S&P MidCap 400 in Focus: AEO KWK

American Eagle's stock declined $1.95, or 12.3 percent, to $13.85, after hitting $13.55 earlier in the session, a level not seen since 2000. An Oppenheimer analyst downgraded shares of the teen retailer, following the departure of its president and merchandising chief and a weak U.S. economy.

Shares of Worthington Industries Inc. declined $2.72, or 11.3 percent, to $21.39, following an analyst downgrade.

Meanwhile, Palm Inc. posted a fiscal fourth-quarter loss, sending shares of the Treo and Centro smart phone maker falling 62 cents, or 9.5 percent, to $5.92 in afternoon trading.

On the rising side, shares of natural-gas and oil producer Quicksilver Resources Inc. gained $2.71, or 7.4 percent, to $39.25.

Forest Oil Corp.'s stock rose $4.38, or 6.3 percent, to $74.14, also on an analyst upgrade.

Shares of security systems company Federal Signal Corp. rose 48 cents, or 3.9 percent, to $12.70.

The S&P MidCap 400 is an index used to track midsized companies which have a market capitalization of $1.5 billion to $5.5 billion. According to S&P, midcap stocks make up roughly 7 percent of the U.S. equities market.

The index declined 1.39 points to 825.22.