Amid all the volatility, can central banks rescue the bull market again?

US stocks were flat last week but the calm waters masked an explosion of volatility below the surface that could represent a turn in the market cycle.

But any negative developments in the ongoing Hong Kong protests, retail earnings or US economic data due out later this week could torpedo stocks once again.

"It feels like a bear market," said Lorenzo Di Mattia, manager of hedge fund Sibilla Global Fund.