Muscles need to tense in order to produce movement. Only under a full narcosis all our muscles are relaxed at the same time, otherwise some of our muscles are active, while others are inactive. When we deliberately tense specific muscles, we are usually aware of the increase in tension of this muscle. We can, however, only sense the change in tension.
When parts of muscles are habitually tense, we lose the sensitivity to notice that they are already active. Especially when parts of the outermost layer of muscles are habitually tensed, problems are most likely to occur. Tension around the spine often leads to a weaking of the innermost layers of muscles that automatically maintain our balance, which makes activity more effortful and straining.
We need tension for movement, but we need it in the right places. When our head is out of balance, parts of the neck and trunk muscles are already engaged, which interferes with any movement our arms do. Efficient movement engages the entire body, which is only possible if habitually tense muscles don't act like a break.