As is being mentioned above, this is common in programs where one trainer is riding many horses. It is how they maintain a training program that size.
Eventing doesn’t allow anyone other than the competitor to ride the horse in competition, but almost every other discipline does allow it. So in eventing, having others warm up the horse is just at home, whereas with other disciplines it’s common in all locations. Owners know their warm-up riders as well as their trainer.
Using other riders to maintain the part of the program that isn’t as sensitive to the ride is, frankly, key to maintaining an income level from having horses boarding and training with them that makes horse training worth doing at all. It is not an easy way to make a living.
Riding is a physical effort. The human body can only do so much in a day, day after day. Especially after years and years of work as a pro trainer.
Many pro trainers in many disciplines have staff or working students warming up the horses that the ‘name’ trainer will ride after warm-up.
The trainer is the most valuable rider, in the difference that they make in the horse. But, it isn’t that important that they do the warm-up. They can teach competent riders how to get the horse ready for a short focused session with the one truly gifted rider in the barn, the pro.
There are trainers who spend only 15, maybe 20 minutes, per horse, for 10 +/- horses per day. But after that 15 minutes, the horse rides like a different animal. The effect is more significant than the other riders and it lasts longer, especially when this happens four times a week or so.
There are trainers who are riding three+ horses per hour for some number of hours per day, on most of their working days at home. And teaching some advanced lessons as well. There is one I’m aware of who will ride 10 horses and teach 5 lessons/coaching-sessions on a normal day, from oh-dark-thirty to way-past-dark. Fifteen minutes per horse, 30 minutes per human lesson, figure the math on the trainer’s day with only brief breaks.
And how much physical effort that is, several days per week. The more horses in training they have in the barn, the greater their earnings, but also the more physical work and coordination is involved. While showing positive results.
It is rare that an owner doesn’t know that this is the routine for their horse. An owner shouldn’t be surprised at this routine, while their horse is in a training barn. Any discipline.
And the ‘name’ trainer may be juggling the competition schedule that has them away from the home barn for periods of time. They have pressure to maintain their competition record to maintain their level of business. Plus just being out and being seen, keeps them relevant to their base of potential customers.
If clients are not satisfied with the progress their horse is making, not satisfied with the end result when they get the horse back, then depending on where they live, yes there are other trainers to choose with a good reputation. Plus the higher-end clients can send their horses to trainers in other parts of the country. Thus expanding the world of competitors, for each trainer.
IMO it is profoundly sexist to judge a female trainer more harshly than the many generations of male trainers who do the same thing. Female trainers who are also incorporating a pregnancy into their realm. While staying relevant to their customer base. The male riders also ‘have’ children but they don’t deal in the same way with the impact of pregnancy, childbirth, and frankly baby and child care later.
Figuring out how to make their whole life work out for them is how female trainers who want a family manage to maintain their riding careers. As is the case in any career field for any woman who also wants her own family, in or out of a barn.