The Cost of Pacing Spikes

Two pacers can finish 10 km in exactly 60:00. One ran every kilometre at 6:00. The other went 5:30, then 6:30, then 5:30 again. On paper their average pace is identical. For the runners behind them, the two races felt completely different. This lesson shows why, using simple numbers.

Why steady is cheaper#

The effort of running does not grow in a straight line with speed. As you speed up, the strain on the body (breathing, heart rate, and especially the share of carbohydrate you burn, see Lesson 1.1) grows faster than the speed itself. Slowing down saves a little, but speeding up costs more than that saving.

So a fast kilometre followed by a slow one does not cancel out. The fast kilometre costs more extra effort than the slow one gives back. On top of that, every acceleration costs energy of its own, like a car using more fuel in stop-and-go traffic than at a steady speed.

A simple demonstration#

To make this visible, we use a deliberately simple and cautious model: the "fuel cost" of a kilometre rises with the square of speed. These are illustrative numbers, not laboratory measurements. In real runners the curve is usually even steeper near threshold.

We set the cost of one kilometre at 6:00/km (10.0 km/h) to 100.

Now compare two ways of running 10 km in 60:00:

The erratic pacer used about 2% more fuel for no gain at all. Even with this gentle model the cost is real. In practice it is larger, because:

• for some runners in a 6:00 group, 5:30/km is close to their threshold, where carbohydrate use jumps;
• each surge adds the extra cost of accelerating;
• surges raise heart rate, and it takes time to settle again.

Over a marathon, those "small" percentages add up to the difference between a strong finish and a long walk from 35 km.

Time lost and time "won back"#

Suppose your group loses 30 seconds at a crowded water station. You have two options:

• Surge: run the next kilometre in 5:30 instead of 6:00. The time is back in one kilometre, but you have pushed the whole group into a hard effort.
• Spread it: run the next six kilometres in 5:55 instead of 6:00. You get the same 30 seconds back, and the change is barely noticeable.

The Support Guide's rule is to keep corrections within about 5 s/km of plan and spread larger gaps over more kilometres (see adjusting pace smoothly).

Key takeaways#

• Effort rises faster than speed, so fast and slow kilometres do not cancel out.
• Even a cautious model shows erratic pacing costs more fuel for the same finish time; in reality the gap is larger.
• Every surge and hard braking action spends energy your runners need later.
• Recover lost time gradually, at no more than about 5 s/km, spread over several kilometres.