Coaches obsess over reaction time. It’s the number on the scoreboard, the first thing announced after the start. But swim block time — how a swimmer moves once the signal fires — usually matters more for actual race outcome.
Reaction time only measures how fast a swimmer responds to the beep. It says nothing about the quality of the movement that follows. Two swimmers can react in the same 0.65 seconds and leave the block with wildly different speed, angle, and distance covered before they even touch water.
What block time actually captures
Block time covers the full push-off phase: the drive off the wall, the flight through the air, and the transition into the water. A swimmer with a slower reaction but a powerful, efficient block can easily beat a fast-reacting swimmer who leaves the block flat or weak.
This is why coaches who only watch the reaction number on the timing system are missing most of the picture. The start is a chain of events, and reaction is just the first link.
Entry position and underwater speed tell the real story
Where a swimmer lands in the water — the entry or landing distance — shapes everything that follows. Land too short, and a swimmer fights drag immediately. Land too far, and the underwater phase starts from a poor angle.
Underwater speed right after the start is often the highest velocity a swimmer hits in the entire race. A start that gets this phase wrong costs more time than a slow reaction ever will. This is exactly the kind of detail that’s hard to judge by eye from the pool deck, but shows up clearly on video with data overlaid frame by frame.
Why this connects to stroke efficiency later in the race
Research backs up the idea that efficiency, not raw speed of movement, tends to separate fast swimmers from the rest. A study by Craig, Skehan, Pawelczyk, and Boomer (1985), comparing U.S. Olympic Trials swimmers, found that in most events, speed gains came from covering more distance per stroke rather than stroking faster. Finalists almost always covered more distance per stroke than swimmers who were 3-7% slower in the same event.
The same principle applies at the start. A powerful, well-timed push and a clean entry set up a swimmer to cover more ground before the first stroke — the underwater equivalent of a longer, more efficient stroke.
How AIM Systems captures the full start
AIM Systems measures block time, start entry distance, and underwater speed after the start automatically, alongside reaction time. Because the fixed camera installation covers the pool from above and below water, coaches get the full sequence — not just the beep-to-movement number.
The system works with the pool’s existing Omega timing setup, and after a swim, the stitched video and full data spreadsheet are ready in about 2 minutes for a 50m race. A coach can compare a swimmer’s start against their own past best, or against a training partner, and see exactly where the time is actually won or lost.
If you want to see what your swimmers’ starts really look like beyond the reaction number, get in touch with AIM Systems — no pressure, just a look at what’s possible.
Frequently asked questions
Is reaction time a good predictor of swim start quality?
No — reaction time only measures response to the signal, not the push-off power, entry angle, or underwater speed that determine how fast a swimmer actually leaves the block.
What is block time in swimming?
Block time covers the full push-off phase, from the drive off the block through flight and entry into the water.
Can AIM Systems measure start performance automatically?
Yes — AIM Systems’ fixed camera installation automatically captures block time, entry distance, and underwater speed after the start alongside reaction time.
Sources
- Craig, A.B., Skehan, P.L., Pawelczyk, J.A., & Boomer, W.L. (1985). Velocity, stroke rate, and distance per stroke during elite swimming competition. Medicine and Science in Sports and Exercise, 17(6), 625-634. https://doi.org/10.1249/00005768-198512000-00001
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