Why can't I do push ups (lol) but I can perfectly do 13 kilos 4series 30 reps, in a tricep or bicep workout? Plus it's a compound movement meaning I'm not enfoquing everything on just one muscle?
If you have recommendations on how to begin on push ups feel free to help me please.
If you can perform 4 sets of 30 reps using a machine, it's time to
Since you can't perform at least one pushup, you can start with a knee pushup.
http://www.munsterbootcamp.com/wp-content/uploads/2015/07/knees-pushup-munster-bootcamp.jpg
Once you can perform about 5 - 10 reps with good form, you can progress to planks.
Try to hold it for about 3 - 5 seconds.
Once you're comfortable with that, switch to half pushup (not sure what it's actually called though). From your lying position, you lift yourself up into the plank position.
The next stage is lowering your body to the ground from the pushup position.
At this point, you have enough strength and ability perform one pushup. Attempt it.
This progression should help you with your push up in no time.
To become proficient at a movement requires practicing the movement. Proficiency in one movement will not necessarily transfer to others even if they seem to be using related muscle groups. Your case demonstrates this. To become proficient at pushups requires practicing pushups.
To become able to do your first pushup, try starting with incline pushups (search the web for this phrase and you'll find plenty of results), decreasing the incline as your ability improves, until you are able to do a pushup on the floor.
You're not telling us how much you weight, but pushups require you to push a quite large portion of your body weight, my own unscientific measurements with my hands on a scale in pushup position tell me ~60%.
Doing 13 kg in bicep/tricep workout doesn't really tell us anything. Different types of exercises vary a lot in how heavy they are due to the different mechanics of the exercises. I can deadlift about 160 kg, but I'm struggling with 8 kg in standing shoulder flies.
Regarding compound lifts, the fact that they use more muscles doesn't mean that the muscles share the burden, it means that the movement is more complex and requires a larger amount of work in total. It's like a chain, every link has to be strong enough, that's why they are popular.
My recommendations would be to do pushups on your knees, negative pushups (just do the lowering, first half of the movement, slowly, you are 40% stronger in the exentric phase than in the concentric so it should work better), dumbbell presses and planks.
Perhaps it could be in part because a push up is using primarily chest muscles, while apparently you are working your biceps and triceps. Maybe those are stronger for some reason for you. Also, 13 kilos(28 pounds) is chump weight(no offense). But if you are doing 30 reps of that, you should be able to lift 1-5 reps of 35 kilos, or 77 pounds, which still isn't a whole lot for a one rep max. I'd say try either eating more if you're skinny or less if you're fat, if you're medium or muscular I don't know how push ups could possibly pose a problem. Also, try doing bench press(using a spotter), begin with just the bar.
