"My horse people"

In one of my gym classes the other day we were doing an exercise aimed at the inner thigh muscles, the adductors. Half the class was grimacing and complaining; the other half was confused and said they must be doing it wrong because they felt nothing. The contrast, which persisted even when controlling for age, size and overall fitness level, was striking. Our instructor supplied the ready explanation: those who weren't feeling it were, not coincidentally, the equestrians in the class, who already have phenomenally strong adductor muscles.