It could be make-or-break time for high-yield corporate bond ETFs that have seen trading volume surge after falling to a key technical support level. A breakdown here would be a worrying sign for credit markets, the S&P 500 and other major U.S. equity benchmarks, according to technical analysts.

The largest ETF in the category, iBoxx High Yield Corporate Bond (NYSEArca: HYG), traded more than 14 million shares on Monday, an all-time high. That tops the recent record of about 11 million shares set last week on May 29. [High-Yield Bond ETFs Slip on Rate, Liquidity Fears]

HYG and SPDR Barclays High Yield Bond (NYSEArca: JNK) have been volatile the past two days near a rising support line that has been in place since late 2011.

A break of support for the high-yield ETFs would “send a concerning message to the S&P 500, especially with the yields on high yield bonds reaching the lowest effective yield in history,” according to Kimble Charting Solutions.

Junk bond ETFs have been extremely popular among investors taking on more risk to boost yield with the Federal Reserve keeping short-term rates near zero. However, the junk debt ETFs have taken a hit recently on rising Treasury yields and hints the Fed may scale back its bond purchases.