Why your sprinter's hips drop on one side
A hip that dips every time one particular foot lands is a stability fault, and it happens in the one plane a side-on camera cannot see.
It usually gets noticed by accident, filming from behind for some other reason, and there it is: one hip visibly drops every time a specific foot is on the ground. Nothing hurts. The athlete has always run this way. It is not obvious whether it is worth doing anything about.
What a hip drop actually is
While one foot is on the ground, the muscles on that side of the hip, mainly the glute medius, are supposed to hold the pelvis roughly level, or tipped slightly up on the swing-leg side. When they cannot quite do that job, the opposite hip drops for an instant every time that leg takes weight. It is a sideways fault, not a forward one, and it shows up on the stance leg that is weaker at holding the pelvis still, which is not always the leg that feels weaker to the athlete.
Why it costs speed and not just looks
A pelvis that tips sideways changes the angle the leg is driving through underneath it, so some of what should be forward push is spent instead on controlling a wobble. It also tends to change exactly where the foot lands relative to the hips on that side, which can feed a small overstride that has nothing to do with reaching and everything to do with the hip that let go a moment earlier.
Where it is visible, and where it is not
- Visible from directly behind or directly in front, at the instant of single-leg stance, watching a line drawn hip to hip.
- Invisible from the side, because a side-on view reads forward lean, the strike point relative to the hips, and knee angles, and none of those change when the fault is sideways rather than forward and back.
What StatLap measures, and what it does not
The sprint event reports trunk lean, elbow angle, overstride, and knee fold and extension, every one of them read from a side-on clip and every one of them a forward-and-back measurement. A hip dropping sideways is a different plane of movement entirely, and a single camera at the side of the track is pointed away from the geometry that would show it. This is not a case of the app missing something it should catch. It is filming from the wrong angle for this particular question.
What to do about it
Film once from directly behind, at an easy jog and again near full speed, and watch the belt line rather than the feet. Off the track, single-leg balance holds with the eyes closed usually reveal which side actually gives out first. Side-lying hip abduction and single-leg glute bridges build the muscle that is not doing its job, and single-leg step-downs off a low box, watched in a mirror, are the closest thing to training the fault in the position it actually happens in.
A side-on camera is the right tool for the faults it was built to read and the wrong one for this. If a hip drop is what you are chasing, point the phone at the athlete's back, and treat a sprint session's numbers from that same clip as answering a different question entirely.
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Related
Why does your sprinter's foot turn out when they run?
A side-on camera cannot see which way a foot points, and from where most coaches film, neither can you.
Read →Why does your high jumper's plant foot never land in the same spot?
A side-on camera reads the vertical arc of a jump well and has no idea where the plant foot actually landed on the ground, which is usually the real question.
Read →Why your high jumper knocks the bar off on the way down
Legs taking the bar after the hips have cleared is a timing fault at the top, and usually a takeoff point that drifted before it.
Read →Try it: Sprint timer app: 40-yard dash and flying sprints with one phone