why, for MTB, **you prescribe and judge from power,
not HR.**

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## The demand profile: intermittent, explosive, anaerobically shifted

XCO (Olympic cross-country) racing is **intermittent and explosive**: a hard start followed by
repeated high-intensity bursts, with roughly **25% of race time spent above maximal aerobic power
(MAP)** in brief 5–10 s efforts, on top of a hard first-lap start (Hays et al. 2018, *Front Physiol*
9:1062; PMID 30158873; DOI 10.3389/fphys.2018.01062). A corroborating dataset reports ~30% of time
in the top zone across 334 efforts averaging ~4.3 s at ~135% MAP (Prinz et al. 2021, *IJSPP*;
PMID 33848975).

Contemporary XCO has **shifted toward greater anaerobic contribution while maintaining high aerobic
demand** — you need both engines (Protzen et al. 2026, *Sports Med Open* 12:16; PMID 41739301;
DOI 10.1186/s40798-026-00976-4).

## Both aerobic and anaerobic power independently predict performance

- **Aerobic:** body-mass–normalized aerobic/threshold indices correlate strongly with XCO race time.
  Reported correlations across the literature sit in the **r ≈ 0.6–0.9** range, but note the classic
  elite-cohort study found raw VO2max/peak power did **not** separate riders — the significant
  correlations were for **mass-normalized threshold** power/VO2 (Impellizzeri et al. 2005,
  *Br J Sports Med* 39(10):747–751; PMID 16183772; DOI 10.1136/bjsm.2004.017236). **Frame the
  0.6–0.9 as "across studies," not one clean value.**
- **Anaerobic:** maximal power in a 5×30 s Wingate protocol correlated **r = −0.79** (p = 0.006) with
  XCO race time — anaerobic power independently predicts performance (Inoue et al. 2012, *J Strength
  Cond Res* 26(6):1589–1593; PMID 21912290; DOI 10.1519/JSC.0b013e318234eb89). *(The correlation is
  negative because higher power = lower/faster race time.)*

**Training implication:** develop both. Aerobic base/VO2max (`vo2max-intervals.md`) *and* anaerobic/
repeated-sprint power and strength (`strength-for-cyclists.md`). Neither alone covers the demand.

## Fatigue resistance separates the field

Higher-performing MTB riders decline **less** under fatigue. Across 693 elite male XCO race files,
power under fatigue fell ~**6–10%** in Top-10 riders vs ~**15–20%** in lower-ranked riders (p from
0.008 to <0.001) — durability is decisive off-road (Sánchez-Jiménez et al. 2025, *Scand J Med Sci
Sports* 35(11):e70170; PMID 41285697; DOI 10.1111/sms.70170). Train it via `durability.md`.

## ⚠️ The HR caveat (read this before prescribing MTB intensity by HR)

In intermittent MTB racing, **HR, power, and VO2 dissociate**:
- **VO2 stays high on descents where power drops** — you're still consuming oxygen while barely
  pedaling, so power under-reads true metabolic cost on descents.
- **HR lags short efforts** — a 5–10 s surge is over before HR responds, so HR under-reads the
  intensity of the bursts that define the sport.
- In XCO, %VO2max did **not** correlate with %HRmax or %MAP (Hays et al. 2018, PMID 30158873).

**Therefore: power governs MTB intensity; HR is unreliable for it.** Prescribe intervals, pacing,
and race targets from power. Use HR only where it is valid — steady-state durability decoupling and
recovery trends — and always screen the confounds in `data-confounds.md`. Do **not** set MTB
interval targets or judge burst efforts by heart rate.

## Discipline nuance: marathon/point-to-point ≠ lap XCO

A 4-hour marathon MTB has **different performance predictors and pacing** than lap-based XCO
(Novak et al. 2018, *J Sports Sci* 36(4):462–468; PMID 28406361; DOI 10.1080/02640414.2017.1313999).
Marathon/point-to-point events (e.g. Iceman) are steadier and more sustained; lap XCO is punchier
and more repeatedly supra-MAP.

> ⚠️ **Don't over-transfer.** The XCO numbers above come largely from **small-n elite** samples.
> A masters marathon rider does **not** inherit an elite XCO rider's ~25%-above-MAP profile or their
> exact fatigue-decline percentages. Keep absolute numbers **discipline- and level-specific**, use
> the *principles* (both engines matter; power governs; durability decides), and **track the
> individual's own race files** via the MCP data rather than importing elite constants.