Many players use both arm and wrist movement in some combination with their picking techniques. It depends on the style of music you're playing. I believe the wrist allows faster alt. picking and better control, others may disagree. Regardless, the most important thing to me when it comes to alt. picking is if I can use "inside picking" vs. "outside picking" to achieve the smoothest transition. I have'nt seen the term posted yet, but the technique does wonders. I'll post a thread on the subject.