Polos just protect the outside of the legs. They do not support the tendons or joints much at all. I do not use any kind of protective boots on our horses for lunging, trail riding, or jumping. My daughter uses splint or galloping boots on her horse, but just for the look of matching boots & saddle pad, lol.
Even "sport boots" like SMBs aren't all that "supportive," especially considering the cost.
The best way to protect her joints and legs is to condition her slowly and do not over-do lunging or jumping. I do not like to lunge for more than 20 minutes total, 2-3 days a week, 4 days max. I also don't do much cantering on the lunge line, mostly walk/trot work.