Its the best blend of both strength and technique really along with speed. Technique has to be learned and developed first though and many force or rush that process and start trying to add speed, strength and power into things too soon. Doing that means they never really develop their technique to the highest level then in a lot of cases and instead its something less than their fullest potential. I do a lot of work on this aspect that your question touches on with fighters for the above reasons.
Also the strength we are talking about is not the same that most develop or train to develop. Many focus on shoulders and biceps and only one of those is the correct one while the other will often times slow your punches down and sapp their power if you are developing them by lifting weights. I can curl as much as my heavier bodybuilding and weightlifting buddies but they truly put me to shame on the incline press or even the bench press. I don't have bulging biceps either but have good strength there as well as good tendon strength in my arms, forearms and wrists as well as my hands. Most fighters that have exceptional power have that and a strong grip. That is why some fighters focus on developing their hand and tendon strength more so.
The other thing about your question is using synergy to add power to your punches. That's when you use muscles other than those involved directly in punching to add power to your punches. Hip action or letting your heels turn when you throw some punches for instance, arching your back when you throw an uppercut, using counter rotation when you duck under and come back up at an angle to your opponent are examples of synergistic movement also that many don't understand or use. Learning these things usually takes formal instruction from a good coach or instructor.
Here is an experiment for you to try and stand sideways to the heavy bag and swing your arm outward hitting it with a chop or a back fist. Now try it again but this time have your body positioned so that you also turn your hip slightly into the motion when hitting the bag. Don't stop there now. Next stand somewhat sideways to the bag again and this time turn your hip away at the last possible fraction of a second on hitting it and you will note that also is stronger than just doing nothing and standing there and striking the bag. This is an example of what I am saying above and so learning about this and how to add that into your technique is critical along with developing your technique to the highest level possible.
See if you can't find someone with some knowledge and skill that can give you some pointers and help you out with this as your question is really pretty general and nondescriptive. It really does not identify what problems you are having but a person who is knowledgeable and skilled can pinpoint problems pretty quickly once they see you doing things and give you some pointers.
Sometimes its not even anything like this but instead something else as simple as what direction your back foot is pointing when you throw things especially with your rear hand. That could be the problem in itself to a large extent so repost with more details or find someone who can help you and give you some pointers.
Strong knuckles do no make you punch harder. It just protects your knuckles so you do not hurt them when you do punch harder. Being able to punch hard can be done different ways. The easiest and the way most people do it will be with strength. It is easy to lift a bunch of weights and muscle your way through it. The other and better way is learning good technique and that takes time and countless repetitions but not a lot of strength. For the first option you lift weights and wail away on a punching bag like most people. It is not martial arts though or at least I would not consider it. For the second option you find a teacher and learn proper stances and footwork, timing, speed and how to generate power without strength and using a lot of your energy.
Make sure you standing at the right place. So your punch doesn't just touch someone, it penetrates their flesh hard. Try to hold them still or trap them in some ways. Rotate at least the hips of your body. Make sure you come into them with plenty of motion, so it's not like a fly or weak snap punch. It depends on whether you want to grapple. If grappling isn't your concern, retract your punch. You can strengthen your knuckles for safety. Though do it under supervision and consult your doctor for any bone issues. Muscles.....if your muscles aren't coordinated, or partially utilized, it does more good at decreasing your power than increasing it.
The best way to improve punching is to punch correctly every time. That means you must do some slowly and exactly as it is supposed to be done. People often say practice makes perfect. Wrong! Practice itself is no good if you do the movements a little off. You only get better at doing it a little off. Learning the punch correctly and making sure you do it that way every time will improve your punching quicker than doing a lot of inconsistent punches. Good punches are a product of how strong you are. some of the hardest punches are thrown by very small Asians. They can often punch harder and faster than bigger stronger people. Their punch relies on proper technique and years of training, not strength.
For the best results, you'd have to use a combination of strength and technique training. To quote from a blog entry I wrote a few weeks back-
"Merriam-Webster defines strength as 'the quality or state of being strong', strong being 'having the power to move heavy weights or perform other physically demanding tasks.' This is perhaps a bit simplistic, but it covers the general concept well enough. But where does this strength come from?
The four 'Main Sources' that the ISSA teaches strength comes from are as such-
Body Mechanics
Metabolism and Energy
Skill, Attitude, and Willpower
External factors [Weather, Gravity, etc]
All of these come together to create a person's physical ability, or strength.
... power is basically the force behind something- And as everyone should know, Force is equal to Mass times Acceleration. As such, in order to improve your power, you must either put more weight behind it or move it faster. To do this efficiently, we must first understand and apply the mechanics of the technique, then seek to maximize the potential force expressed through each motion. A punch which draws from the strength of the legs and core as well as the arms will be stronger than one that only uses the arms, so improving your technique will improve your power rapidly. Weightlifting helps by improving the strength of each of these individual muscles even further, thus allowing them to express even more strength than they would without conditioning. Thus, Performance conditioning in regards to punching- And in martial arts in general- Should consist of technique/skill training, and weight training to maximize one's speed, strength, and agility."
In the end, there are several ways to make your punch stronger- And nothing is actually stopping you from taking every single one of those methods and using it.
Use Your leg & ankle muscles,Throw Your weight into the punch.
I found it helpful to study Bruce Lee's one inch punch.
With proper technique, using your entire body, learning to relax and by striking softer.
Your body moves fort and being controlled by your body wipes out and strikes
Google how to snap your punches. A real punch feels effortless.
Technique; tighten your muscles as you land the punch.
Isometric muscle training. Look it up.
Is it muscle, is it technique or something else, do I need to strengthen my knuckles or anything like that?
