|As someone who specializes in movement education and awareness for riders, I try not to get caught up in all the names of the different muscles of the body. Instead I try to focus on teaching riders the correct sensations of contraction, relaxation, tone and isometric bracing of the muscles. I try to bring awareness to the rider of the effect of muscular effort, whether conscious or unconcious, on the skeleton, posture, and movement--and on their horse.
I try to find images that will help the rider to ride well without trying too hard or thinking too much and thus twisting or contorting or stiffenting the body into undesirable postures. Every rider has experienced someone telling then to change (shoulders back!) without ever experiencing lasting results. When I have changed someone's self-image and perception they will truly find lasting improvements--because their mind and body will become more closely integrated with their actions.
However, sometimes it does help to see images of your muscles and know their function. I like this little group of muscles shown below because they can be thought of as the smile muscles of the seat. These deep little horizontal muscles at the bottom of our sacrum attach our pelvis to our thighs around the hip joint. When relaxed and working for us, they allow internal rotation and lengthening of the thigh and they allow our seat bones to widen so they are full contact with the saddle. When they smile our horses will smile too!
When contracted they bring the thighs away from the horse by externally rotating and abducting the thigh. When tensed they also work with the gluteus maximux to pop us out of the saddle (pinching your cheeks!). They get turned on when we get tense and fearful, try to hard to drive the horse by tucking our tail, or try to bring our seat bones closer together.
Allowing your seat to smile is essential to attain a deep seat. This is explained in many books, and clearly in the Principles of Riding: "The weight of the body should be distributed equally over the two halves of the seat and the inside thigh muscles. The muscles should be free from tension: any tension in the seat or thigh muscles will cause the rider to lever himself out of the deepest point of the saddle and sit 'above the horse'.
If the rider's seat is supple and free from tension, the thighs can turn inwards slightly so that the knees rest flat against the saddle." (The Principles of Riding, German National Equestrian Federation 2003).
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