Suppose you are the most uncoordinated person in California.

And suppose the most athletic thing your spouse has ever done is drift-boat fishing.

Does this mean your 8-year-old child is destined to a lifetime of Nintendo and being the last kid picked for kickball at recess?

Not necessarily. But a new study--sure to dishearten some parents--points to heredity as a strong determinant of physical fitness and athleticism.

In the study of 105 pairs of 10-year-old twins and their parents, researchers found that fitness scores on such tasks as arm strength, jumping ability and aerobic capacity were remarkably similar between parent and child.

"What is clear is that genes have something to say, and they will determine what range [of fitness] you can expect yourself to end up in," says Hermine H. Maes, a geneticist who conducted the study in Belgium. "If you want to be a top athlete, for instance, you had better have a pretty good set of genes."

Using an intricate mathematical model that considered both genes and environment, the researchers concluded that heredity contributed to:

* Three-quarters of a child's ability to pull weights with the arms and do bent-arm hangs.

* Two-thirds of a child's vertical-jumping ability.

* Two-thirds of the ability of male children on aerobic capacity tests; almost 90% in female children.

The study was published this month in Medicine and Science in Sports and Exercise, a journal of the American College of Sports Medicine.

The researchers cautioned that the study only predicts the likelihood that a child will be predisposed to physical prowess. It nevertheless raises very practical questions about how to promote fitness and athletic endeavors among the children who may be destined to find soccer, softball, ballet or even P.E. class difficult and frustrating.

"There are definite health benefits to being fit. And if genes play a huge role, what does that leave for those not born with the genes that make you fit?" Maes notes.

Indeed, the study and others like it may shed some light on why so many families encourage their young children to participate in sports only to see them drop out soon thereafter, frustrated and embarrassed by their lack of ability.

According to one study, about 20 million school-age children enter organized youth sports each year in the United States. But by age 13, according to a Michigan State University study, the vast majority of them have dropped out. Besides stating that the activity wasn't fun, a majority of the kids surveyed said they quit because they lacked the ability to perform.