→ Full technical version: Methodological Framework and Monte Carlo Validation — White Paper
Every time you finish a ride, your Garmin, Wahoo, Coros or Polar saves a .FIT file. Most cyclists upload it to Strava, check the map and time, and forget it. But that file contains between 7,000 and 14,000 records of cadence, GPS speed, barometric altitude, and power. Data that, properly analyzed, reveals whether your bike is correctly adjusted, whether your pedaling technique changes under load, and whether your aerobic capacity is improving or declining.
The BikeLab FIT Analyzer processes that file directly in your browser — without uploading anything to any server — and delivers four metrics validated with Monte Carlo simulation. This guide explains what each one measures, why it matters, and how to interpret the results.
The .FIT (Flexible and Interoperable Data Transfer) format was created by Garmin but is today the industry standard. A FIT file from a 2-hour ride at 1 Hz contains approximately 7,200 samples per channel: cadence in rpm, speed in m/s calculated from GPS, barometric altitude in meters, and power in watts if you use a power meter.
The problem is that raw data is noisy. GPS has an accuracy of ±3 m/s under normal conditions, worse in urban canyons or under tree cover. Cadence has sensor latency. Barometric altitude drifts with changes in atmospheric pressure. The analyzer applies statistical filters and segments the ride by temporal continuity and quasi-stationary regime before computing any metric.
Compatible with .FIT files from Garmin, Wahoo, Coros, Polar, Suunto, and Bryton. Also works with FIT files exported from Strava (Settings → Download your data → activity.fit). All processing happens in your browser: the file never leaves your device.
Cadence CV is the standard deviation of cadence divided by the mean, expressed as a percentage. It measures how consistent your pedaling is at steady state: if your cadence fluctuates between 78 and 92 rpm on flat ground at constant power, the issue could be saddle position, crank length, or simply a technical weakness in your pedaling stroke.
CV_c [%] = (cadence standard deviation / cadence mean) × 100
CV_c below 5% indicates very uniform pedaling. Values between 5–10% are normal. Above 12% at steady state suggests a technical or mechanical issue worth investigating — often saddle height or cleat alignment.
The CSI combines the coefficient of variation of speed and cadence on climbing segments with gradient above 3%. It is the most useful metric for detecting drivetrain problems: a rider with a worn chain, misaligned indexing, or loose cranks will see a high CSI because the drivetrain doesn't convert effort uniformly.
Low CSI (near 0) = mechanically efficient pedaling on climbs. High CSI = the speed/cadence ratio varies under load. Common causes: worn chain, incorrect indexing, bottom bracket play, or suboptimal climbing posture (excessive lateral trunk sway).
VI is the ratio of Normalized Power (NP) to Average Power (AP). Normalized Power applies a fourth-power rolling average to weight intense efforts, capturing the real metabolic cost of an irregular ride. A VI of 1.00 is mathematically impossible in practice; the normal range for base training is 1.02–1.08.
VI can only be calculated if your FIT file contains power data. Without a meter, the analyzer reports N/A for this metric. VI < 1.05 indicates very steady pacing (ideal for aerobic base). VI between 1.05–1.15 is typical on rolling terrain. VI > 1.20 on flat ground can indicate very irregular pedaling or a setup issue generating asymmetric effort.
Cardiac decoupling (or aerobic decoupling) measures the change in the power/heart-rate ratio (or speed/HR in the absence of power data) between the first and second halves of the analyzed segment. If in the first 30 minutes you need 150 W to maintain 145 bpm and in the last 30 minutes you need 140 W for the same heart rate, your heart is working harder for the same result — that is positive decoupling.
Decoupling < 5% = good aerobic capacity for the effort intensity. Between 5–10% is acceptable for long rides in heat or altitude. Above 10% indicates the effort exceeded your aerobic threshold or you hadn't recovered sufficiently before the ride. Don't confuse with heat: hot weather elevates HR independently of metabolic decoupling.
| Metric | Optimal | Normal | Investigate |
|---|---|---|---|
| Cadence CV | < 5% | 5 – 10% | > 12% |
| CSI (climbing) | < 0.04 | 0.04 – 0.08 | > 0.10 |
| Variability Index | 1.02 – 1.05 | 1.05 – 1.15 | > 1.20 |
| Cardiac Decoupling | < 5% | 5 – 10% | > 10% |
* Base thresholds for road bike, 2x drivetrain, 700c wheel. The analyzer adjusts automatically for full-suspension MTB, 1x, and 29" wheels.
// Free tool — no account — no server
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