Alternate picking is an extremely efficient picking technique for melodic patterns that ascend or descend on a single string or a group of strings. However, when you encounter a situation where there is only one note played per string you will find sweep picking to be much more effective.
An arpeggio is produced by playing the notes of a chord one-by-one. Arpeggios are often played with just one note per string. Instead of using alternate picking to play arpeggios, you will find sweep picking to be more efficient.
In sweep picking, you use as many continuous down or up-strokes as possible. If the arpeggio changes direction, you simply reverse your picking.
In Sweep Picking Exercise #1 you will see an E major arpeggio. In this fingering for the arpeggio, there is one note on each string. The way to play this is to use one continuous down-stroke for strings 5-1 and then one continuous up-stroke for strings 2-4.
Sweep picking gets its name from the way it looks when performed—it looks like you are sweeping across the strings with your pick.
Sweep Picking Exercise # 1
E major arpeggio
Sweep Picking Exercise # 2
E major arpeggio
In some arpeggios you will need to play two notes on a string. To do this use alternate picking on the strings that have two notes and sweep pick the rest.
In Sweep Picking Exercise #2 you will see that we have added in one additional chord tone for the arpeggio. There are now two notes played on the first string. This means you will need to use a combination of sweep picking and alternate picking.
For more powerful guitar technique building exercises check out my courses How I Got Killer Guitar Chops While I Was Still in High School: Confessions of a High School Shredder and Mega Chops: Scale Mastery Beyond Hanon.