Inflows into the three biggest exchange-traded funds that track the S&P 500 Index spiked as the benchmark U.S. stock gauge enjoyed its best week of the year amid technical factors that may have juiced activity.

State Street, Vanguard and BlackRock’s offerings pulled in more than US$11 billion last week, the most since the five days ended Sept. 21. That stretch included the S&P 500 Index’s record close of 2930.75.

“The flows show confidence in an accommodative Fed is starting to outweigh concerns over the lateness of the cycle for many investors, although some are still choosing to play this a bit more safely with quality and low-volatility ETFs,’’ said Eric Balchunas, senior ETF analyst for Bloomberg Intelligence. “In a sense we are now able to see more natural sentiment -- and it’s clearly getting more bullish.’’

This appetite for a broad basket of U.S. equities would stand in stark contrast to many of 2019’s early trends. Despite the Federal Reserve pausing interest-rate increases, year to date, these funds have endured outflows on a cumulative basis. In addition, stock-picking was said to be in fashion, according to data from Bank of America.

Quadruple witching -- the expiration of futures and options on indexes and individual stocks that typically spurs a lot of trading activity -- may have contributed to these flows. Indeed, the all-time equity highs in September were reached the session before this quarterly event. However, in neither of these episodes did most of the inflow activity appear to come on Friday, according to data compiled by Bloomberg. After September’s quadruple witching, the three products saw fund outflows the following week that failed to erase the recent intake.