to strengthen
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to strengthen
To make someone or something (e.g. the body, the muscles, the economy) stronger and more resilient.
Regelmäßiges Training soll die Rückenmuskulatur kräftigen.
Regular training is meant to strengthen the back muscles.
The verb 'kräftigen' (to strengthen) is derived from the noun 'die Kraft' (strength/power). It literally means to 'make something or someone strong' or to give them strength.
Diese Übung ist gut, um die Arme zu kräftigen.
This exercise is good for strengthening the arms.
You can use 'kräftigen' transitively to make something else strong (etwas kräftigen). You can also use it reflexively when you regain your own strength (sich kräftigen).
Schwimmen kräftigt den ganzen Körper.
Swimming strengthens the whole body.
When strengthening something, typical objects are body parts such as 'die Muskeln' (the muscles), 'den Rücken' (the back), or 'das Herz' (the heart). It can also be used abstractly, for example 'das Immunsystem kräftigen' (to strengthen the immune system) or 'die Wirtschaft kräftigen' (to strengthen the economy).
Gezieltes Training kann die Bauchmuskulatur kräftigen.
Targeted training can strengthen the abdominal muscles.
to build up
'aufbauen' and 'kräftigen' can both mean that someone or something becomes stronger. The difference is what kind of strength is meant. 'Jemanden aufbauen' (to encourage someone / build someone up) is often used emotionally: you comfort or encourage a person so that they regain courage. 'kräftigen' is more suitable for physical or general strength, for example muscles, the body, or even the economy. Learners may confuse the words because both sound positive and strengthening. When it is about feelings and courage, 'aufbauen' is more natural. When it is about strength, the body, or resilience, 'kräftigen' fits better.
Greta macht jeden Morgen Übungen, um ihre Rückenmuskeln zu kräftigen.
Greta does exercises every morning to strengthen her back muscles.
Nach der schlechten Nachricht baut ihre Freundin sie mit ruhigen Worten auf.
After the bad news, her friend encourages her with calm words.