Some horses are losing muscle mass during winter, what kind of exercices do you do in order to get them fit again?

If you can still get out on trails where you are then doing some trotting and walking intervals on the trails are nice. If you are stuck with arena work then trot poles and cavaletti are a great tool to use. Backing up is also good to do as well. Just make sure you do it on the ground in hand as well as in the saddle as all to often when you back up in the saddle your horse puts their head up rather than leaving it down and this can hollow out the top line and not engage the hind properly. Stretches also help.
I would suggest to do cavaletti training and regular exercices for improving fitness/stamina like hacking out / hill training.
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