Can a phone measure a sprinter's reaction time off the gun?
Yes, but not automatically: Race Replay times the gap to the frame you point at, not the frame it finds itself.
A coach asks this because the stopwatch never could. A hand timer reacts to the gun and then reacts again at the finish, so any reaction number off a stopwatch is really two human reaction times stacked on top of the real one. A phone removes the timer from that loop entirely, which is the whole case for using one. Whether it can isolate reaction time specifically, apart from the rest of the race, is a different question, and the honest answer is yes, with a person still doing part of the job.
What reaction time actually is
It is the gap between the gun and the athlete's first movement out of the set position, not the gap between the gun and the first step landing. Under the usual rule, anything under 0.100 seconds counts as a false start, because no nervous system reacts to a sound that fast. Everything slower than that is a real reaction, and shaving tenths off it is a real part of training a sprinter's start.
What Race Replay does with a filmed start
StatLap's Race Replay tool finds the gun two ways: it scans the clip's audio for the spike, or you scrub to the frame with the flash or smoke and mark it by hand. If the camera stood some distance from the starter, you enter that distance and the tool corrects for it, because sound only travels about 3 metres every hundredth of a second, and a camera 30 metres down the track hears the gun noticeably late.
- Scan the audio, or mark the flash frame, to fix when the gun actually fired.
- Jump to that frame, then step forward one frame at a time until the athlete visibly moves.
- Mark that frame. The gap between the two becomes the reaction time, to the thousandth.
The part that is still a human judgement
That middle step is not automatic. The tool does not scan the video for movement and flag the frame itself; a person steps through the clip and decides which frame shows the first real motion, as opposed to a sway, a blink, or the wind catching a jersey. Two people stepping through the same clip can land on frames a frame or two apart, which at typical frame rates is a few hundredths of a second. What the tool removes is the sound-delay error and the stopwatch-reaction error. What it does not remove is the judgement call about what counts as movement.
A single side-on camera can tell you when a body first moved and compute the gap to the gun precisely, once you have pointed at the right frame. It cannot tell you which part of the athlete moved first, a hand lifting off the track or a hip starting to rise, without a second angle, and it is not a certified measurement for any rule book. It is a training number, read the same honest way every time, which is what makes it worth watching get faster across a season.
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 keep false starting?
It is rarely a speed problem out of the blocks, it is a guess in the tenth of a second before the gun even lands.
Read →How do you know if your sprinter is anticipating the gun?
World Athletics calls a false start under 100 milliseconds for a reason, and a slow-motion clip will tell you roughly where your sprinter actually sits against it.
Read →How accurate is phone timing, really?
An honest answer, including where it is worse than a hand time and why that still beats a stopwatch.
Read →Try it: Sprint timer app: 40-yard dash and flying sprints with one phone