the length of the seat should allow about one hand's width both behind the rider's seat and in front of the pelvis. A saddle that is too small is uncomfortable to ride in and does not allow the rider the security provided by sitting deep in the saddle. A saddle that is too big does not provide any support for the rider and allows the seat to slide around too much. However, flap size and length of the rider's thigh bone also influence the length of seat needed. Twist or Waist: the narrowest part of the saddle needs to fit the rider's pelvic structure so that the seat bones properly support the rider. This varies by age, weight and gender. If either too wide or too narrow, considerable discomfort may result. Some saddle twists are designed more for the pelvic structure of men than for women and thus may be uncomfortable for the other gender. Saddle flap: with the stirrups at the appropriate riding length (which will differ according to experience and riding discipline), the knee should not come too far behind the flap (so the flaps do not provide any support), or too far in front (which will force the lower leg back and severely disrupt the balance of the rider. Position of the stirrup bars: The bars must be properly balanced under the saddle so that the rider is not put off balance when rising in the stirrups (such as when posting or jumping) Riders also should check that the stirrup bars are properly recessed and do not stick out in such a way that the buckles of the stirrup leathers will cause bruising or rubs on their legs. Finally, particularly with used saddles, the rider must verify that the safety release mechanism works properly to release a stirrup leather in the event a rider falls and is tangled in the stirrups. Consequences of poor fit Long term, poor saddle fit may cause multiple back problems for the horse. It is possible for the horse's topline muscles to deteriorate, or for the horse to develop the wrong muscles. The muscles of the back just rear of the withers may atrophy, causing hollows right behind the shoulders, giving the withers the appearance of being higher and sharper. Horses may also lose muscle tone from traveling with a hollowed back, leading to increased risk of lordosis ("swayback"), kissing spines, or