Stiffness is the lack of freedom, ease in actions, in manifestation, in the expression of something (movements, thoughts). As a rule, stiffness is associated either with fears or with muscle clamps (muscle enslavement).
If this is situational stiffness, then you need to deal with the situation: most often this happens when a person does not know how to act or is afraid to act, afraid to make a mistake. If this is your child’s stiffness, talk to him and give him simple, understandable instructions on how to proceed. Show him what to do, do it several times with him, cheer him up — the stiffness will gradually go away.
Stiffness is also another, general stiffness of movements in the whole body — always. Restricted movements, constrained gait, inexpressive facial expressions. What to do? Develop mobility, train relaxation, learn free movements. In most cases, it makes no sense to figure out “where it comes from”: either the child was frightened in childhood, and he froze like that, or they simply didn’t play sports with him, and he was used to walking stooped and stiff. Stiffness will quickly go away from simple exercises aimed at releasing muscles: do facial gymnastics, practice any stretching of the arms and legs (twine, yoga, Pilates), alternate soft smooth and explosive movements, run with acceleration. Dancing and martial arts classes are very useful: it is clear that a shackled person will not demonstrate high achievements, but the task is something else — to remove the stiffness, and the stiffness will begin to go away. In dances and martial arts, it is important to pay attention to the development of biting movements, when an arm or leg flies out in a state of complete relaxation, as if shaking off water abruptly. The alternation of relaxation and tension helps to overcome muscle stiffness, movements will become more free every day.
If the stiffness seems to be more of a psychological nature, the most effective is reliving the problem situation, positive anchoring and working with submodalities — all tools to help remove the block and negative anchoring that happened in the past, replace with a neutral or positive vision.