This is the 10188th most frequent Korean word.
관절에
"관절에" means "in the joint" or "to the joint."
The term "관절에" in this sentence refers to the joints, indicating the part of the body that shouldn't experience strain due to exercise.
관절에 무리가 가지 않게 운동을 해야 합니다.
You should exercise in a way that does not strain your joints.
Here, "관절에" refers to the location where the stiffness sensation is felt, which is the joints.
아침엔 관절에 뻐근함을 느꼈어요.
I felt stiffness in my joints in the morning.
"관절에" in this case specifies the area (joints) where problems might occur due to aging.