Why a sudden stop loads the knee
When you run, your body carries forward momentum—the tendency to keep moving. If your foot suddenly plants to stop you, the ground pushes back while the rest of your body is still traveling forward. Your leg has to absorb that force quickly.
A bent knee can flex a little and help spread the braking force across the muscles and joints. But if you stiffen or “lock” the knee straight, it has less room to give. The muscles around the knee must tense sharply to keep you upright and steady, creating the sudden strain behind the popping sensation described here.
The pop is a sensation, not a diagnosis
A pop can be a sound, a feeling, or both. It may come from movement within or around a joint, but the sensation alone does not identify what caused it. The important part of the movement is the abrupt transfer of force: your leg goes from carrying you forward to trying to stop you almost at once.
That is why a controlled stop is easier on the leg than freezing mid-stride. Letting the knee bend slightly gives the muscles more time and distance to absorb the change in motion—instead of making a rigid leg take the whole jolt.
