The reference of "thick with saddle muscles" is a crude description of the shape and strength of the women's thighs from frequent horseback riding. While astride a horse, both Western and English contemporary riding techniques require the rider to use their thighs to stabilize the body to the horse's movements. Personally, I wouldn't write "thick saddle muscles" or "thick with saddle muscles" as a description; however, "thick with saddle muscles" is grammatically correct.