Undated (AP) _ A spike is something pounded into a railroad tie, but it can also mean a 10-ounce volleyball hammered at you at 120 mph if you are facing the powerful world champion Americans at the Olympics in Seoul.

Trying to block the soaring spikes of 6-foot-5 Steve Timmons, offset the scrambling digs of Karch Kiraly and subdue the rest of the American squad will be the unenviable task of the 11 teams trying to unseat the defending Olympic titlists in Seoul.

The main challengers are expected to be the Soviets, who boycotted the 1984 Los Angeles Games and have since been unable to reclaim their traditional dominance in international volleyball.

Although volleyball was invented in the United States in 1895, it took the 1984 gold medal to help give the sport respectability in the eyes of many Americans.

No longer just an excuse to go to the beach, volleyball now has its own pro leagues with stars able to bank as much as $150,000 a season.

In a sign of the sport's growing popularity, the American games are scheduled in the morning in Korea so that NBC can broadcast them in prime-time in the United States.

''Since 1984 there has been a tremendous increase in popularity and visibility of the team and players,'' coach Marv Dunphy said. ''Volleyball is legitimate now.''

Heading the American contingent in Seoul will be the 29-year-old Timmons, one of four carryovers from the 1984 squad. A flashy redhead with a 40-inch vertical jump, Timmons has few peers as a spiker. His ''kills'' pack the explosive force of an overhead smash by Ivan Lendl or a slam dunk by Michael Jordan.

''Other teams have to prepare to stop him,'' says Dunphy. ''If they don't, they pay the price. At the same time, if they concentrate too much on him, it opens it up for somebody else.''

That could be Kiraly, a 6-foot-3 swing hitter who has been cited by the international volleyball federation as the best all-around player in the world; or 6-foot-8 middle blocker Craig Buck, described by Dunphy as ''the quiet hero of the team.''

The Americans - 11 southern Californians and one transplanted Coloradan - are paired with France, the Netherlands, Japan, Tunisia and Argentina in one Olympic pool. The Soviets are in the other bracket with Sweden, Italy, South Korea, Brazil and Bulgaria.

Since winning its first ever Olympic gold in 1984, the U.S. team went on to complete volleyball's triple crown by winning the 1985 World Cup in Tokyo and the 1986 World Championships in Paris. In 1987, the U.S. squad captured the Pan Am Games and the prestigious Savvin Cup in the Soviet Union.

So far this year, through late July, the Americans had compiled an overall record of 34-4, including a 7-0 mark against the Soviets.

''We were once the teacher and they the students,'' Soviet Coach Gennadi Parchin said earlier this year after the Americans won the USA Cup in Los Angeles. ''Now we're the students and they are the teachers.''

''We've been on the high side the last few years,'' said Dunphy, former coach at Pepperdine who took over the national team in 1985 from Doug Beal. ''But it's to the Soviets' advantage to play us as often as they have. They're benefitting from playing a better team.''

Bob Ctvrtlik, who was a security guard at the volleyball venue in 1984 and is now of one of the six starters on the U.S. team, sees the head-to-head competition with the Soviets as an advantage.

''I feel like I know everyone of their players inside and out,'' he said.

While the U.S. team blends speed and agility, the Soviets are known more for their straight-ahead power game, led by Yaroslav Antonov and Alexander Sokolet.

''They average about two inches taller per player,'' Ctvrtlik said. ''They pretty much tell you what they're going to do and they try to do it - through you and over you.''

Besides the Americans and Soviets, Dunphy views the best medal hopefuls as France, Argentina and Holland in one bracket, and Bulgaria and Brazil in the other.

France has come on strong lately, but even superstar setter Alain Fabiani has said that the Americans and Soviets are locks for the gold and silver and ''the rest of us are just going for the bronze.''

In the women's division, the strong favorite is China, the 1984 gold medalist and the defending world champions. The Chinese are pooled with the United States, Peru and Brazil. Japan, the Soviets, East Germany and South Korea are in the other bracket.

The U.S. women's team has undergone a complete revamping since winning the silver medal in 1984. With a new coach - German-born Taras Liskevych - and all new players, the squad has struggled at times. But the team came to life in 1987 by placing third at the Pan Am Games and beating China twice during the year.

END ADV for Release Aug. 13 and thereafter