In the quest for stronger, shapelier legs, the calf muscles are often ignored in favor of building up the powerful thigh muscles. But poorly developed calves, fitness experts say, can seriously affect performance in any exercise or sport.

"Although the calves are not the most powerful muscle group in the body, they are used constantly," said Sue Thompson, an exercise physiologist and fitness consultant in Rolling Meadows, Ill.

In walking, running or jumping, it is usually the calf muscles that work the hardest. They help to keep the body from falling forward.

The calves -- located on the back of the lower leg -- are made up of two major muscle groups -- the gastrocnemius and the soleus. The gastrocnemius, which visibly contracts when a person stands on tiptoe, starts behind the knee and makes up the bulky part of the calf. The soleus, which lies directly under the gastrocnemius, helps to form the Achilles tendon.

Weak calf muscles can also contribute to a common exercise complaint -- leg cramps.

"When the muscle fatigues, it contracts and starts to spasm," Thompson said. "It needs to be stretched out then."

People who are deficient in potassium -- a mineral involved in the contraction of muscles -- or who fail to drink enough fluid during exercise are more likely than others to experience muscle cramps.

To strengthen the gastrocnemius, Thompson recommends slowly rising up on the toes or ball of the foot while standing -- and repeating this several times. For the soleus, do the same toe lift exercises while seated. This allows the gastrocnemius to relax.

To maintain muscle balance and help prevent shin splints -- a tearing of muscle fibers -- exercise physiologists strongly suggest strengthening the shin area as well as the calves.

Place a five-pound weight around the ball of the foot and, while seated, lift the front part of the foot toward the shin, recommends Louis A. Sorto, a podiatrist in Chicago. Sorto suggests doing three sets of toe lifts with 15 to 30 repetitions per set.

Strong calf muscles can also lead to a more graceful looking and well-proportioned leg. "People who want to improve the shape of their legs," said Thompson, "don't realize that if they build up the calf muscles, the whole leg -- including the thigh -- will look better."