V4 versus lactate minimum speed in horses: what each test measures

V4 is the speed at which a fitted blood-lactate curve reaches 4 mmol/L during an incremental exercise test. It is useful for tracking the response to a repeatable protocol. It is not a direct measurement of an individual horse's maximal lactate steady state—the highest sustainable intensity at which lactate production and removal remain broadly balanced.

What an incremental V4 test tells you

In a standardized exercise test, the horse works through stages of increasing speed. Heart rate and blood lactate are recorded at each stage, and V4 is estimated from the resulting speed–lactate relationship. The 4 mmol/L cutoff is a convention, not a physiological switch that occurs at precisely the same concentration in every horse. A model's V4 depends on the observed stages and the curve-fitting method, so compare results from the same protocol and method.

What lactate minimum speed adds

A lactate minimum speed (LMS) test first raises lactate with a hard effort, then follows it with stages at increasing speeds. The lowest point of the resulting U-shaped lactate curve is the estimated LMS. It uses a different physiological question from V4: where does circulating lactate fall to a minimum during this particular protocol? LMS is an estimate, not a guaranteed stand-in for maximal lactate steady state [1].

A study of six young Standardbred mares compared a conventional incremental test and a modified LMS protocol before and after eight weeks of harness training. VLa1.5, VLa2 and VLa4 increased after training; the authors also reported a wider ‘aerobic window’ and a flatter lactate-minimum curve. The small sample, single breed and specific training program limit how far these findings can be generalized [1].

Which should a stable use?

• Use repeatable incremental tests when you need to follow the same horse's V4 and other fixed-concentration speeds over time.
• Consider LMS only with a qualified team and a protocol that can be reproduced; its initial lactate-raising effort is still demanding.
• Do not compare an LMS number directly with a V4 number as if they were interchangeable speeds.
• Ask a veterinarian or equine exercise specialist to help interpret results when the horse is returning from illness, shows poor performance or cannot safely complete intense exercise.

Neither test establishes training zones with certainty for every individual. Their strongest use is as part of a consistent record alongside clinical context, workload and heart rate—not as a stand-alone verdict on fitness.