NEW YORK ( TheStreet) - The big theme among U.S. banks this earnings season was the boost to the bottom line from the release of loan loss reserves. That trend is likely to accelerate as credit measures continue to improve.

Over the three years ended Dec. 31, 2009, Citigroup ( C) and Bank of America ( BAC) quadrupled their loan loss reserves, while JP Morgan Chase ( JPM) and Wells Fargo ( WFC) more than tripled theirs.

For U.S. Bancorp ( USB) -- the fifth-largest domestic bank holding company by total assets - reserves more than doubled.

During the second quarter, four of the five largest holding companies reported net charge-offs - loan losses less recoveries - exceeding provisions for loan loss reserves. This "releasing" of reserves directly affects earnings, and for Citigroup, the second-quarter reserve release was more than half of net income.

U.S. Bancorp bucked the trend, as the Minneapolis lender continued to build loan loss reserves.

During a discussion with TheStreet on the prospects for the largest banks over the next few quarters, Jamie Cox, managing director at Harris Financial Group in Colonial Heights, Va., said "the reserve reduction provides enormous earnings power going forward."

Cox thinks that with the uncertainty created by financial reform softening the market, "this is a fantastic time to accumulate," adding that JPMorgan is his favorite pick as a long-term play. He also disclosed personal holdings in Citigroup and Bank of America, with a shorter-term outlook.

The following is a summary of reserve growth, coverage and loan loss activity for the group of five.

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