The funding levels of pension plans sponsored by large, publicly held U.S. employers improved slightly in 2010 due to strong investment returns and hefty contributions, according to a Milliman Inc. survey released Tuesday.

Defined benefit plans offered by 100 U.S. employers with the largest pension programs were, on average, 83.9% funded during 2010, up from 81.7% in 2009 and 79.3% in 2008, according to the survey.

Solid investment gains and increased employer contributions were responsible for the turnaround, Milliman said.

On average, plans earned 12.8% on assets last year, down slightly from 14.1% in 2009 but a huge improvement over 2008 when investment losses averaged 18.9%.

In addition, employers contributed a record $59.4 billion to their plans last year, up from $54.1 billion in 2009 and $29.8 billion in 2008.

“This was a record year for pension contributions, though the number could have exceeded $60 billion if a few things had gone differently,” John Ehrhardt, co-author of the “2011 Milliman Pension Funding Study,” said in a statement.

“Pension funding relief enacted last summer helped reduce the funding burden, along with positive investment performance. If interest rates remain at current levels (or decline), contributions will be even higher in 2011,” added Mr. Ehrhardt, who is a principal and consulting actuary in Milliman’s New York office.

In all, the market value of the plans’ assets increased about $115 billion to just more than $1.2 trillion. The value of plan obligations increased nearly $103 billion to about $1.43 trillion, Milliman said.

Even with the improvement in funded status, plans’ funded ratio was the fourth-lowest since the Seattle-based actuarial consulting firm began the surveys in 1999. The lowest average funded ratio was 79.3% in 2008, when the equities markets’ slump sharply reduced the value of plan assets. The highest average funded ratio was 130% in 1999.

The Milliman analysis, available at www.milliman.com/expertise/employee-benefits/products-tools/pension-funding-study/index.php, was based on financial reports of publicly held companies sponsoring the 100 largest pension programs for which full-year data was available.