A recently published study has shown the importance of a fast digesting protein (like WPI).
“The present study demonstrated that protein composition has a large influence on muscular performance after prolonged resistance training. More specifically, as compared with placebo or micellar casein, soluble milk protein (fast digestible) appeared to significantly reduce muscle fatigue induced by intense resistance exercise.”
Not only is the type of protein important but when it is consumed impacts on it’s effectiveness.
Full article here: