A saddle sitting even an inch too low or too high rarely feels dramatically wrong while simply sitting on the bike, but that small a difference is enough to meaningfully change the angle at the knee joint at the bottom of every single pedal stroke, and that’s usually where the resulting pain actually shows up.

Why a Small Height Error Changes Knee Angle Specifically

The geometry: at-the-bottom-of-the-pedal-stroke-the-leg-is-nearly-fully-extended-so-even-a-small-change-in-saddle-height-produces-a-disproportionately-large-change-in-knee-flexion-angle-at-that-point (at the very bottom of the pedal stroke, the leg sitting nearly fully extended, meaning even a small change in saddle height produces a disproportionately large change in the actual knee flexion angle at that specific point in the pedal cycle, per the cycling-for-beginners logic — at-the-very-bottom-of-the-pedal-stroke-the-leg-sitting-nearly-fully-extended-meaning-even-a-small-change-in-saddle-height-produces-a-disproportionately-large-change-in-the-actual-knee-flexion-angle-at-that-point), and the reframe (a seemingly minor height error concentrating its full effect at one specific, repeated point in every pedal cycle)).

Why the Error Is Easy to Miss by Feel Alone

The blind spot: sitting-on-a-bike-at-a-standstill-and-even-riding-for-a-short-distance-rarely-produces-an-obvious-uncomfortable-sensation-from-a-one-inch-height-error (simply sitting on a bike at a standstill, and even riding for a short distance, rarely producing any obviously uncomfortable sensation from a one-inch saddle height error, since the effect is subtle and cumulative rather than immediately sharp, per the bike-fit-basics-comfort logic — simply-sitting-on-a-bike-at-a-standstill-and-even-riding-for-a-short-distance-rarely-producing-any-obviously-uncomfortable-sensation-from-a-one-inch-saddle-height-error-since-the-effect-is-subtle-and-cumulative), and the frame (the error staying invisible by feel until repeated pedal strokes over miles accumulate into actual knee pain)).

How to Check Saddle Height With a Leg-Extension Test

The test: at-the-bottom-of-the-pedal-stroke-with-the-heel-on-the-pedal-the-leg-should-reach-nearly-full-but-not-locked-extension-and-adjustments-should-be-made-against-that-specific-reference (checking that, at the bottom of the pedal stroke with the heel resting on the pedal, the leg reaches a position of nearly full but not fully locked extension, and making saddle height adjustments against that specific, repeatable reference point rather than against general comfort while sitting still, per the bike-fit-basics-comfort logic — checking-that-at-the-bottom-of-the-pedal-stroke-with-the-heel-resting-on-the-pedal-the-leg-reaches-a-position-of-nearly-full-but-not-fully-locked-extension-and-making-adjustments-against-that-specific-repeatable-reference-point), and the frame (a specific, repeatable test replacing a vague comfort check that a one-inch error can easily slip past).

At the very bottom of the pedal stroke, the leg sits nearly fully extended, meaning even a small change in saddle height produces a disproportionately large change in the actual knee flexion angle at that specific point, an error that rarely produces any obviously uncomfortable sensation from simply sitting on the bike or riding a short distance. Checking that the leg reaches a position of nearly full but not fully locked extension at the bottom of the stroke with the heel on the pedal, and adjusting saddle height against that specific reference point, catches an error that a general comfort check can easily miss.