To prepare for his first marathon last year, Michael Nolan trained for six months with the New York Road Runners Club, running up to 20 miles a day five days a week.

Nevertheless, he finished with an average speed of about 11 minutes a mile, a full 60 seconds a mile off his target pace. This year, he vowed to be faster. “I didn’t want to take that long again,” he said.

So he got a personal trainer. Now as Mr. Nolan prepares for this year’s New York marathon, he is leaner, stronger and “easily” averages eight-and-a-half-minute miles on training runs, he said.

Mr. Nolan’s new workouts are not coached by a running guru, but by iPhone applications that show video workout instructions and tabulate every set of burpees, a full-body exercise for strength training, and step-ups.

The sports and health industries are just beginning to tap the computing power of smartphones. Applications range from simple calorie counters to heart-rate monitors that use complex metabolic calculations.

These apps can help an athlete achieve a personal best, but some doctors say that more important is their ability to produce no-fail routines for the sedentary and obese, which could improve health and drive down medical costs. Here are some of the popular fitness apps out there.

WEIGHT LOSS In theory, losing weight is simple: Just burn more calories than you eat. Martin Gramckow, an avid bicyclist who lives in California, had considered that fact since a cyclist he met on a ride bragged about losing 50 pounds. “I’m huffing and puffing trying to keep with him, while he leisurely pedals along and tells me how he did it,” said Mr. Gramckow.

The answer was calorie-counting. But Mr. Gramckow thought logging every morsel that passed his lips “was always too much work.” Then he saw Calorie Tracker an affiliate of the Lance Armstrong Foundation.

As with similar apps, Mr. Gramckow can search for foods by name or meals from restaurant chains and the app calculates and stores all of the nutritional information.

“Sure enough I find a couple of things in my meal plan that are out of whack,” said Mr. Gramckow, who trimmed 10 pounds in less than a month. “I’m not far away from being the fittest I’ve been in a long time,” he said.

Calorie Tracker, which is also available for the BlackBerry, won’t give you a breakdown of proteins, carbohydrates and fats (unless you sync it and view the result on your computer). The app also allows you to enter the ingredients of your own recipes and store a list of meals you commonly eat.