The word "clenched" means holding tightly or gripping something firmly, often refers to tightly closing or squeezing a body part or object. Full definition
You probably have seen babies fling their arms up in the air with clenched fists while bringing their knees close to their chest. (rookiemoms.com)
As much effort as it took for me (lots of clenched teeth here, too), in the end I don't feel like it had a huge impact on their sibling relationship... but I don't know since I didn't try it any other way. (hobomama.com)
Dragging me into the house, he threw me to the floor and began pummeling me with clenched fists like I was another man. (annvoskamp.com)