Knee angle in bike fitting: the 25–35° rule
Knee angle at the bottom of the pedal stroke is the single most useful number in a bike fit. It tells you whether your saddle height is right, and it is easy to measure at home from one side-on photo.
By Maxence Poutord·Last updated
What knee angle means
In bike fitting, knee angle usually means knee flexion: how far your knee is from fully straight when the pedal is at the very bottom of the stroke (the 6 o'clock crank position). A completely straight leg is 0° of flexion; a right angle at the knee is 90°.
Fitters measure it at 6 o'clock because that is where your leg is most extended. If the knee works in a good range there, saddle height is close to right for the rest of the stroke too.
The target range: 25–35° of flexion
Most published fitting systems (Holmes, Retül-style dynamic fits, and the ranges Pedalyze uses) put the target at 25–35° of knee flexion at the bottom of the stroke, for both road and TT positions. A band of roughly 20–40° is tolerable; outside that, most riders lose power or load the knee.
There is no bonus for hitting exactly 30°. Anywhere inside the range is fine — pick the spot that feels stable and pain-free.
| Knee flexion at 6 o'clock | Verdict | What it usually means |
|---|---|---|
| 25–35° | In range | Saddle height is close to right |
| 20–25° or 35–40° | Borderline | Adjust the saddle 5–8 mm and re-check |
| Below 20° | Too straight | Saddle too high — lower it 10–15 mm, then fine-tune |
| Above 40° | Too bent | Saddle too low — raise it 10–15 mm, then fine-tune |
Annotated example: one leg, three saddle heights
| Panel | Knee flexion | Verdict | What to do |
|---|---|---|---|
| Left | 15° | Fail: too straight | Lower the saddle 10–15 mm, re-shoot, then fine-tune in 5 mm steps |
| Middle | 30° | In range | Nothing. Anywhere in 25–35° is fine |
| Right | 45° | Fail: too bent | Raise the saddle 10–15 mm, re-shoot, then fine-tune in 5 mm steps |
Leg too straight: saddle too high
Under about 20° of flexion, the leg is nearly locked out at the bottom of every stroke. Typical signs on the bike: rocking hips, pain behind the knee or in the hamstrings, numb feet, and reaching for the pedal with a pointed toe. The knee pain guide maps each place it can hurt to the setting to check first.
Lower the saddle in 5 mm steps and re-measure after each change. Small steps matter — 5 mm at the saddle changes the knee angle by roughly 2–3°.
Leg too bent: saddle too low
Above about 40° of flexion, the knee stays deeply bent through the power phase. This overloads the front of the knee (a common cause of pain around the kneecap) and shortens your effective stroke, so you produce less power at the same effort.
Raise the saddle in 5 mm steps. If you have been riding low for years, spread the change over a few rides — your muscles adapt to the position you give them.
How to measure it from a photo
You do not need a goniometer. Sit on the bike on an indoor trainer, put the crank at 6 o'clock, and have someone take a photo from directly side-on at hip height. The angle between hip–knee and knee–ankle lines, subtracted from 180°, is your knee flexion.
Pedalyze does this automatically: upload the photo and it detects the hip, knee and ankle, measures the flexion, and checks it against the 25–35° range. Photo quality drives accuracy, so shoot it carefully — see the photo guide below.
Measurement pitfalls
Knee flexion is the most robust angle in a fit, but the same saddle can read 5–10° differently depending on how the photo was taken. These are the failures we see most, and what each one does to the number.
