Seduction will take you only so far. If you want your sex life to sizzle, stretch--with your partner.

At least, that's the gist of "Sex Flex," a how-to fitness book that aims to put the play back into foreplay. Its 80-odd exercises are intended to increase flexibility, self-confidence, communication and body image.

CROSS-ARM TRIANGLE: Sit facing each other, legs apart, soles of your feet touching your partner's. Keep your buttocks on the floor and spine straight. Cross wrists and grab each other's hands. Lean back while your partner leans forward. Hold for five to fifteen seconds. Return to the starting position and reverse direction. Repeat three to six times.

Put them together, claims Deborah David, Ph.D., who wrote the book with fitness expert Paul Frediani, and you get better lovemaking.

Or, as Frediani puts it: "When you're giggling and laughing and doing something beneficial, you're laying the groundwork for a more intimate relationship."

Titillating title aside, "Sex Flex," out this month from Hatherleigh Press, is no "Kama Sutra."

THE BRIDGE: Sit facing each other, knees bent. Place the soles of your feet against your partner's. Hold hands and lift your legs to form a bridge. Hold for five to 15 seconds before returning to starting position. Do three to six times. Body parts worked: upper shoulders, back, hamstrings, front of hips (hip flexors).

Written in clear, easy-to-follow prose, the book features wholesome-looking husband-and-wife models in coordinated workout clothes who illustrate each pose. But even when you're clothed, you'll want to practice some of these moves in private--with someone you know and trust.

Trust, Frediani says, is a must.

"The possibility for injury is high," warns Frediani, a personal trainer at Manhattan's Equinox Fitness Club and author, too, of "Golf Flex," "Surf Flex" and "Net Flex."

"You have to be able to communicate. If you're angry or upset with your partner, you don't want to do this program."

So much for safe flex. Elsewhere in the book are a few cautionary notes about shoulder or jaw problems. For the most part, through, the stretches are safe for all, though it helps if one partner doesn't significantly outweigh the other.

Beginning with the right way to breathe--through the abdomen, with your hand just south of your partner's belly button to check it--the book charts a steady course in flexibility for every body part. It takes such standard routines as leg lifts and back stretches and adds a touchy-feely element: i.e. your partner.

CHEST PRESS: Lie on your back, legs straight, arms to the side, palms up. Bend elbows 90 degrees. Your partner straddles you, places hands over yours, leans forward. Press your partner away, straightening your arms--10 to 20 times. Change places. Body parts worked: chest, shoulders, arms.

In other words, who needs a gym when you've got each other?

Then again, you many not get very far: Some exercises are simply too... effective.

Instead of doing calf raises on a machine, for instance, "Sex Flex" suggests you site your mate on your lap--and raise him (or her) up and down a few times.

Leg lifts may get a rise out of both of you when you're using your partner's leg for resistance. And chest presses provide an added tingle when he's lying on his back and you're straddling him, hoisting yourself up and down, palm-to-palm.

Some of the exercises spring from yoga, like the Lion Roar, a move you don't want to try on a first date--or a second or third, or even in a marriage without a pre-nup.

After making a prune face (pulling your face in tight), open your mouth and eyes as wide as you can.

True, it does "get the blood flowing," as Frediani says--but it can also send the meek fleeing. Best to do this one standing side by side, instead of face to face, as pictured.

As far as sheer fun goes, Frediani suggests the feet-to-feet, cross-arm triangle--a kind of "give and take" stretch in which you sit on the floor, facing each other, holding onto each other's wrists and taking turns leaning forward and back. Do it three to six times, he says, and it not only works the inner thighs, hamstrings and lower back, it's bound to make you giggle.

But does all this flexing really help sex?

"Undoubtedly," says Frediani, who's 48 and looks a decade younger. "The bottom line is, if you're more flexible, you'll be able to enjoy different positions without getting a cramp. And when you feel more flexible, you're also willing to try a variety of positions."

Besides, he adds, workouts tend to make people feel good and

"And what's a better time to feel good," he points out, "than when you're with someone you care for?"