The Dow industrials and the S&P 500 are both on the brink of multiyear highs, but volumes across U.S. markets this week hit their lowest levels of the year. Market watchers attribute the sluggishness to a host of factors: a summer slowdown, nervousness about where markets may be headed and how they operate, and waning participation from electronic traders.

So far in August, an average 4.88 billion shares have changed hands each day, according to the WSJ Market Data Group. That compares with 8.17 billion shares last August--when markets were roiled by a U.S. debt downgrade--and 5.9 billion in 2010. This past week, 21.92 billion shares changed hands, the lowest count for any nonholiday-shortened week this year.

Average daily trading volumes peaked in 2009, bolstered by traders reshuffling positions to deal with the fallout of the worst global economic slowdown since the Great Depression.