I am asking how to develop strong punches, kicks, knee strikes and elbow strikes. I have attempted training my biceps, chest and back, but I feel as if my striking force is the same as before...
First, let's cover some basic context.
Coordination of muscle is key
It helps to have muscle, but... the real key is coordinating your muscles to work together to generate power. It's the same reason power lifters, who can undeniably lift and move great weight, don't make good baseball pitchers - the pitcher's ability is about coordination to generate power, not just muscle strength. Striking power is the same way.
Damage comes from good shots at places that count
Boxers often say, "It's the punch you're not ready for that knocks you out." If the person isn't protecting one of the better places for a knock out blow, it's quite possible they'll go down quickly - and most striking arts spend quite a bit of training and focus into trying to find ways to get to those spots.
In streetfights, the majority of damage comes in once someone hits the ground - stomping and downward blows put the full bodyweight behind the strikes and the target is unable to give any with the strikes- they are pressed into the ground so the full force hits them.
Many ways to get striking power
Different arts use different ways to get power. The most common is twisting the hips and pivoting on a foot - the boxer's cross, Muay Thai's roundhouse, many movements use this. The twisting moves your center of gravity and your striking limb gets the advantage of your weight transfer going into the target.
Some arts use a springing forward action to drive a thrusting attack - fencers, boxers with a jab, etc. Some add a small straightening of the back to get the torso muscles added. Many styles push off the back leg and let that small forward momentum provide the power.
Some styles use gravity - you strike as you step forward or to the side, "falling" into your strike. Your bodyweight falling is what powers the strike. Jack Dempsey wrote about it, and it shows up in a variety of fighting styles as well.
There's also styles that use things like specific muscle coordination, tightening core muscles, hard breathing, spiraling motion to get a little extra muscle power, and so on. When you dig into a variety of sports, you also find overlap with some of these techniques as well.
Contextual to what you're doing
So all these different methods? Generally are contextual to the type of fighting you're training for. The power generation of boxing has overlap, but differences with Muay Thai, which is why you see differences in both guards and foot placement. Arts that focus on using weapons or the possibility of weapons also have different guards and ways of dealing with things.
Whatever you're training, start with getting your form and delivery of your attacks better. Find out when and where you're supposed to be shifting weight, when and where you tense up, and when your strike is supposed to land in ALL of that. Practice the hell out of that timing to get the most force. It can help to deliberately play with the timing a bit - try doing things a little too early, a little later, just so you can find where the actual "perfect" spot is in that timing chain for you.
Then find out from your instructors/coaches if there are better situations for WHEN to deliver this strike on a target. "If you catch them mid-step, perfect!", "This is a great counter punch", etc. Then set up drills to get that down.
From a physics perspective, one way to think about striking is impulse:
To have an effective strike, you want to:
- Maximize mass. The normal advice is use your whole body to strike. This means that you do not want to punch simply by using your arm. Most styles deliver more mass behind a strike by rotating the hips. You can also step while striking. It helps to be more massive too, but obviously you do not have as much control over changing your body mass as moving it effectively.
- Maximize velocity. The simplest way to increase velocity is to swing fast. If you are rotating about your central axis with constant angular speed, you get more velocity with a striking limb or weapon that is farther way from your central axis. You can also whip your body, which generates high velocity at the tip of the whip, but whipping is harder to learn. You can add stepping to increase velocity.
- Minimize time of force. This is what makes contact a strike and not a push. Deliver the maximum force in the minimum amount of time. If you do bag work, you want your strikes to deliver all of their momentum immediately upon contact and not with a push after contact. When striking, you want all of your mass to arrive at the same time. This is hard; for a punch, you have to coordinate your feet, hips, shoulders, arm, and hand so a strike hurts your target, but not you.
You have to work out for yourself what motions best combine these elements for your body. Your hook punch may have maximum velocity, but it may be hard to get your mass behind it effectively.
Remember that power or impulse is not everything. You can maximize striking power with an American football tackle, which uses your full body mass at running speed to make contact with all of your mass at the same time, but this is very easy to counter.
A second factor is energy dissipation from an inelastic collision. The energy from a strike, E=mv^2, must be accounted for in the movement of the target and damage internal to the target. With this consideration, velocity is more important than mass.