Next time you sit, practise this drill and see if you can avoid getting stiff while you are not moving your skeleton muscles. There is a opening to your self mastery here.
When we inhale the flat horizontal muscle which inflates your lungs contracts downwards, moves down and creates a wave of fullness in the torso which also affects the movement of the hips and shoulders and neck. The same happens in reverse during the exhale. Use this to relax your muscles and align without tension by directing the pressure/fullness toward the location where you feel your muscles are holding an uncomfortable alignment. Fore example inhale and direct the wave toward your lower back and move it lightly to relax into a more aligned and relaxed hips and spine. This in turn will help you move more naturally and survive strikes by letting them move through you. Practise and find where you need to do this and remember that as long as you breathe you can move.