I have 2 kids. One that is five and the other is one. My 5-year-old son has no idea how strong he is. I have bruises in places I prefer not to mention. I have been doubled over in pain, had my ribs/feet/hands crushed and been head butted. You name it, I've suffered it. My daughter, on the other hand, is small and gentle.
But last night, I pulled a muscle in a place I didn't know that I could pull a muscle.