Got a smart phone? Put it to good use on your runs.

Running used to be simple. All you had to do was head out the door with a pair of shoes and a watch. Now, however, most runners have added GPS devices, heart rate monitors, and smart phones to their list of essentials.

A Nielsen Company study on smart phones concluded, “Most Americans can’t imagine leaving home without [them].” Runners are no exception.

But the study also argued that the No. 1 problem for smart phone users is finding relevant apps amid the thousands in the marketplace.

That’s particularly true in the crowded fitness space, where there are apps for every health-related thing you can think of and plenty you can’t. Here’s a list of five (well, six) running and fitness apps that are worth checking out.

MapMyRun and RunKeeper

When it comes to running and GPS tracking apps, we’re declaring a tie. Austin-based personal trainer Karen Shopoff Rooff, who recommends both MapMyRun and RunKeeper to her clients, said that the primary fitness app people ask for is something to track activities, store information, and create statistics over time.

While there are a number of these kinds of logs — among the most popular are Nike+, Endomondo and Strava — the two that runners gravitate to are MapMyRun and RunKeeper.

Both apps are free (though you can upgrade to a paid version) and both use the GPS in your phone to record your activities, including pace, distance and location. Both also sync with an online logging system that creates long-term graphs of your workouts, and both encourage you to share with friends.

RunKeeper also has a number of coaching plans to pick from. MapMyRun has more maps and routes, since it is born out of the popular mapping Web site, as well as a place to store your nutrition data.

“Having all of this information further motivates people by holding them accountable,” Shopoff Rooff said.

RELATED: Are GPS Watches Worth The Money?

MyFitnessPal Calorie Counter

The second most common thing people look for in fitness apps is a calorie counter or diet helper. MyFitnessPal, which has a food and exercise tracker to record calories in and out, is one of the most popular and addictive weight loss apps.

The free app brags that it has more food items in its database than any other calorie counter, which is important when it comes to logging multiple meals a day. The app also lets you scan a barcode to log an item. The usual challenges with logging creative ad-hoc meals are still inherent in this calorie counter, but you input recipes and have it measure the nutritional value.

With a primary focus on dieting, the app gives you the ability to track exercise and create goals that can be shared with friends — adding additional motivation.

A word of warning, however: don’t overstress about the targets it sets. Eat healthy and trust your instincts.

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Zombies, Run!

If you’re looking for more motivation from your smart phone, then Zombies, Run! may be the app for you.

Choose either the “epic adventure” for $7.99 or the more structured 5K zombie training plan for $3.99. Each version plays like a video game, forcing you to run away from zombies and save civilization. The app comes with audio cues and a story narrated over your music. It also syncs to a Web site for the game, where you can see how many zombies you avoided, what supplies you picked up, and how many people you saved.

“The app distracts you so much that you forget about running and, in the end, you run farther and faster than you may have originally planned,” said Wendy Flynn, a running coach and author of the One Tough Mother Runner blog.

And it works. According to the company, over 300,000 players have logged over 6 million kilometers and avoided 350,000 zombie hordes. (Note: as long as you run slightly faster when it tells you to, it’s hard not to avoid the zombie hordes.)

Pocket Yoga

Not strictly a running app, Pocket Yoga is (obviously) a yoga app, but it is easily the top yoga app in the marketplace. And, the convenience of having a yoga guide in your pocket might encourage more runners to actually do the stretching that they know they should.

For $2.99, Pocket Yoga includes a number of illustrations and instructions for each pose. You can either choose to whip out your phone and do a few poses, or select one of 27 full sessions, based on difficulty and length, and follow along.

Incredibly simple, the app has illustrations and explanations of each pose, as well as a dictionary for all those yoga terms. The sessions are narrated and designed by yoga instructors.

Plus, it’s cheaper than an actual yoga class.

RELATED: What Is Yoga Rave?

Charity Miles

Less than a year old, the Charity Miles app does one thing and one thing only: it raises money for every mile you run, walk, or bike.

After logging into the free app, simply choose one of the 20 participating charities and then start running. Every mile — tracked with the phone’s GPS — earns money for the charity you picked, at a rate of 10 cents/mile biked and 25 cents/mile walked or run. Corporate sponsors have backed the donations up to $1 million and, presumably, other sponsors will step up after that $1 million mark has been hit.

The app, which won an award at the South by Southwest Festival when it was introduced, has become incredibly popular primarily because of its ease of use. Just throw it in your pocket when you head out and you made a donation.

“I use Charity Miles for almost every run, at least I try to,” Flynn said.

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About The Author:

Kelly Dunleavy O’Mara is a journalist/reporter and former professional triathlete. She lives in the San Francisco Bay Area and writes for a number of magazines, newspapers, and websites. You can read more about her at www.sunnyrunning.com.