Final answer:
A common physical sign of stress buildup is muscle strain in the back, neck, and shoulders, as the body's fight-or-flight response to stress often causes muscle tension.
Step-by-step explanation:
One common physical warning sign associated with stress buildup is muscle strain in the back, neck, and shoulders. When the body is exposed to stress, it responds with a 'fight-or-flight' reaction, which, among other physiological reactions, causes muscle tension. If stress is prolonged or chronic, this can lead to muscle strain, which is one of the many potential indicators that the body is under stress and trying to adapt to the pressure it is facing.