(CNN) "Seven of the ten most common chronic diseases are favorably influenced by regular physical activity," the US Department of Health and Human Services says in its latest report on physical activity. Let's pause there and just consider just how essential exercise is for optimal health and disease prevention.

Yet nearly 80% of Americans don't meet the agency's weekly recommendations: 150 to 300 minutes of moderate exercise or 75 to 150 minutes of vigorous exercise. In addition to either of those, it is recommended that you do two or more days of muscle-strengthening exercises involving all of the major muscle groups: legs, hips, back, abdomen, chest, shoulders and arms.

If you are one of the minority of people who meet these guidelines, congratulations and keep up the good work -- for the rest of your life. If not, it's never too late to start or increase your weekly exercise regimen. And even if you don't meet the full recommendations, any amount of exercise, in addition to less sedentary behavior (i.e. sitting), is beneficial. Whether you are a gym junkie or a novice, exercising correctly is essential for improving fitness, optimizing health and preventing injury.

As you commit or recommit to this life-extending goal, here are three common mistakes to avoid.

Under-training

Without specialized fitness testing, it is impossible to know exactly how intensely you are or should be exercising, and the intensity of a specific activity can vary based on your fitness level. In general, though, a person in the middle of moderate-intensity aerobic exercise should be able to talk but not sing. And a person doing vigorous activity should not be able to say more than a few words without pausing for breath.

To optimize the health benefits of aerobic exercise, make sure you're at least meeting this level of intensity, even if you're not always hitting the recommended frequency and duration. Also, the fitter you become, the less intense any given exercise becomes for you, so your workouts should be constantly evolving. You should try to push yourself beyond your threshold (not necessarily during every workout) by changing the intensity, type or duration of aerobic exercise, recommends Los Angeles-based cycling instructor and studio owner Shirley O'Connor.

We often don't physically work hard enough to build muscle strength and power. Many women fear "bulking up," for example, and older adults may have a fear of injury. Falling short can be due to inadequate resistance (from weights, bands or even body weight, as in a pushup) or an insufficient number of repetitions.

Muscle-strengthening exercises should be performed to the point at which it would be difficult to do another repetition. According to the American College of Sports Medicine, one set of eight to 12 repetitions of each exercise is effective for enhance muscle strength, although two or three sets may be more effective.

Training too specifically

Having too narrow a training scope is common in athletes and in veteran and novice exercisers. According to former World Cup-winning soccer player Brandi Chastain, who is now coaching young women, this is even a problem in kids who are being asked to choose one sport early on.

The specificity of a sport is causing overuse problems from repetitive movements, leading to an increase in the likelihood of injuries. This can be seen with vigorous forms of aerobic exercise, which is why it is important to combine strength training with balance and flexibility, to build or maintain muscle, tendon and joint health and to minimize your risk of injury.

In addition, many people train only specific muscle groups to improve their appearance or performance, but it is critical to work all major muscle groups, including opposing muscle groups (chest and back, biceps and triceps, hamstrings and quadriceps), in order to avoid muscle imbalances which lead to pain and injury.

Pushing too hard too fast

Trying to do too much too soon is a common cause of muscle soreness and injury, especially during the initial weeks of an exercise program. And it's often due to unrealistic goals and timelines.

It can take weeks to months to build a solid cardiorespiratory (heart and lungs) and musculoskeletal (muscle, joints, tendons) foundation, especially if you've been inactive or are starting a new or more intense exercise program. If you're just starting an exercise program, the American College of Sports Medicine recommends that you "start low and go slow" by starting with lower-intensity activities and gradually increasing how often and how long activities are done.

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"The risk of injury to bones, muscles and joints is directly related to the gap between a person's usual level of activity and a new level of activity," the college explains in its official position paper on prescribing exercise.

For strength training, adequate rest (48 to 72 hours) between sessions is also important to allow optimal muscle development. Personal trainer and gym owner Ami Jampolis (my sister) also notes that a lot of injuries occur due to inadequate or improper warmup and cool-down. She recommends dynamic warmups that are multi-directional to get your body moving in different planes of motion, and static stretching after the workout.

Doing your best to avoid these common mistakes, along with eating a healthful diet and getting adequate sleep, is essential in reaching your health, fitness and performance goals.