Stifle question

[Wizpop]

Posting here in the hope that more will see.
Has anyone experience of a horse having some sort of stifle problem- possibly arthritis, who really struggled to get canter lead in school but no problem on grass?

 

[tristar]

how fit is the horse, how old and has it had any previous problems, ie. catching locking in extension

had stifle issues but not picking up lead problems