What frame rate do you need to film a sprinter?
Ordinary video is fine for checking posture. Ground contact time and cadence need enough frames to actually catch the foot going on and off the track.
A coach films a sprinter on a normal phone video, sends the clip to the Film Room, and wonders why the ground contact number looks jumpy from rep to rep while the trunk lean number looks solid every time. The two measures are not asking the same thing of the camera, and that is why one is steady and the other is not.
Some measures need one good frame. Others need many
Trunk lean and elbow angle are read off single moments in the stride, the position of the body at foot strike or at the top of the arm swing. A normal video has plenty of frames to find a clean one of those. Ground contact time and flight time are different: they are the gap between two events that happen close together, and the camera has to have caught enough frames inside that gap to measure it honestly.
The actual numbers
- A sprinter's foot is on the ground for roughly 80 to 120 milliseconds per step at speed.
- Standard video at 30 frames a second takes a frame every 33 milliseconds, so a contact that short is covered by only 2 to 4 frames.
- Slow motion at 120 frames a second takes a frame every 8 milliseconds, which gives 10 or more frames across the same contact.
- 240 frames a second, the slow-motion setting on most modern phones, halves that gap again.
Cadence is more forgiving than either of those, because it is counted across several strides rather than timed within one, so a slightly thin sample in any single contact washes out over the whole clip.
What this changes about how you film
If a session is about ground contact, flight time or anything timed within a single stride, film it in your phone's highest slow-motion mode and accept that you will need more light for it, since slow motion trades light for frames. If the session is about posture, trunk lean or arm position, ordinary video is enough and you gain nothing by shooting slow motion you then have to sit through.
A higher frame rate fixes how finely time gets sliced. It does not fix what a single 2D camera was never going to see: a foot that lands at a slight angle to the lens can read flatter or steeper than it actually is, and whatever happens on the far side of the athlete, the trail leg, the opposite arm, never reaches the frame at any frame rate.
Time your next meet with the phones you already have
Timing, the Film Room and the speed gates are free forever, with no account. A Club plan, $71 a month, puts your meets on your own results page, and one plan covers every phone.
Related
Why does my sprinter's stride get longer but their time doesn't improve?
A longer stride on video is not the same thing as a faster stride, and the gap between the two usually shows up in ground contact time, not in the tape measure.
Read →Why is my sprinter's ground contact time longer on one side?
It might be a real limp worth checking. It might also be a question the Film Room cannot currently answer, because the number it reports is not split left from right.
Read →Can you check a sprinter's form on a treadmill with the Film Room?
Most of what the Film Room measures does not care whether the ground is moving, but the one number that needs real distance is the one thing a treadmill never gives up.
Read →Try it: Sprint timer app: 40-yard dash and flying sprints with one phone