There is clear evidence that many horses are apparently sound in hand, even after flexion tests, and also trotting in a small circle on a soft or firm surface, and yet they show pain when they are ridden, manifest as performance problems for the riders. (Dyson & Palmer, Harmonious horsemanship, 2023)
A 2021 study of 148 horses, revealed that while “28% were lame in hand, (...) 62% were lame ridden”. Being assessed in hand or tacked up, with the weight of a rider and performing demanding movements, are two very different efforts.
Moreover, lameness is often so low level that it is hard to see for the human eye. Compensatory patterns of your horse’s mouvement can also mask the lameness. A horse may be lame on more than one limb. Limping on the two same-sided legs or in diagonal makes the horse still move symmetrically, making classical lameness assessment nearly irrelevant. This is why behavioural assessment is critical to detect pain in ridden horses.