Do yoga and stretches on my rest days hinder muscle growth?
I'm afraid if I try yoga that I'll lose the muscle I've worked so hard to gain.

Yoga and stretching will not hinder your muscle growth. In fact, they will do quite the opposite.
After an intense strength workout, your muscles have a build up of lactic acid and adhesions, or knots. Stretching and yoga is the perfect way to break up that lactic acid build up while kneading out the knots. This is crucial as adhesions can cause muscle imbalances. Even worse, if not dealt with, these same muscle imbalances can lead to postural distortions such as an arched back or rounded shoulders. Yoga and stretching will ensure you avoid this.
Yoga itself is an actual workout. The poses that you must hold challenge your stability, coordination, endurance, and strength. When practiced regularly, you will see the following benefits:
- Improved flexibility (a better range of motion for when you are lifting weights)
- Improved strength and endurance (you'll be able to lift more for a longer period of time)
- Sleep better (this will allow the maximum amount of repair hormones to be released during your R.E.M. cycle)
- Reduced stress & blood pressure (keeping cortisol levels down in your blood is essential to maximize muscle)
Yoga and stretching should supplement any weight lifting routine. They will protect your body from injury while helping you achieve that dream physique.