Knee Extension at BDC
Target≈140°–150° (≈30°–40° of knee flexion)
included hip–knee–ankle angle with the crank at bottom-dead-centre (6 o'clock).
Why this matters
Knee extension at the bottom of the stroke is the single most important saddle-height metric. Too little extension (a knee that stays bent) wastes power and loads the front of the knee; too much (a near-straight knee) makes the hips rock to reach the pedal and strains the back of the knee.
What good looks like
At BDC the leg is nearly — but not fully — extended, leaving ≈30°–40° of flexion. The hips stay level on the saddle; the ankle is roughly neutral.
Common faults
- Saddle too low / too far forward — knee stays well flexed (< 140°), patellofemoral load climbs.
- Saddle too high / too far back — knee over-extends (> 150°), hips rock and the posterior knee is stressed.
Coaching cues
- Adjust saddle height in small 3–5 mm steps and re-measure; bike fit is sensitive.
- Confirm the hips stay quiet on the saddle from behind — rocking is the classic too-high tell.
How to draw
- Pause with the measured leg's crank straight down (6 o'clock).
- Thigh line: greater trochanter → lateral femoral condyle (knee).
- Shank line: knee → lateral malleolus (ankle).
Related measurements
- Knee Flexion at TDC — the other end of the stroke.
- Knee Tracking — confirm the knee also tracks straight under load.
General bike-fit guidance, not a clinical recommendation. Optimal values vary with discipline and anthropometry.
source · sports/cycling/learn/cycling_knee_extension.md