The S&P 500 is trading above the key 1,400 level and on track for a 12% gain for the first quarter, but ETFs that invest in the basic materials sector and copper are flashing weakness.

The $1.8 billion Materials Select Sector SPDR Fund (NYSEArca: XLB) is actually down 2% over the past month, while iPath Dow Jones-UBS Copper ETN (NYSEArca: JJC) is also in negative territory with a 3% decline.

Some technical analysts watch materials and copper ETFs to gauge the health of the global economy.

In the past, a lack of upside follow-through by the basic materials sector and copper has been a sign that the economy lacked strength going forward, says Chris Kimble at Kimble Charting Solutions.

“Most materials firms produce commodity products and operate in a highly cyclical industry,” says Morningstar analyst Robert Goldsborough in a report on Materials Select Sector SPDR Fund (XLB). “This fund struggled in 2011, significantly underperforming the broader market amid global economic woes.”

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