If you have too much tension in your muscles (they are contracted), even the ones that are needed at the time, then your movement will be restricted. An easy movement from a toned yet relaxed body is desired. The problem often stems from forcing movement instead of allowing the natural forces of gravity and inertia to allow the body to swing.
The skeleton, when minimally supported by surrounding muscles, is very adept at countering gravity and supporting the weight of the body. Tensing muscles and trying to hold up the body with too much muscle usage does not allow the skeleton, whose separate parts are quite strong, to bear the brunt of the work of opposing gravity, and tires the muscles while at the same time restricting otherwise free-swinging movement, as stated above.
Think of a natural walking motion. If you walked with tense muscles even a short distance, your legs and back and everything involved would be worn out, and your walking motion would not be natural, but appear forced, as if you were consciously placing each step. Instead, the natural and efficient way is for the impetus of movement to come from a perspective of moving the body (skeleton), and allowing the legs to fall/swing under it.
