Every number FitX shows you has a citation.

Critical Force (CF) is the maximum finger-flexor force a climber can produce indefinitely without fatiguing. Below CF, recovery matches demand; above it, you burn anaerobic reserves (W′) until failure. FitX fits a hyperbolic Force–Time curve to your max-hang and repeater data, reports CF as a percentage of bodyweight, and classifies it into research-backed zones. The curve is the model Monod and Scherrer introduced for muscle groups in 1965, adapted by Giles and colleagues in 2006 for finger-flexor endurance specifically.

Finger strength stagnates when you run the same protocol for more than three weeks — the nervous system adapts, then stops responding. FitX’s hangboard scheduler rotates between max-hang (85–100 % load, 7–10 s), repeaters (7/3 intervals at 60–80 %), and no-hang grip work on a three-week cycle, matching the periodization frameworks in López-Rivera’s research on finger-strength adaptation and MacLeod’s earlier work on climbing-specific endurance.

Your body doesn’t “get stronger” at a planche — it shortens the lever arm until it tolerates more torque. FitX quantifies each progression (tuck, advanced tuck, straddle, full) as a percentage of full-lever torque based on EMG and kinematic data from Marchetti’s lab, so you train at the actual muscle length and load your body can sustain. Skipping to straddle before you own 70 % torque is how elbows and biceps fail.

A 7/10 squat on Monday isn’t a 7/10 on Friday — sleep, stress, and accumulated session volume shift the ceiling every day. Zourdos and colleagues validated a rep-in-reserve RPE scale in 2016 that lets lifters train to consistent effort instead of a rigid percentage of 1RM. FitX’s autoregulator adjusts your target load each set based on your previous RPE inputs and the validated Zourdos scale.

Hypertrophy adaptations scale with weekly volume until they don’t. Below the Minimum Effective Volume (MEV), you maintain. Between MEV and the Maximum Adaptive Volume (MAV), you grow. Above MAV and up to the Maximum Recoverable Volume (MRV), you grow but debt accumulates. FitX tracks weekly sets per muscle group per Schoenfeld’s 2017 dose–response meta-analysis and the MEV/MAV/MRV framework formalised by Israetel and colleagues, and flags when you run chronically above MAV.

A three-minute round draws roughly 30 % alactic, 50 % glycolytic, 20 % aerobic — but the split inverts completely over eight-plus rounds of sparring. FitX’s boxing block periodises conditioning based on Chaabene’s review of amateur boxing physiology and Smith’s earlier energy-system studies, separating heavy-bag intervals (alactic–glycolytic) from roadwork (aerobic base).

Harris–Benedict was published in 1919 — on subjects whose body composition and activity patterns look nothing like modern athletes. Mifflin and St Jeor re-derived BMR in 1990 with a sample closer to today, and Frankenfield’s 2005 validation study showed Mifflin–St Jeor estimates resting energy expenditure within 10 % for 82 % of healthy adults — beating every older equation. FitX uses Mifflin–St Jeor with activity multipliers calibrated for multi-discipline athletes.