It's not a surprise, kind of like proving water is wet, but the American Journal of Sports Medicine recently published a study that concluded, "The use of prophylactic knee bracing appears to have a beneficial effect in preventing medial collateral ligament and anterior cruciate ligament injuries as well as overall knee injury occurrence. These findings may be applicable to other sports that involve similar forces and mechanics."
The study was conducted at the Sanders Clinic for Orthopaedic Surgery and Sports Medicine in Houston. The researchers define "prophylactic knee braces" as "any knee brace worn by a rider with the intent of preventing an injury to the knee.
The study examined data from 2115 motocross riders. The athletes were divided into two groups: riders that wore a motocross knee brace and non-braced riders. The non-braced group experienced a statistically significant higher rate of knee injuries when compared to the braced group. The most common injuries involved the ACL, MCL, and menisci.