I was a harness racing groom for five years: as others have said blinders help horses focus on the direction ahead rather than anything else. That includes the driver, as some horses will take the opportunity to kick if they see the driver raise/move the whip, or if they see another horse coming up behind/along side them.
There are also ear hoods and ear plugs that can be used to reduce audible stimulation, pop out plugs being strategically popped out to give a flood of noise when needed to get a brief extra boost of speed.
Latest Answers