An Analysis of Reaction Times

This article was originally published in the 2019 Spring Issue of SwimSwam Magazine. With so much discussion on reaction times throughout the broadcast of International Swimming League (ISL) matches this season, we thought it’d be valuable to offer our readers a statistical look at the true impact reaction times have on a race.

Every swimming race starts the same way: with the sound of a horn.

Swimmers hear that and begin the race. However, they don’t all leave at the same time. Some react quicker than others. Some get off the blocks faster. We have a stat for this: reaction time.

At high-level meets, there is a pad on the block that measures how long it takes for a swimmer’s feet to leave it.

That reaction-time number isn’t just a measure of how quickly swimmers register the horn and begin moving, but of their start mechanics. A swimmer who does an old-school two-foot start will take longer to get off the blocks than one who does a modern track start.

So let’s dig into some data and see what we can learn about starts. I grabbed times data from every World Championships meet, long course and short course, since 2011.

In events off the blocks (no backstroke starts), swimmers had a median reaction time of 0.7 seconds, with a standard deviation of 0.063 seconds (19,215 data points).

Men were a tiny bit faster than women off the blocks. Men had a median reaction time of 0.69 seconds and standard deviation of 0.059 seconds (10,573 data points), and women had a median reaction time of 0.72 seconds with a standard deviation of 0.062 seconds (8,459 data points). Those small differences are statistically significant, p < 0.0001.

Sprinters were quicker off the blocks than distance swimmers. The median reaction time for women in 50s and 100s was 0.7 seconds, and reaction times got progressively slower as the distance went up. Each higher increment of distance saw swimmer getting off the blocks about 0.02 seconds slower: 200s swimmers took a median of 0.72 seconds to get off the blocks, 400s swimmers took 0.73 seconds, 800s swimmers took 0.76 seconds, and 1500s swimmers took 0.78 seconds.

The men saw a similar pattern: 50s swimmers had a median reaction time of 0.65 seconds, 100s swimmers took 0.68 seconds, 200s swimmers took 0.69 seconds, 400s swimmers took 0.71 seconds, 800s swimmers took 0.75 seconds, and 1500s swimmers took 0.74 seconds.

There are a lot of swimmers at Worlds that FINA lets swim despite their lack of a qualifying time — they’re called “universality selections.” They’re from smaller countries, and the goal is to encourage growth of the sport in those places. It’s a nice program, but the presence of those swimmers skews the data a bit, with their slower starts pulling down the averages. Removing all prelims swims from the data removes their reaction times, lowering the overall medians by a hundredth or two of a second at every distance. Table 1 has all the details.

Table 1: Reaction times by distance at World Championships since 2011

Swimmers in shorter events were better off the blocks than swimmers in longer events. This makes quite a bit of sense: The shorter the race, the more the tiny improvements to start mechanics matter to a swimmer’s final time. Also, sprint-oriented athletes tend to be able to generate more power than distance athletes, allowing them to get off the block quicker.

Elite swimmers are pretty consistent with their starts. The average change in a swimmer’s reaction time from one round to the next (prelims versus semifinals, semifinals versus finals, same swimmer, same event) was only 0.02 seconds (standard deviation 0.02).

A good or bad reaction time has only a tiny correlation with how a swimmer’s time changes from one round to the next. Changes in reaction times accounted for less than 2 percent in the variability of a swimmer’s first 50 from one round to next. The same was true in individual 50s (the 50 free, the 50 breast, and the 50 fly). The change in a swimmer’s reaction time in 50s accounted for about 2 percent of the swimmer’s time change from prelims to semis or semis to finals.

The observation that a swimmer typically gets off the block faster than their competition is useful, but the observation that they got off the block 0.05 seconds faster in finals than they did in prelims is borderline useless.

How long it takes a swimmer to leave the block after the starting horn isn’t the only reaction time reported on most meet results — there are also relay reaction times.

Men had an average relay reaction time at Worlds of 0.38 seconds, while women had an average reaction time of 0.41 seconds.

Swimmers tightened up their exchanges in later rounds of relays. In finals, men had an average reaction time of 0.36 seconds, and women had an average reaction time of 0.40 seconds. I’ll leave it to someone else to psychoanalyze why men appear to be more aggressive off the blocks on relay starts, even by a small amount, at the highest level of the sport.

Swimmers were also more aggressive on relay starts in the shorter relays. Men had an average reaction time of 0.33 seconds in the finals of 200 relays (contested only at Short Course Worlds), 0.36 seconds in 400 relays, and 0.38 seconds in 800 relays. Similarly, women had an average reaction time of 0.37 seconds in the finals of 200 relays, 0.39 seconds in 400 relays, and 0.44 seconds in 800 relays.

In the case of relays, we can go deeper. Because the relays also exist as individual events, there’s something to compare with. To do that, I took swimmers in the individual 100 free and compared their times with non-lead-off relay splits at the same meet. I used the best individual time and relay split that a swimmer put up at a meet.

Swimmers who swam the individual 100 had faster reaction times on 400 relays than swimmers who did not. Men had an average 400 relay non-lead-off reaction time of 0.24 seconds, and women averaged 0.30 seconds. Each group was about a tenth of a second better than the overall average.

Men’s times on relay splits were on average 0.61 seconds faster than their individual swims. Their reaction times were on average 0.44 seconds faster, leaving 0.17 seconds of improvement down to improved momentum from a relay start and whatever energy swimmers got from the relay atmosphere.

Women saw a smaller relay bump: They improved their times by an average of 0.44 seconds, but of that, 0.41 was from improved reaction times, meaning they improved by only 0.03 seconds from the extra start momentum and relay intangibles.

This is an odd result. Why would women benefit much less from a relay start than men? To see if it’s repeatable or the result of some other unknown confounding factor, I gathered more data, from the NCAA Championships over the past four years.

Men in the individual 100 free had an average reaction time of 0.67 seconds, and women had an average reaction time of 0.69 seconds — totals similar to what we saw in the World Championships data.

In the NCAA data, swimmers were more conservative in their reaction times than at Worlds. The men had an average relay-start reaction time of 0.51 seconds, and men who also did the individual 100 had an average reaction time of 0.40 seconds, about a tenth of a second worse than Worlds swimmers.

Women saw the same trend. They had an average relay-start reaction time of 0.50 seconds, and swimmers who also did the individual 100 had an average reaction time of 0.39 seconds.

Despite the worse relay reaction times, NCAA women saw the same improvement in their relay splits versus individual events as Worlds women. NCAA women’s 100 free relay splits were an average of 0.46 seconds faster than their individual events, and 0.30 seconds of that was from improved reaction time.

Men’s relay times were 0.44 seconds faster than their individual swims, and 0.28 of that came from reaction time.

Unlike in the Worlds data, NCAA men and women got very similar benefits from relay starts. I’m not sure what the cause of the differences in the data is. I assume the reaction-time gap between the relay starts and flat starts couldn’t be caused by an equipment difference, because the reaction times in the individual events were almost identical.

The bottom line seems to be that the extra momentum from the arm swing and step on a relay start is worth about 0.15 to 0.17 seconds, but the biggest reason by far that relay splits are better than individual swims is the improved reaction time off the blocks.