Lameness is the commonest problem in horses. The honest test is simple — a straight trot on a hard surface — but interpreting what you see takes a little method: watch the head for a front leg, the hips for a hind leg, and let the sound side teach you what the lame side is doing wrong.
Set up a fair test
Choose a flat, hard, even surface — concrete or tarmac, not deep arena footing, which masks mild lameness — and a handler who can trot the horse on a loose rein at a steady, unhurried pace. A tight rein or a handler hauling the head to one side invents asymmetry that is not lameness. Trot the horse away from you and back, at least twenty or thirty strides each way, and repeat it; subtle lameness may only show once the horse is moving freely. Standing the horse square first and looking at how he rests his feet is part of the test: a horse who persistently points one front toe forward or rests one hind is already telling you something before he moves.
Reading a front-leg lameness
The head is the tell. As a sound horse trots, the head stays fairly level, bobbing slightly and evenly. When one front leg hurts, the horse throws his head up as the sore leg lands, lifting weight off it, and lets the head drop lower as the sound leg takes the stride — 'down on sound' is the yard shorthand, and the leg opposite the biggest downward nod is your suspect. Watch several passes and commit to what you see before you start second-guessing: the nod is easier to spot watching the horse come towards you. Front lameness also often shortens the stride on the sore side and can make the horse land noticeably harder — you may hear the unevenness before you see it on a hard yard.
Reading a hind-leg lameness
From behind, compare the hips. Both should rise and fall evenly; with a lame hind leg, the hip on the sore side hikes higher as that leg swings and drops, because the horse is snatching the leg through and avoiding pushing off it. The stride on that side shortens, the toe may drag or wear unevenly, and the horse may swing the leg in an arc or place it wide. Hind lameness is easier to miss than front, and circles help expose it: trot a large circle each way, and a hind problem usually worsens with the sore leg on the inside. Watch too for the horse who looks lame in front but is compensating behind, and the one who is bilaterally sore and shows only a general shortness and stiffness — no nod, no hike, just a horse who is not striding out anywhere.
Hands-on checks at the stable
Before concluding anything, pick out all four feet and inspect them: a stone wedged in a shoe, a thorn, or a hot, tender sole area explains many sudden lamenesses, and a bounding pulse in one fetlock points you at that foot. Run your hands down both front legs and both hind legs comparing heat and swelling side to side, flex nothing forcefully, and note exactly where any reaction comes.
Rest, monitor, or call the vet
A horse who will not put weight on a leg at all, who has obvious swelling, a wound near a joint, or heat and a strong pulse in one foot needs a vet the same day — do not trot him up repeatedly to be sure. A mild, first-time shortness with no heat or swelling can reasonably be met with box rest, cold hosing if there is any warmth, and a fresh look in a day or two; no improvement, or improvement that vanishes when work resumes, earns a vet visit and a proper workup. Save your trot-up clips and notes in KinsLe's Pet Passport each time you check him, and the vet inherits a film of the lameness as it was, not as it happens to look on the day.