You need to work on your movements. I would suggest do a bunch of animations of just one guy runngin, jumping, kicking, etc for a while until you get them down a bit better. It helped me a lot.
Im going to point out one major thing with the running. When you run, it is actually a series of jumps. you hit the ground with one foot and have the next foot follow past the first one. The second foot then gives the momentum for a mini leap, which entails both feet being off of the ground for a short time. This shoud also be the time where the arms are at their fullest distance from the body. Now, im no expert at running, but im sure the more experienced animators here would say the same thing.
The next thing is easing/anticipation. Easing is the slowing down after a movement, and anticipation is the slow period just before the next one. If this doesn't make sense, there are multiple tuts here on it. Anticipation can also dramatically increase the force (the "strength" as sunnygo put it).
Overall, its decent for a first. Its kinda funny that I made the exact same mistakes on my first animation lol. But keep goin at it!
