Riding is a very unique sport for the human pelvis, it requires symmetry when applying aids bilaterally (with both sides) but also when conducting more lateral aids the pelvis is required to accommodate a more asymmetric position when one leg moves independently of the other.
It has been well documented within scientific literature that many riders are aware of the terms 'crookedness' or 'collapsing' at their hip. This is likely to be produced as a result of a flexibility or strength issue within the pelvic musculature, and may be more evident in novice vs experienced riders. Over a period of time this can influence laterality and asymmetry in our horses (Martin, et al., 2016).
This asymmetry is likely to be further exacerbated as the speed increases due to the forces and momentum produced by your horse along with the influence of acceleration and deceleration. When we look at rising trot, riders will to some degree instill some stability through the stirrups, no-stirrups increases postural instability to a greater degree as the rider is unable to counterbalance using the stirrups.
In a study looking at rider asymmetry, riders were treated using simple massage, techniques aimed to improve motor control (muscle control and activation) and limb specific rotational movements. Following intervention, left and right symmetry was greatly improved with a more equal pressure distribution noted on pressure sensors (Nevison & Timmis, 2013).
Physiotherapists can assist you in identifying your pelvic weakness or flexibility and prescribe a tailored, individual and discipline specific exercise programme which can improve your stability and symmetry.
A couple of exercises you can have a go at to improve your pelvic strength include the Pilates 'Clam'. You can increase its level of difficulty through adding resistance bands or varying this exercise by extending your leg and rotating your toes to the ceiling. Remember - don't let your pelvis tip backwards!
Prior to riding why not have a go at 'waking up' your gluteal muscles by pressing the outside of your knee against a wall (see the picture). Press as hard as you can against the wall but try to keep yourself straight (and don't cheat by using your arms!) - if you've got this exercise correct you will notice a slight ache in the muscle on the outside of your hip. None of these exercises should cause you any pain.