Most people think of the recognizable muscles (such as biceps) in strength training. The superficial muscles are closer to the skin surface and clearly visible. These muscles, however, serve more for "gross motor movements". Those who train martial arts should not neglect the deep muscles.
Deep muscles have a very important role and stabilize our spine, pelvis and trunk. Especially for martial arts this muscle has a very important function. Unfortunately, one cannot consciously use and control deep muscles; the contraction takes place through reflexes. Normal strength training therefore does not address these muscles properly.
How do I exercise deep muscles correctly?
The best way to use these deep muscles is to train on a shaky surface. This puts the body in an unstable state. As an example, you can use a large gymnastics ball that you can sit on. Sitting on this gymnastic ball puts the body in an unstable position. The body must now constantly stabilize to keep the balance. This is done by small balancing movements, which are controlled reflexively. These balancing movements train the deep muscles and stabilize joints and spine.
Static exercises at the beginning
You should not start by choosing the most difficult exercise, but by starting with a static exercise. Balance exercises on one foot are a good start. As a small increase you can use the second free leg by moving it slowly (forward and backward, left and right).
Plank-training or push-up positions with a longer hold are very well suited as further increases and as well with this kind movements or simple exercises can be used as additional increases (push-ups, squats etc.).
Gymnastics balls are a very good training tool if you already have some practice. Sit on the gymnastic ball and train simple exercises: Bending the torso, stretching one leg, etc.
Other examples of unstable surfaces: balance boards, sling trainers, surfboards on the water, etc. …