Why do my muscles tremble like a leaf when I do the slightest effort with them?

Weakness. Trembling during an activity that requires strength suggests muscle weakness. I'd recommend continued dedication to a fitness routine that includes strength training 3-4 days weekly, flexibility 3-5 days weekly, and cardiovascular/aerobic activity 3-5 days per week to build on your endurance. Gradually increase your time and intensity and over several months, you should see improvement.