Remember how alive and energized you felt after your first week of yoga or Spinning classes? In an effort to recapture that bliss, you've stuck to the same routine ever since, setting yourself up to hit a fitness plateau, burn out, or — even worse — suffer a debilitating injury.

“We're creatures of habit, so the routine is easy,” says Christopher Mohr, RD, an exercise physiologist and dietitian in Louisville, Kentucky. “But a routine means you'll never grow fitter; your body needs constant change to progress.”

With help from some of the top fitness experts in the country, we put together a simple plan that will fit easily into your lifestyle, whether you've been exercising for five weeks or five years. Try it; your body and your mind will be glad you did.

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CURRENT REGIMEN: Cardio, 2-4 times/week

Regularly making your heart pound helps pump oxygenated blood to your brain and muscles. “Minute for minute, you enjoy the biggest health benefits from cardiovascular exercise,” says Mohr. “And high-intensity cardio work is the best way to burn calories.”

Plus, if you're running or cycling, you're probably outside enjoying the fresh air and reconnecting with the natural world, says Tom Holland, a certified athletic trainer and endurance-sports coach based in Darien, Connecticut. Even in a city, says Holland, going outside keeps the routine fresh and attractive.

What's missing: Flexibility and balanced strength. “When you do just one sport,” says Holland, “you overdevelop the muscles needed to do that sport, while other muscles grow weak by comparison. It's a classic setup for injury. Runners have overdeveloped hamstrings [the muscles on the back of their thighs]. Cyclists overdevelop their quads [the muscles on the front of their thighs].”

Performing only one type of exercise causes your range of motion to shrink, as well. Because the body is smart about minimizing the energy and muscle it needs to do a certain activity, any movement outside of the one it's used to doing can strain both those big, developed muscles and the ones that get little use. The result can be a painful muscle pull or nagging joint pain.

The fix: For runners, bikers, and elliptical fans, Holland recommends twice-weekly trips to the yoga studio for Ashtanga yoga, or “power yoga,” which focuses on lunges, push-up-type positions, and balance to build strength and flexibility.

Lindsay Hyman, a coach to triathletes, runners, and cyclists at Carmichael Training Systems in Colorado Springs, Colorado, also suggests that cardio addicts take one of their scheduled exercise days to go for an easy bike ride. “So many people burn out on their sport because they go all out all the time in their quest to burn maximum calories,” says Hyman. “But the body — and mind — need to recover in order to bounce back feeling fresh.”

CURRENT REGIMEN: Strength training, 2-3 times/week

“The older we are, the more important strength training becomes,” says Mohr. That's because working through exercises that exhaust your muscles keeps them strong, shores up the connective tissue around your joints, and builds the bone density that wards off osteoporosis. Studies have shown that a regular weight-lifting routine can significantly slow the creaks and pains of aging — even among people who engage in regular aerobic exercise.

It also helps you enjoy daily activities. “I work with accomplished triathletes and runners who want to work out with weights so they have the strength to pick up and play with their kids,” says Hyman. “It doesn't necessarily help them in their sport, but it definitely improves their quality of life.”

What's missing: While there is a cardiovascular component to lifting weights — the panting that comes at the end of a hard set of lifts is your heart going into overdrive — you're missing out on the sustained aerobic benefits enjoyed by runners, swimmers, cyclists, elliptical-trainer users, and even walkers.

“A long, steady run of 30 to 60 minutes builds endurance,” says Hyman. By building stamina, you're teaching your body to handle sustained physical stress as efficiently as possible. And — wouldn't you know it — the better you can handle physical stress, the better you can handle mental stress.

The fix: There are a couple of options. First, turn your weight routine into a cardio workout. “If you're doing weight circuits [in which you move from exercise to exercise to complete one circuit, instead of doing three or four sets of the same exercise before moving to the next], don't rest between exercises,” says Holland. By not letting up, you'll keep your heart rate elevated throughout the circuit. “Just be careful to not push yourself too hard,” he warns. “You don't want to be out of breath. These aren't sprints.”

Second, take a brisk walk on a hilly route for an hour or so, ideally every day. That's easy if you're a dog owner. Hyman also recommends swimming. “In the pool, you take the weight off your body so it can enjoy a break, yet you still get a great aerobic workout.”

CURRENT REGIMEN: Yoga, 2-4 times/week

The practice of yoga has stood the test of time because it does an excellent job teaching the body total balance and flexibility, as well as tuning you in to the connection between breath and movement. It strengthens the torso's core muscles and, from there, allows you to connect, say, the power of your legs to your ground strokes on the tennis court. Yoga also heightens the mind-body connection, says Mohr. “And being in tune like that makes every movement easier.”

What's missing: “Yoga's not a weight-bearing exercise,” says Hyman. “And that's a critical need for women — especially postmenopausal women, who need to worry about osteoporosis. The impact of running spurs your bones to grow stronger.”

Although moving through yoga poses does provide some aerobic benefit, it doesn't compare to the high-intensity cardiovascular workout that comes from Spinning class or a run. Those efforts do more than nuke fat and leave you breathless; they strengthen the most important muscle in your body — the heart.

“A good, hard bit of cardio completes what I call the circle of fitness,” says Hyman. “Endurance, flexibility, strength, and a strong heart are all necessary to grow healthier.”

The fix Hyman recommends running at least three times a week for 30 minutes or longer. After a 10-minute warm-up, do 15 minutes of high-intensity intervals, then take 5 to 10 minutes to cool down. Vary the length of your intervals: One week, do 1-minute sprints followed by 1 minute of easy jogging. The next week try 3-minute sprints followed by 3 minutes at a slower pace.

If running is too hard on your joints, do intervals by hiking up a steep hill. “You'll get your heart rate up, and the hike down will provide some stimulus for bone growth,” says Hyman. Or get your interval workout on a bike, elliptical trainer, or in the pool. To strengthen your bones, though, you need to do a couple of sets of multijoint weight-bearing exercises such as lunges. Start with ten lunges per leg and work up to 30, making sure your front knee never bends past your ankle. Keep your back straight and your chest open. For a bigger challenge, hold 5- to 10-pound free weights in each hand.

Fitness and health writer Grant Davis runs marathons, races mountain bikes, and does 100-mile bike rides from his home in Colorado Springs.