rider's muscles

rid·er's mus·cles

the adductor muscles of the thigh, which come into play especially in horseback riding.
References in periodicals archive ?
The most common hybrid vehicle of the future may not be the now familiar four-wheel automobile combining an internal combustion engine and electric motor, but the electric bicycle that can be powered either by a rider's muscles or energy stored in a battery.
As the horse moves, the rider is constantly thrown off balance requiring that the rider's muscles contract and relax in an attempt to rebalance.