Certain parts of my body shake/tremble at certain positions. Why?

yirara

I would simply guess non-use of muscles and having a bodypart in a less favourable position for maintaining strength. Like if you hold a shopping bag in an arm hanging down and lift it up close to your body it's a lot easier than doing this with an outstretched arm. And as such, some everyday movements or positions requite a bit more strength overall.

To, try to build up your exercising slowly but continuously. you're young and it's not too late.