DETROIT (AP) _ The economy is booming, unemployment is low and union demands are high.

Thousands of workers are marking the Labor Day weekend on strike, while others are thinking about walking a picket line. Labor experts say unions are growing more aggressive and more willing to walk out in an effort to win back concessions from past contracts.

The experts say it is a byproduct of nine straight years of economic growth and a string of high-profile strikes.

``Employers are having a hard time finding enough employees, so this is an opportune time to push up wages in the trenches,'' said Peter Feuille, head of the University of Illinois' Institute for Labor and Industrial Relations.

He said the Boeing contract, reached this week, was an example of unions taking advantage of the booming economy.

``We can be an example to the labor movement,'' Bill Johnson, president of Aerospace Machinists Industrial District Lodge 751 in Seattle, said Wednesday after his union approved the Boeing contract. ``You can take on a major corporation and be successful.''

The Boeing machinists had struck twice during the previous three negotiations, including a 69-day walkout in 1995, and appeared to be ready to walk out this year. Instead, the 44,000-member union overwhelmingly voted to approve the deal.

The contract offers two annual wage increases of 4 percent and a third-year 3 percent raise, as well as a 10 percent signing bonus worth $4,400 to the average machinist. In addition, Boeing dropped demands for a seven-day work week and more worker contributions to health-insurance premiums.

But other recent contracts agreed to by union leaders have been rejected by members who want better deals.

About 6,100 of 11,000 Northwest Airlines flight attendants voted last week to reject a proposal that would have raised their average pay by 14 percent. And this week, about 9,200 teachers for Detroit public schools went on strike, rejecting their leader's request for a contract extension.

One Northwest flight attendant who campaigned against the contract said his colleagues wanted to break a pattern of concessions to the company. Attendants took pay cuts for three years beginning in 1993 to help keep Northwest out of bankruptcy, and wages are now only 3 percent higher than before the cuts.

``All kinds of things have evolved to make this a position we've never been in before,'' said Andy Damis of Seattle. ``We've got a tight job market and the pilots went on strike last year, so we think Northwest is really reluctant to go through that again.''

The strike last fall by 6,200 Northwest pilots was part of an overall increase in work stoppages, including strikes and lockouts. The U.S. Department of Labor said there were 34 major stoppages, each involving 1,000 workers or more, that affected 387,000 workers last year. That's up from 29 major strikes in 1997 that idled 339,000 workers.

``The work force can look back at a time where they gave concessions or took profit sharing,'' said John Revitte, a professor at the Michigan State University School of Labor and Industrial Relations. ``Meanwhile, in the last couple of years they've seen incredible CEO compensation packages and mergers. In some of those instances, the work force says it's our turn.''

It's not clear how such concerns might affect negotiations between the United Auto Workers and U.S. automakers that cover 407,000 workers. The contract between the UAW and General Motors Corp., Ford Motor Co. and DaimlerChrysler AG expires Sept. 14.

The union, which has released no information about the progress of the talks, hasn't held a national strike during negotiations since 1976. It has used plant-level strikes as a bargaining tool, though, the last in 1996 against GM during the last national negotiation.

But Feuille said labor actions now are much less frequent than in the 1950s, '60s and '70s, where no year had fewer than 200 major work stoppages.

``There may be a few situations where unions are willing to strike more than in the last 10 years,'' he said. ``But unions have realized the strike is a weapon that can play more into the employer's hands.''