Muscle fitness can mean you have muscles that can lift heavier objects (strength) or muscles that will work longer before becoming exhausted (endurance). As you increase your muscle fitness, you will notice that you can carry heavy grocery bags more easily, pick up children without feeling as much strain, or carry heavy items longer before becoming too tired to continue. Having stronger muscles also protects your joints.

One part of muscle fitness is strengthening the muscles of your trunk. This strengthening is called core stabilization. It can help you have better posture and balance, and help protect you from injury.

Getting started on core stability

Muscles become stronger through a 3-step process:

1. Stress
2. Recovery (rest)
3. Repeated stress

When you exercise against resistance, you stress your muscles slightly but not to the point of serious damage or injury. When you rest, your body rebuilds the muscles and the connective tissues between them (joints, tendons, and ligaments) in a way that prepares them for the next time they will be stressed. When you stress the same muscles again, the process is repeated, and the muscles gradually become stronger.

A resistance-training program to increase muscle fitness can include:

• Basic muscle-conditioning exercises such as push-ups, leg lifts, and other familiar exercises.
• Resistance training with surgical tubing or stretchable bands .
• Weight training with free weights ("dumbbells") or weight-training equipment. See an illustration of muscle-strengthening exercises using weights.
• Doing housework and yard work, such as scrubbing the bathtub, washing walls, tilling the garden, or pulling weeds, on a regular basis.

Experts advise people to do exercises to strengthen muscles at least two times each week.⁷ Examples include weight training or stair climbing on two or more days that are not in a row. For best results, use a resistance (weight) that gives you muscle fatigue after 8 to 12 repetitions of each exercise.

Swimming, cycling, rowing, and skiing are activities that improve both muscle strength and aerobic fitness.

When you begin your muscle-conditioning activity, try to take 5 to 10 minutes to walk, jog in place, or do other activities to warm up, and do some stretches. Learn and pay attention to the proper form for all exercises.

As you build muscle strength and endurance, you will notice that you can do more and more of each exercise. Some people will see a change in the way their muscles look, but others will not see a change for a long while. A more important sign of progress is how many repetitions and sets of an exercise you can do, or how much easier it feels to do them. This means your muscle fitness has improved.

Talk to your health professional before starting a resistance-lifting program, particularly if you have high blood pressure, heart disease, or joint problems.

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