Most people's arms are extremely tense and they don't even realise it. Why? Why are people's arms tense?
Your hand is the primary tool employed by the mind for the exploration of reality. You touch, you hold, you press, you pull, you manipulate most things using your hands.
Jacob Bronowski wrote "The hand is the cutting edge of the mind." There's your answer. A tense arm is the by-product of a mind that is not in any way relaxed and at ease.