IT'S a cold and rainy day, better suited for napping than pushing yourself to the edge of exhaustion on a bicycle. Nevertheless, Shelley Sarmiento is hammering away atop a stationary bike in the lab room of Fitness Concepts, a personal-training and endurance-coaching facility in Fairfax.

The speedometer on Sarmiento's handlebars reads 26 mph. As if the blistering pace isn't enough to exhaust her, the 38-year-old mother of two must also contend with a nose clip and a plastic mouthpiece connected by tubes to a computer known as a metabolic analyzer.

"Just 30 more seconds," says Ken Mierke, who is an exercise physiologist, triathlete and coach. "Keep it up, Shelley."

As Sarmiento bears down for the final half-minute, Mierke checks out columns of numbers being displayed on the analyzer's monitor. Heart rate. Oxygen consumption. Ventilation. Respiratory quotient. These and other readings are being picked up from the mouthpiece and a heart-rate monitor that is also attached to Sarmiento.

It might appear that Sarmiento is a cardiac patient undergoing some type of stress test. But what she's actually taking part in is a 20-minute VO2 max test, which is similar to tests that world-class athletes take in order to help formulate and monitor their training programs.

To put it simply, VO2 max equals a person's aerobic capacity. In layman's terms, it's all about the heart's ability to pump much-needed quantities of blood to the muscles during strenuous work or exercise.

Sarmiento doesn't consider herself an elite athlete. Though she has completed 20 marathons, she only recently entered the world of Ironman competitions. She is among a small number of weekend competitors who are trying to maximize their training efficiency through the use of VO2 max testing. Exercise physiology labs at universities and hospitals sometimes make these tests available to the public, but they are not a common offering at fitness facilities. In fact, Sarmiento has traveled from Annapolis to take the test.

The cost of the VO2 max test at Fitness Concepts is $75 ($60 for people enrolled in their coaching program). The test is non-invasive, though more elaborate testing offered at hospitals and universities often includes the drawing of blood. In addition to being tested, Sarmiento will pay $75 a month for Mierke and fellow trainer Eric Sorensen (who has twice completed the Ironman Hawaii Triathlon) to design and monitor a training program based on the test results.

"We call it a VO2 max test, but there's much more to it," Mierke says. "We use the results to set realistic but aggressive training goals."

Mierke thinks that many people train too hard, paying a high cost in extended recovery time while gaining few benefits from the pain. Working with the various numbers from the computer, his goal is to formulate a "low-cost, high-benefit" training program that will improve a client's endurance and overall performance without wasting any of their precious time.

Clients use heart-rate monitors as guidelines for exercise intensity. By monitoring their heart rate during training sessions, they can stay within parameters that are designed to benefit their aerobic capacity and endurance.

"There are certain intensities that produce excellent results without being overly demanding on the body," Mierke says. "A heart rate monitor allows the athlete to exercise precisely at the most efficient intensity for the purpose of each workout."

Of course, many weekend athletes continue to train both safely and successfully without the aid of a heart-rate monitor and without even knowing the definitions of terms such as "lactate threshold" (let's just say it's an important factor in physical endurance). Plenty more runners and cyclists are content to simply estimate their VO2 max and lactate threshold using formulas involving maximal heart rate and previous race paces. Does this mean low tech is the way to go?

"If you're willing to invest the time and money, an accurate [lab] test will help you come up with a precise prescription for what your pace and various training intensities should be," says Scott Douglas, a former editor-in-chief of Running Times magazine and co-author of "Road Racing for Serious Runners." "Still, it's not pointless to use the estimates. It's not like you have to be tested or else it's not worth it."

For skeptics, one way to gauge a test's precision is to look at the results of the accompanying training prescription.

"It's an issue of test and re-test," says CC Cunningham, American Council on Exercise (ACE) spokesperson and a personal trainer. "Take the test, get an exercise program, then re-test later and see if there are improvements. Look at the training and see if it's working or not working."

The test, itself, is no walk in the park. At Fitness Concepts, clients start off at an easy pace and increase it by 1 mph each minute. The goal is for the client to reach their VO2 max within a 10- to 20-minute time span.

"When they can't hold the speed they're supposed to hold, I tell them to close their eyes and sprint for a minute," Mierke says. "The numbers I get within that minute are what I'm looking for in determining the VO2 max."

Sarmiento survives the test intact, complaining mostly of the annoying mouthpiece. She hopes the test results and tailored training routine will help her get the most out of her limited time. Having recently finished her first Ironman race in Florida, she's about to begin preparing for the Ironman California Triathlon scheduled for May. She plans to take the VO2 max test again in a few months to help chart her progress.

"I don't have a very structured training schedule," she says after she climbs off the bike. "With kids and a job, I basically squeeze it in when I can. So, I have to maximize my ability to train efficiently."

FITNESS CONCEPTS -- 8301 Arlington Blvd., Fairfax. 703/560-7846. Web site: VO2 max testing is $75 ($60 if enrolled in coaching program). Coaching is $75 per month.