sitzend
sitting
In a position where the buttocks rest on a surface such as a chair or the floor.
Er verbrachte den ganzen Tag sitzend am Schreibtisch.
He spent the whole day sitting at his desk.

sitting
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sitting
In a position where the buttocks rest on a surface such as a chair or the floor.
Er verbrachte den ganzen Tag sitzend am Schreibtisch.
He spent the whole day sitting at his desk.

'Sitzend' (sitting) is the present participle of the verb 'sitzen' (to sit). This form describes an ongoing action or a state and is used like an adjective.
Der sitzende Gast wartete auf seine Bestellung.
The seated guest waited for his order.
The word can stand as an adjective before a noun or as an adverb describing an activity. As an adverb, it describes how an action is performed.
Die sitzende Katze beobachtete die Vögel.
The sitting cat watched the birds.
In a professional context, the combination 'eine sitzende Tätigkeit' (a sedentary job/activity) is very common. It describes a job that mainly takes place at a desk or a similar workstation.
Viele Büroangestellte haben eine sitzende Tätigkeit.
Many office workers have a sedentary job.
standing
'stehend' and 'sitzend' both describe body positions. 'stehend' means that someone is on their feet and is not sitting or lying down. 'sitzend' means that someone is sitting on a chair, a bench, or the floor. The words are opposites, but they can be confused because both often appear in descriptions of people, courses, or events. A practical rule: if the feet are supporting the person, the person is 'stehend'; if the buttocks are resting on a surface, the person is 'sitzend'.

Nach dem langen Spaziergang trinkt sie sitzend ein Glas Wasser.
After the long walk, she drinks a glass of water while sitting down.
Bei dem Konzert mussten viele Besucher stehend warten.
At the concert, many visitors had to wait standing up.