angespannt
tense
To be in a state of inner restlessness, nervousness, or tension.
Vor dem wichtigen Vorstellungsgespräch war sie sehr angespannt.
Before the important job interview, she was very tense.

tense
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tense
To be in a state of inner restlessness, nervousness, or tension.
Vor dem wichtigen Vorstellungsgespräch war sie sehr angespannt.
Before the important job interview, she was very tense.

'Angespannt' (tense) is the past participle of the separable verb 'anspannen' (to tighten / to tense). The verb 'spannen' means to pull something tight. The prefix 'an' intensifies this, and the basic idea of 'Anspannung' (tension) or 'Druck' (pressure) connects all meanings of the adjective.
Er musste die Seile für das Zelt fest anspannen.
He had to tighten the ropes for the tent firmly.
The context shows which meaning is intended. If 'angespannt' (tense) refers to a person, it means nervous. If it refers to a situation, it means critical. If it refers to a body part, it means tight or cramped.
Der Redner wirkte vor seinem Vortrag sichtlich angespannt.
The speaker seemed visibly tense before his talk.
When a situation is difficult, 'angespannt' (tense) is often combined with abstract nouns such as 'Lage' (situation), 'Situation' (situation), 'Verhältnis' (relationship), or 'Atmosphäre' (atmosphere).
Das finanzielle Verhältnis zwischen den Geschäftspartnern war angespannt.
The financial relationship between the business partners was tense.
In the physical meaning, the word is usually used with muscles or body parts that are often affected by stress, such as 'angespannte Schultern' (tense shoulders), 'ein angespannter Kiefer' (a tense jaw), or 'angespannte Muskeln' (tense muscles).
Durch die Massage lösten sich ihre angespannten Schultern.
The massage loosened her tense shoulders.
quiet / calm
'ruhig' and 'angespannt' are clear opposites when describing people. Someone who is 'ruhig' is inwardly calm and not nervous. Someone who is 'angespannt' feels pressure, restlessness, or nervousness. 'angespannt' can also describe the body, for example tense shoulders or cramped muscles. So 'ruhig' fits when someone reacts calmly. 'angespannt' fits when you can tell that someone is under pressure inwardly or physically.

Vor dem Termin ist sie angespannt und spricht kaum.
Before the appointment, she is tense and hardly speaks.
Mein Freund bleibt ruhig, weil er gut vorbereitet ist.
My friend stays calm because he is well prepared.