To gain strength or vigor; to become invigorated or encouraged. Similar in meaning to 力がつく (to gain strength).
入院していたが、リハビリのおかげでだんだん力づいてきた。
He was hospitalized, but thanks to rehabilitation he gradually regained his strength.
毎朝ジョギングを続けたら、体力がつき力づいた気がする。
After continuing to jog every morning, I feel like I've grown stronger and more energetic.
若手選手は試合経験を積むことで次第に力づいていった。
The young player gradually gained strength as he accumulated match experience.