Transversus Abdominis (TVA)
Your Transversus Abdominis (TVA) these are seated quite deep in the abdominal muscle groups, located underneath the rectus abdominis (your six pack muscles to you and I) and attach from the lower ribs to your pelvis. It is therefore a key part of stabilising your spine through many movements and activities. Training how to engage these muscles then allows a better mind to muscle connection/recruitment which means then you don't have to "activate" the muscle.
Why are the transverse abdominis so important?
The TVA's main function is to activate the core musculature and control stability of the pelvis and low back prior to movement of the body. It can often be referred to as your own internal corset or girdle. A weak TVA can often result in the rider suffering from lower back pain. By improving your TVA it will provide a strong foundation for any movement:
Think of absorbing that sitting trot without collapsing
Your head will therefore be still, no head bobbing.
Your seat is secure and deep so no bouncing on your horse's back.
It's supports a range of pelvic motions, example is the canter pace.
Rotation of your upper body, during a show jumping round.
I often describe activating the TVA by bracing your core, one analogy I use is to imagine I was to poke you in the stomach, also think of how your core acts as you sneeze, this is bracing. This action is "activating" that corset.
Some people think that pulling in the stomach, sucking back is how to engage the core, this is incorrect. This just moves the pressure upwards or downwards (think of squeezing a tube of toothpaste). You need to learn and feel how to brace the corset thus tightening those TVAs.
Correct engagement of the TVA will feel like an upward and inward pull.
So as you can see this muscle has a large part to play in helping us be stable and secure especially when our horses can often be unpredictable. Through training your TVA and core in general you transfer momentum through muscles efficiently and less through joints and your back, this can then help prevent injuries.