Gaining lean muscle and burning fat is all about smart lifting! Over the years I've built a solid amount of natural muscle and have stayed injury free all because I've kept smart and avoided ego lifting..
Meaning I've always lifted using great form with controlled movements to achieve these gains. Now it can seriously mess up with your mind when you're lifting lighter, to ensure you're using the right form, but it can pay off big for you in the end, when you start feeling:
1. A better pump at the gym.
2. Increased lean muscle gains.
3. Motivation from seeing the extra results.
Now if you're shooting for muscle gains a general 8-10 reps works well. And 10-12 reps works great if you're aiming for fat loss.
The lower reps works well to really break down the muscle fibres better under heavier weight, achieving better muscle gains.
The higher reps allow you to stay under tension a little longer, getting your heart rate higher, helping you sweat more and burn more fat eventually.
Moral of the story is, lift smart and drop the weights slightly, improve your exercise form and achieve the best muscle gains!