The problem is caused by you setting the axis of the joint negative. This causes it's usual movement to be reversed. Meaning, when you contract, it actually extends and vice versa (You can also read this from the bottom of the screen where it says what the joint is currently doing).
You can set the axis to be positive, but this will cause the visual presentation of the joint to be opposite, which in turn will also throw people off. This however, in your particular case, can be fixed by rotating the bodypart 180 degrees on y-axis. Or atleast that's my theory, I don't think I've ever really tried it out.

You can`t rotate joints as far as i know :c And i hate it

Edit: I just had the stupid idea to switch the left and right arm (including pecs) entirely.
Then it will show everything correctly, but it will say left ellbow for the right one an vice versa

That's exactly why you have to rotate the bodypart attached to the joint.

Edit: I just tested my way, it seems to work.

Thank you, i didn`t know that. I will play around with that and try to work that out.
I had the idea to write a makro in Excel (because that`s the only thing i know) that will convert the QUAT and POS information from replays into a format that can be put into the modmaker. But i quickly dropped that idea, because the joints would be rotatet weirdly (and also because i am a lazy human).
Maybe i will make it some day, but still it won`t look perfect.

I tried it, but if you don't make z axis range negative how do you make the arm contract/extend in line with the bicep? It becomes crooked.

Edit: nvm I'm an idiot. You da real MVP Obazu

how do u guise make an object inanimate where it can like float in the sky without falling and shit

set it to static

how bout how to make spinny weapon obstacle things like in raid_challenge and etc

You have:
object 1: it`s the weapon so you can shape it as you want and set it to weapon. It is not static.It is one of the first 16 objects.
object 2: the weapon is attached to it. You can make it however you want and place ot anywhere. It only has to be static and one of the first 16 objects.

Go to joint objects
click on the 1 in the white box and then check enable. Then check the two numbers that are your objects.
the settings are:
upper: 5, lower: -5
axis: x: 0, y: 0, z: 1 (or whatever you want)
speed: whatever you want
strength: whatever you want (preferably something high)
The position has to be the same as the rotating object.

Did you even try once to make that by yourself?

As Ukephile said ^

These are things you could have figured yourself.. Opening raid_challenge, and looking how the joints are made, what is attached to what etc.

I'd like to precise that the axis you want to enable will differ depending on the direction you want your thing to spin on. So if z = 1 doesn't work for you, try enabling another one. (typically, the z axis will make it spin on an horizontal way.)