Fifty-two million workers participated in 401(k) plans at the beginning of 2013, holding accounts totaling $3.5 trillion, or 18 percent of all retirement assets, according to an annual snapshot of the plans released by the Employee Benefit Research Institute.

The average account balance at the beginning of 2013 was $63,929, up from $58,991 12 months before. However, that average was inflated by big accounts, which overstated the wealth most people have in these plans. The balances of more than three-quarters of the workers were less than the 2012 average and nearly 40 percent of the workers had less than $10,000 saved.

The percentage of employees holding company stock in 401(k) plans continued to decline. Roughly one out of every four plan participants now invests in his or her company. About a decade and a half ago, the ratio was almost three out of every five.

The percentage of employees whose 401(k) plans included balanced funds with a mixture of debt and equities inched up in every age class except for 20-somethings, who showed a slight decline. However, the percentage of workers in their 20s holding balanced mutual funds is still higher (at 70.8 percent) than it is in all other age groups.

Balanced funds as a whole increased from 21 percent of the value of the average 401(k) to 22 percent during the year.

The share of 401(k) account holders who borrowed from these investments stood steady from the start of 2012 to the start of 2013 at 21 percent. However, the magnitude of the loans declined from 14 percent of the balance to 13 percent.