Why you can't chase players down in the second half, even though you're not slow

Plenty of footballers know this: your 30 m time in fitness testing is pretty quick, but in the second half, when an opponent makes a run in behind you, you just can't catch them. The usual first reaction is that you're not fast enough, so you add a few more sets of sprints.

My view is that these players usually aren't short of one-off speed – what they're missing is the ability to recover after a sprint. In testing you're fully rested and run once; in a match, you sprint, catch a few breaths and have to sprint again.

One study tracked 42 professional players in competitive matches: high-intensity running in the final 15 minutes was 35% to 45% lower than in the opening 15 minutes. The 13 substitutes who came on actually covered 25% more in the final 15 minutes than those who played the full match. Fresh legs can still run; what holds you back is the fatigue that builds up sprint after sprint.

Another study had 18 top professional players do a set of repeated sprints and looked at their average time. The better-scoring half covered 235 m of sprinting in matches, the weaker half 164 m – 71 m more.

Why you can't recover after a sprint comes down to the energy in your legs: the fastest source is mostly used up within a few seconds and has to be topped up during the few dozen seconds of rest. When 8 people did two back-to-back cycling sprints, nearly half the energy in the second sprint came from the aerobic system. That was a cycling experiment, but the direction is the same: with a poor aerobic base, you drop off more on the second and third sprints.

So what should you train? Four things.
Build the aerobic base first: for the first 8 weeks, aerobic runs, aerobic intervals and air bike.
Sprint, then sprint again: 6 seconds all-out, 24 seconds walking, 6 times in a row; or a 20 m sprint every 20 seconds.
Vary the pace: interval runs of 15 seconds running, 15 seconds rest.
Every rep all-out: stop when your sprints slow down.

At what point in the second half do you usually start losing the chase? Tell me in the comments.