Physical:
* Tense: This is the most direct opposite, implying physical tightness and stress.
* Stiff: Describes a lack of flexibility or ease of movement.
* Stressed: Refers to mental or emotional tension.
Mental/Emotional:
* Anxious: Describes a state of worry and unease.
* Stressed: Refers to mental or emotional strain.
* Uptight: Suggests excessive worry or tension.
* Nervous: Indicates a feeling of apprehension or unease.
* Agitated: Describes a state of restlessness and unease.
Behavioral:
* Energetic: Describes a high level of activity and enthusiasm.
* Busy: Indicates being occupied with tasks or activities.
* Focused: Describes a state of intense concentration.
* Alert: Suggests being watchful and responsive.
The best opposite word will depend on the specific situation and what aspect of "relaxed" you want to contrast.