Personally, I wouldn't use them in the hunter ring either. I've heard so many judges talk about "tradition, tradition, tradition",,and although it's not illegal, it may rub someone wrong.
I understand classes shouldn't be won on the color of one's stirrups, but in big classes in good company, the overall impression of the horse, rider and turnout can influence the judges score.
I heard the reason they're not allowed is because it messes with the way the judge interprets where the riders leg is and they cues they are using. The traditional silver stirrup makes it easier for the judge to see, considering they're looking across the ring and very specific things.
What is the advantage of composite stirrups, aside from the fact that they are "in" right now?
I am an older rider with knee problems. The wider foot bed and the Dupont "give" to the stirrups for some strange reason allow me to continue to ride without knee pain.
The metal foot grates help with grip too. I know my regular irons with the rubber pads can be slippery. I did change my metal irons to the metal grated foot pad but the non flex of the metal stirrup hurts my knees.
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