In "Personal Best," Mariel Hemingway, who was Woody Allen's teen-age girlfriend in "Manhattan," has traded her milkshakes for beer and grass.

Hemingway plays an Olympic hurdler and pentathlete taking up with an older woman athlete who becomes her mentor as well as her lover, and later, her competitor. The affair is subordinate to the athletics: Conflicts in the relationship just fizzle out. After all, there's a race to be run.

This is an instructional film, a docudrama approach to what goes into preparing for the Olympics: the training, the competitiveness and the sexism the female athlete encounters. It even has a few pearls, such the coach's description of one of the five events in the pentathlon: "The high jump is a masochist event that always ends on failure." What lends the movie authenticity is that most of the people in it really are Olympic athletes and record-holders, and they show that they know what they're doing. The second lead, Patrice Donnelly, is a former Olympic hurdler.

But "Personal Best" only jogs along sometimes: It suffers from fleshing out. The most-repeated shot in the movie is thighs thundering up hills, over hurdles and around the track; the second is calves muscling, ditto scenery, and the third is bodies sweating in the steamroom used by the Olympic hopefuls in women's track.

The film could easily do without the details that elicit its R rating -- the pointless nude scenes in the steamroom and the superfluous barrages of swearing, which may be attempts at humor. Cut out the scene where Mariel Hemingway helps her boyfriend go to the bathroom. Then no one will have to explain this gratuitous scene, or answer questions such as: Does this demonstrate her budding heterosexuality or is it nothing more than a case of penis envy?

"Personal Best" is a running movie: The big question in ending the lesbian relationship is not so much will they date other people and the other sex, as will they continue to work out together?

PERSONAL BEST -- At the AMC Academy, Dupont Circle, K-B Cerberus, K-B Congressional, NTI Landover, NTI State and Old Town.