I spanked my son once and he said that didn't hurt, so I responded, that was a warning, you know my strength, you really want me to make it hurt? He went quiet. Never said that again.
He was throwing a fit in my arms one time and kicked and would have fell if I didn't grip his leg hard, only issue is that at 190lb grip strength I left 5 bruises on his leg, one for each finger. Daycare had to note it and have me sign it when I dropped him off so I couldn't blame them.