die Anstrengung
exertion
The use of physical or mental energy that is experienced as strenuous or tiring.
Nach der langen Wanderung spürte sie die Anstrengung in ihren Beinen.
After the long hike, she felt the exertion in her legs.

effort / exertion
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exertion
The use of physical or mental energy that is experienced as strenuous or tiring.
Nach der langen Wanderung spürte sie die Anstrengung in ihren Beinen.
After the long hike, she felt the exertion in her legs.

'Die Anstrengung' (effort / exertion) is a noun derived from the verb 'sich anstrengen' (to make an effort / to exert oneself). The ending '-ung' turns the action of the verb into a feminine noun.
Das Lernen für die Prüfung erfordert viel Anstrengung.
Studying for the exam requires a lot of effort.
The meaning often depends on the number. In the singular, 'Anstrengung' usually describes general exertion, often physical. In the plural ('Anstrengungen'), it almost always means targeted attempts or measures to achieve a goal.
Allein die Anstrengung beim Treppensteigen machte ihn müde.
The exertion of climbing the stairs alone made him tired.
In the meaning of 'Bemühung' (effort), the word is very commonly used in the plural with the verb 'unternehmen' (to undertake). The fixed expression is 'Anstrengungen unternehmen' (to make efforts).
Das Unternehmen unternimmt enorme Anstrengungen, um nachhaltiger zu werden.
The company is making enormous efforts to become more sustainable.
When referring to the use of energy, 'Anstrengung' (exertion) is often used with prepositions that describe the degree. Typical phrases are 'mit großer Anstrengung' (with great effort), 'unter Anstrengung' (with exertion), or 'ohne die geringste Anstrengung' (without the slightest effort).
Er konnte das schwere Paket nur mit letzter Anstrengung heben.
He could lift the heavy package only with one last effort.
work
'die Arbeit' and 'die Anstrengung' can both describe something that takes energy and effort. 'die Arbeit' is more general: it can refer to an activity, a task, or the work involved. 'die Anstrengung' puts more emphasis on needing a lot of physical or mental energy and perhaps being tired afterward. When you are talking about the doing itself, 'Arbeit' often fits. When you want to emphasize how difficult or tiring something was, 'Anstrengung' is more precise.

Nach dem langen Lauf merkte Clara die Anstrengung in den Beinen.
After the long run, Clara felt the exertion in her legs.
Die Vorbereitung für die Prüfung war viel Arbeit.
Preparing for the exam was a lot of work.