RoadFitter°
Knee angle for cycling
Knee angle at the bottom of the pedal stroke is the one part of your riding position with real scientific consensus behind it. Here are the target values, how to measure them — and, unusually, which of the other numbers are only experience.
The short answer
30 to 40 degrees — measured at the bottom of the stroke, when the pedal is at its lowest. Here 0° means a fully straight leg, so the knee stays clearly bent at the bottom.
The app additionally marks 33° as a target inside that corridor. That is our own choice, not a study result — the literature gives a range, not a point value. We say so explicitly, because an invented decimal would be exactly the false precision we criticise in others.
- Angle too large (above 40°) — the leg never extends enough; the saddle is probably too low. Common company: pressure at the front of the kneecap, heavy quads.
- Angle too small (below 30°) — the leg extends too far; the saddle is probably too high. Common company: rocking hips, a pull behind the knee or in the Achilles.
- Inside the range but near the edge — no need to rush. The band is deliberately wide, because people are built differently.
Why at the bottom — and why while pedalling
At the bottom of the stroke the leg is at its most extended. That is where saddle height decides the outcome: every millimetre at the saddle shows up directly in this angle.
What matters is that it is measured while you pedal, not standing still. A leg you stretch out while seated sits differently from one putting power through a pedal: the pelvis rotates, the foot drops, the heel moves. Values taken statically therefore regularly disagree with dynamic ones — they are not wrong, they simply describe a posture that does not occur while riding.
And because a single moment says nothing about a movement, what counts is the median across many crank revolutions, not one still frame. Otherwise a single slip or a dropped heel skews the whole result.
Every target range — and what is actually proven
These are exactly the values RoadFitter works with. We publish them together with where they come from, because the provenance is the real difference: only for the knee is there a solid body of evidence. Everything else is practitioner experience — useful, but not a law of nature. Presenting these numbers without that distinction sells a precision that does not exist.
| Angle | Target range | Target* | Basis |
|---|---|---|---|
| Knee bottom of stroke |
30–40° | 33° | Peer-reviewed. Ferrer-Roca et al. 2012 (dynamic, at the bottom of the stroke); review by Husband et al. 2024: saddle height is the only position variable with scientific consensus. |
| Torso from horizontal |
40–50° | 45° | Practitioner consensus (BikeDynamics, hands on the hoods). Fintelman 2015 shows a lower torso costs power and oxygen. The same paper notes the aerodynamic gain usually outweighs that cost in time trialling — it is a trade, not a mistake. |
| Shoulder | 80–95° | 88° | Practitioner consensus (~90° on the hoods). Provisional. |
| Elbow | 15–30° | 22° | Three numbers get muddled here, so plainly: the classic textbook value is 20–30° (Burt 2014). But Holliday & Swart 2021 measured 19° ± 8 in the real world — the average sits below the textbook. We grade 15–30° so a rider at a perfectly ordinary 19° is not flagged as wrong. |
| Hip | 85–110° | 97° | Deliberately not graded. Measurement conventions contradict each other (top vs. bottom of stroke, hip vs. torso), and different methods correlate poorly. RoadFitter reports the value but does not score it. |
*The "Target" column is our own marker inside the corridor, not a study value. The sources give ranges; a point value helps with fine-tuning but is not evidenced.
A tolerance of 2.5° is built into all of it — a 2D measurement from the side is accurate enough to decide a direction, but nobody should haggle over the last degree.
Does the target change with the bike?
For the knee: no. The 30–40° band holds for every seated position, from a time-trial bike to a Dutch roadster — because it is set by saddle height, and legs do not know which frame is underneath them. That includes the gravel bike explicitly: the knee-angle target stays at 30–40°, only the torso angle shifts — usually a bit more upright than on a pure road bike, for control on loose surfaces and long days in the saddle.
What does shift is everything above. On a time-trial bike a flat torso is the goal; on a city bike the same posture would be a mistake. That is why you pick the bike type in the app — road (comfort or aero), gravel, time trial and triathlon, spin, city, step-through and kids' bikes — and the target ranges for torso, shoulder and elbow move with it. The knee angle stays put.
Measuring it yourself, without a studio
No turbo trainer and no appointment needed. Lean the bike against a wall or have someone hold it, camera side-on at roughly hip height, whole body in frame, pedal steadily — backwards is fine, bottom-of-stroke detection works in both directions.
RoadFitter finds your joints in the image, measures the angle across the whole pedalling sequence and converts the deviation straight into millimetres of saddle adjustment. Everything runs on the device — no account, no upload, no trackers. For iPhone and iPad, free to try, Pro is a one-time €39.99 with no subscription.
Next: measuring saddle height instead of calculating it — why the familiar inseam formulas are only a starting point.
Can knee pain mean my saddle height is wrong?
Sometimes, yes. Studies link certain knee-pain patterns to saddle position — pain at the front of the knee is more often associated with a saddle that sits too low or too far forward; pain behind the knee with one that sits too high or too far back — though the two directions are not equally well evidenced in the literature. RoadFitter measures the part that is objectively measurable, the knee angle at the bottom of the stroke, and reports whether it falls outside the 30–40° range. It does not diagnose or treat anything, and it does not promise pain relief — for pain that persists, the right next step is a doctor or physiotherapist, not an app.
What a side-on camera cannot see
Knees tracking sideways, cleat position, leg length differences, a tilted pelvis: all of that happens across the camera and cannot be measured from a side view, in principle. An app therefore does not replace a good bike fitter — it does the part that can be measured objectively, and says so when it does not know. For persistent pain, the answer is a professional, not an app.