This 12 weeks intervention study on either soy or whey protein providing 2g of leucin found significant increases in muscle strength and lean body mass in response to resistance training.
There was no difference in muscle strength between individuals supplemented with soy protein or with whey protein. The main driver for building muscle strength is the combination of protein intake with sufficient leucin and resistance training.
Amongst athletes there exists sometimes a misconception that animal proteins, particularly whey, are more effective than soy protein at building muscle mass and strength. This study shows that consuming plant or soy protein can support muscle strength and muscle mass comparably to whey protein, when consumed in amounts that provide sufficient leucine.