| Pitfall | What it does to the reading | How to avoid it |
|---|---|---|
| Crank not at true 6 o'clock | A crank 15° short of the bottom lifts the pedal and bends the knee. The reading comes out a few degrees too bent and you lower a saddle that was right. | Pedal backwards slowly and stop when the crank arm is vertical. Take several frames and keep the one with the pedal lowest. The video mode picks that frame for you. |
| Pointed toe or dropped heel | Plantarflexing the ankle (toe down) drops the ankle landmark and straightens the leg; a dropped heel bends it. The same saddle height can read 5° apart with two foot styles. | Hold the foot attitude you actually pedal with. Never pose the foot for the camera, and compare photos shot with the same ankling. |
| Static pose versus pedalling | A leg held still at the bottom reads straighter than the same leg mid-stroke, because riders do not reach full extension while pedalling. Studies find the two methods give substantially different numbers. | Do not compare a posed reading against a dynamic target or vice versa. If you want the pedalling number, use the video mode, which reads the moving leg. |
| Hip hidden by clothing | The hip is the hardest joint to locate. Baggy shorts or a jersey hanging over the hip push the landmark down and back, which alone can move knee flexion by several degrees. | Wear tight shorts and tuck the jersey. The hip landmark should sit on the greater trochanter, the bony point on the side of the hip. |
| Camera low or off-axis | A lens below hip height foreshortens the shin; a lens ahead of or behind the rider shortens whichever segment points toward it. Both bend or straighten the drawn knee without the leg moving. | Lens at hip height, square to the bike, several metres back with zoom. The capture gates reject strong tilt and off-axis shots for exactly this reason. |
| Reading the far leg | The far leg is partly hidden behind the bike and its knee is estimated, not seen. Its flexion is noisier and its confidence lower. | Always read the camera-side leg. The report does this automatically and shows the side it used. |
| Mixing angle conventions | Some fitters quote the interior knee angle (145–155°) rather than flexion (25–35°). A 150° interior angle is 30° of flexion, not a knee that is too bent. | Convert before comparing: flexion = 180° minus the interior angle. Pedalyze always reports flexion. |
Frequently asked questions
- Is 25–35° the same target for road and TT bikes?
- Yes. Saddle height relative to the pedals follows the same rule in both positions. What changes on a TT bike is the hip, torso and elbow geometry, not the knee flexion target.
- How much does saddle height change the knee angle?
- As a rule of thumb, 5 mm of saddle height changes knee flexion at the bottom of the stroke by about 2–3°. That is why fitters adjust in 5 mm steps and re-measure.
- Should I measure with the crank in line with the seat tube instead?
- Some methods measure with the crank pointing along the seat tube (maximum leg extension). The 6 o'clock convention is more common and easier to reproduce in a photo; just be consistent — do not mix targets from one method with measurements from the other.
- My knee angle is in range but my knee still hurts. Why?
- Saddle height is only one variable. Fore-aft position (see the KOPS guide), cleat placement, saddle tilt and crank length all load the knee. Pain at the front of the knee often points to a low saddle or a knee far forward; pain at the back points to a high saddle.
References and further reading
The 25–35° range is not a Pedalyze invention. These are the sources the range and the advice on this page trace back to; PubMed links go to the abstract.
- Holmes JC, Pruitt AL, Whalen NJ. Lower extremity overuse in bicycling. Clinics in Sports Medicine 13(1):187–205, 1994.
The clinical paper most fitting systems cite for the knee-flexion range at bottom dead centre; treats saddle height as the primary lever against overuse knee injury.
- Bini R, Hume PA, Croft JL. Effects of bicycle saddle height on knee injury risk and cycling performance. Sports Medicine 41(6):463–476, 2011.
Systematic review of saddle-height methods. Concludes the knee-angle method (25–30° at the bottom of the stroke) is preferable to inseam formulas for limiting knee load.
- Peveler WW. Effects of saddle height on economy in cycling. Journal of Strength and Conditioning Research 22(4):1355–1359, 2008.
Cycling economy was better with the saddle set to 25° of knee flexion than to 35° or to 109% of inseam.
- Ferrer-Roca V, Roig A, Galilea P, García-López J. Influence of saddle height on lower limb kinematics in well-trained cyclists: static vs. dynamic evaluation in bike fitting. Journal of Strength and Conditioning Research 26(11):3025–3029, 2012.
In well-trained cyclists, inseam-based saddle heights left a quarter of riders outside the recommended knee angle; measure the knee, do not just apply a formula.
- Fonda B, Sarabon N, Li FX. Validity and reliability of different kinematics methods used for bike fitting. Journal of Sports Sciences 32(10):940–946, 2014.
Compared 2D video, 3D motion capture and an electrogoniometer at bottom dead centre: all dynamic methods differed from the static goniometer, so never mix static and dynamic readings.
Related guides
Related tools
- Saddle height calculator — Estimate your saddle height from inseam with the LeMond method, measured from the bottom bracket to the top of the saddle.
- Saddle setback calculator — See how far behind the bottom bracket your saddle sits, and how that compares with the range road and time-trial riders use.
Pedalyze measures every angle on this page from two side-on photos of you on your bike — and tells you what to adjust. The first analysis is free.