After seven years, the Consumer Product Safety Commission is going to have its way -- or at least most of it -- with lawnmowers.

The U.S. Court of Appeals for the Fifth Circuit yesterday backed the commission in its effort to impose a new set of safety standards on walk-behind lawnmowers. The commission had tussled since 1973 over the regulations with the industry's trade group, the Outdoor Power Electric Institute. The institute probably will not appeal the ruling, a spokesman for the organization said yesterday.

The changes, which are scheduled to go into effect in 1982, will boost the average price of lawnmowers by about $35, costing consumers a total of $189 million per year, the commission estimates.

With the safety group's victory, walk-behind mowers will have to include a device to stop the blade from turning no more than three seconds after the operator's hands leave the controls, as well as a foot shield around the rear of the machines. The foot shield will have to be strong enough to withstand heavy use, and the mowers also will have to carry warning labels.

Lawnmower blades cause more than 77,000 injuries each year, according to the safety commission.

Dennis C. Dix, executive director of the equipment institute, said his group will request an extension of the January 1982 starting date to June 30, 1982.

Dix said his group has never been opposed to a brake "clutch" to halt mower blades after three seconds but just "never found a reliable" one.

The institute probably will not appeal the decision because of the cost of the court fight -- about $150,000 in the last two years alone -- he said.

The group's directors will make a final decision on the matter within two weeks, he added.

The appeals court accepted all but one of the safety commission's proposed regulations: a requirement that the mowers pass a test to assure that users cannot put their feet in the discharge chute of the machines.

The dispute between the commission and the mower manufacturers began in August 1973, when the trade group asked the regulatory body to adopt the industry's own voluntary set of safety standards.