It seems like when I punch and need to punch with knuckles first, I have to angle down, otherwise, it hits the bottom of fingers first and not the knuckles, when practicing against heavy bag.
Check the angle of pictures below,
is this true for you all? Or how do you deal with the angle of knuckles and first contact? See how it hits the green line below.
Stock photos above (not of me, which show what I mean) :
It is normal for the middle knuckles of your fingers to lead your first knuckles (top knuckles). This protrusion of the middle knuckles is exaggerated when wearing gloves. Your middle knuckles will be pushed back in towards your palm upon impact whilst your first knuckles will move very little, providing your wrist is firm and straight.
Do not compensate by tilting your wrist, or you will be at far greater risk of wrist injury and much of the potential power of your strike may be lost by your wrist buckling upon impact.
Concentrate on aligning the bones which run along the top of your hand - from the knuckles of your first and second fingers to your wrist - with the bones of your forearm. Focus on maintaining this alignment at the point of impact and on directing your power through the top knuckle of your second finger alone, or in combination with the top knuckle of your first finger. The sharper the tip of the spear, the more easily it will penetrate the body.
Remember that in a fight, there are many target areas of a body which protrude, unlike a heavy bag's relatively flat surface. This, along with punch angle, means it is sometimes possible to land cleanly with your first knuckles alone, but it is otherwise normal for your middle knuckles to reach the target first.
You are right, the fist on moment of contact must be very slightly angled downward, just to the point to make sure that upper knuckles are making contact first. This is good, also, because it provides a greater contraction of forearm muscles on impact.
