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Why does my runner's feet cross an imaginary line down the middle when they run?

Crossover gait is a sideways fault, which is exactly why a side-on video of it so often looks fine.

Watch a runner from behind rather than the side and sometimes a foot that looked fine landing underneath the hips turns out to be landing across an imaginary line down the middle of the body, almost in front of the opposite hip rather than under its own. Coaches call it crossover gait, and it is common enough in distance runners that it is worth knowing what it actually costs.

Where it usually comes from

• Weak hip stabilisers, particularly the muscles on the outside of the hip, which let the pelvis drop and rotate instead of staying level stride to stride.
• Trying to lengthen the stride by swinging the leg further sideways rather than driving it further forward, which produces a longer-looking step that is actually a narrower, crossed one.
• Fatigue late in a race, when the muscles holding the hips level tire before the ones doing the running do, and the crossing gets worse in exactly the miles where form matters most.

Why it costs more than it looks like it should

Running is meant to move the body in one direction. A foot that lands across the midline sends part of every push sideways instead of forward, which is energy spent correcting a wobble rather than covering ground. It also loads the hip, knee and ankle at an angle they are not built to absorb repeatedly, which is why crossover gait shows up so often in the injury history of runners with chronic IT band or knee pain rather than in a fast time trial.

Why it is easy to miss on film

Crossover gait is a frontal-plane fault: it happens side to side, across the body, not forward and back. A side-on clip, the angle every sprint setting in the Film Room is built around, shows trunk lean, knee fold and overstride clearly because those are forward-and-back faults. It will not reliably show a foot drifting inward, because from the side that foot is simply landing somewhere underneath the runner, and the camera cannot tell you where underneath.

This is coaching guidance, not a diagnosis. The Film Room's running measures come from a side-on clip, which is the right angle for lean, knee fold and ground contact time and the wrong angle for a fault that happens across the body. Seeing crossover gait at all means filming from behind or straight on, a different clip than the one the app's sprint setting is built to read.

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