It's as certain as taxes in springtime. The exerciser's spirit is willing, but the flesh is winter-weak.

As a result, many people who enthusiastically embrace an exercise program in April are back on their duffs by May. Most exercise drop-outs quit for one of three main reasons--injury, motivation and schedule problems--say sports medicine experts at the University of Virginia, one of a handful of institutions nationwide offering masters degree programs for fitness specialists.

"Recreational athletes catch the spring fever and take off too hard, too fast and too often," says U.Va. orthopedic surgeon Dan Kulund, author of The Injured Athlete (Harper/Lippincott Medical Division, 526 pages, $42.50). "So they often wind up with sore muscles, shin splints and other problems."

Most of these "injury drop outs" could be prevented, says Dr. Kulund, "by approaching an exercise program in a systematic, gradual way" and "warming up properly first."

But sooner or later--no matter how careful--virtually every exerciser wakes up one morning feeling like a 2,000-year-old turtle ready to crawl back into its shell and go belly up.

Soreness is commonly the result of injury to the soft tissue--tendons, muscles and ligaments--says Kulund who offers this five-step home treatment plan:

1. Soak the sore part--foot, elbow, knee or entire body--in warm, not hot, water for 10 minutes.

2. Massage area for seven minutes. (Why seven? "That's my lucky number.") Use lotion or oil to fight friction and aid circulation.

3. Wash off lotion with rubbing alcohol.

4. Stretch the area gently. "We show people specific stretches for their particular injury."

5. Ice it. Hold an ice cube in a wash cloth so one side is exposed to the skin of the sore part. Rub in small circular motions until the cube melts or for two minutes--whichever comes first. Repeat. Pain Plan

Pain experienced during an activity--as opposed to soreness several hours later--could signal a more serious problem. "If you go out running, for example, and come back with shin pain," he says, "use moist cold to penetrate the injury and counter swelling." His advice:

1. Run cold water into the center of a towel.

2. Fill a plastic bag with ice cubes.

3. Wrap bag in the towel and place it on the injured part so the wet towel is against your skin and the rest of the towel wraps it in place against your body.

It's important to consult a trained specialist, he stresses, if you have:

* Acute pain. Stop immediately or risk further damage.

* Trouble moving or using the part.

* A chronic problem or one that doesn't get better in a few days of home treatment.

People who have pain from exercise, says Kulund, should REST: "Resume Exercise below the Soreness Threshold." So if you're working out with weights, you may need to reduce the amount of weight or the number of repetitions. If you're running a 7-minute mile you may want to slow down to a 9-minute mile and work back up slowly until you can run faster without pain.

Among his other pain-killing tips:

* Counter "nautilus numbness" by ensuring that the machine is set properly for your body size. If pain persists, reduce weights and/or repetitions and try shifting positions.

* Remember that warming salves may provide a psychological boost, but have "no proven healing benefit."

* Try two aspirin tablets--which reduce inflammation--20 minutes before exercise if you are experiencing mild pain during the activity.

* Consider running in water to rehabilitate an injury and stay in shape: The water's antigravity effect reduces stress on the body. Kulund will have a "runner's pool"--modeled after those used with injured thoroughbreds--in his new Sports Medicine Center opening this July in Charlottesville. If you don't have access to such a pool, try wearing a water-skiing vest and running in the deep water of a regular pool. Motivation Boosters

In addition to injury, "the other two big exercise discouragers," says U.Va.'s head athletic trainer Joe Gieck, are "picking something you don't enjoy" and "not finding or making a convenient time to exercise on a routine basis."

In setting up individual fitness programs for executives at Charlottesville's Federal Executive Institute, Gieck has learned, "If people don't enjoy their exercise, it's only a matter of time before they quit.

"Also, you've got to pick a time when you can exercise regularly that fits your body and your life style. A night person who tries to run first thing in the morning may not make it past the second week."

To maintain motivation he suggests:

* Work out with a friend or family member. Make it something you anticipate and enjoy.

* Vary your route. You don't have to stick to a premeasured course. Think in terms of time exercised--say 30 minutes of swimming or walking--as opposed to counting laps or miles.

* Listen to music or watch TV (for indoor cyclists, rope-jumpers, rebounders).

* Don't overdo it. Beginners might want to limit workouts to every other day. Advanced athletes should give their bodies at least one day of rest per week.

* Reward yourself for a workout (or week of workouts) well done. If you're dieting, pick a non-food reward like a record, massage, flowers, long bath or morning of leisure.

* Consider entering a race or group event. If you don't enjoy competing against others, try competing with yourself by setting a goal to achieve by a certain date.

* Daydream. When the going gets tough, switch mental gears to your favorite vacation spot or the memory of a happy event.

* Join a class or gym with good, professional instructors. Test Your Teacher

Considering a fitness class or club? Be aware that untrained instructors and irresponsible facilities can be hazardous to your physical and financial health, warns medical physiologist Larry Wolfe, director of U.Va.'s Adult Fitness Program. Before enrolling in a program, he suggests testing the instructor's knowledge and commitment to client welfare with these questions:

* Are you trained in CPR (cardiopulmonary resuscitation)? Certification in this life-saving technique is relatively simple to get, and vital since "there is a small, but very real statistical chance that someone will have a heart attack during a workout."

* Do you require any medical screening for participants? At minimum, anyone over 35 should be required to present a physician's approval. If there is only a waiver of responsiblity--that you won't sue if injured--"that's a bad sign."

* What is your training? Do they have a physical education degree and/or certification by a professional group such as the American College of Sports Medicine or the National Athletic Trainers Association.

* Show me a good warm-up stretch. "If they start bouncing it's a bad sign, since ballistic stretches can set you up for injuries. Slow, steady static stretches are what you want for warm-ups."

* Do you know the meaning of these fitness physiology "buzzwords": "The overload principle" (if you give a muscle more to do than it's used to, it will improve) and "training specificity" (you'll get good at whatever your practice).

* Show me how to do a sit-up. If they don't bend their knees, "walk away." Straight-leg sit-ups work unnecessary muscles and can strain your back.

* What exercise will help me lose weight? Aerobic activity is the best calorie burner. "Weight training will increase strength and improve muscle tone but does little for the cardiovascular system or weight loss."

* Can I see a sample program? It should incorporate muscles throughout the body, start off easy and progress gradually. "If they work your triceps but not your biceps, you wind up with 'gorilla-bent' arms."