"You really need to contribute to your account, because those contributions have an equal weighting to the market appreciation over the long term," says Beth McHugh, vice president of market insights at Boston-based Fidelity.

Investment earnings and contributions can grow tax-free in an employer-sponsored 401(k) account, which is a key reason why they're a popular way to save for retirement.

The amounts that investors have saved through their 401(k)s vary widely depending on a participant's age. Fidelity said the average year-end balances were $143,300 for participants 55 and older and not yet retired; $120,400 for baby boomers born from 1946 to 1964; $59,100 for Generation Xers born from 1965 to 1978; and $15,400 for those in Generation Y, born from 1979 to 1991.

Overall, the average employee contribution in Fidelity-administered 401(k) plans has remained steady at around 8 percent of annual pay for the past four years. Factoring in employer matches, the total savings rate rises to 12 percent. In the latest quarter, 5.8 percent of participants increased the amount of their 401(k) paycheck deductions, while just 3.1 percent decreased the amount.

The number of employees increasing their deductions has exceeded the number decreasing them in each quarter since early 2009. That indicates employees feel more comfortable about their financial situations than during the recession, although wage growth has been slow and unemployment remains high.

Yet workers who have stayed in the market long term have found it difficult to rely solely on investment gains to build up 401(k) savings. The S&P 500 remains about 3 percent below its historic peak in October 2007.

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