While aerobic exercise increases endurance and helps the heart, it does not build upper body strength or tone muscles. Strength-training exercises provide the following benefits:

Build muscle strength while burning fat

Help maintain bone density

It is also associated with a lower risk for heart disease, possibly because it lowers LDL (the so-called “bad” cholesterol) levels.

Strength exercise is beneficial for everyone, even people in their 90s. It is the only form of exercise that can slow and even reverse the decline in muscle mass, bone density, and strength that occur with aging.

Types of Muscle Contractions. There are three types of muscle contractions involved in strength training:

Isometric contractions do not change the length of the muscle. An example is pushing against a wall.

Concentric contractions shorten muscles. An example is the “up” phase of the biceps curl.

Eccentric contractions lengthen muscles. An example is the “down” phase as weights are lowered.