Reading the lactate curve: VLa2, V4 and the rightward shift

Plot lactate against speed from a step test and you get a curve that stays flat at low speeds and then climbs steeply. Two points on that curve are especially useful.

VLa2: the aerobic threshold

The speed at which lactate reaches 2 mmol/L. Below it, work is mostly aerobic and can be kept up for a long time. Most base conditioning sits here.

V4: the onset of blood lactate accumulation

The speed at 4 mmol/L. Above it, lactate builds faster than the body can clear it. In racehorses and eventers, V4 is one of the most widely used field measures of aerobic fitness.

Example

Say a young eventer has a V4 of 520 m/min in March. After twelve weeks of steady conditioning, the repeat test shows 575 m/min. The horse now works at a higher speed before lactate builds up: a clear, measurable gain.

When results fall

A drop in V4 is not a diagnosis, but it's worth looking into. Check for lameness, respiratory disease, a change in bodyweight, or a heavy competition schedule.