There are really four factors that influence how much exercise a child will do:

• Natural ability

• Physical conditioning

• Personal interest

• Medical problems

Natural ability refers to the capabilities a child is born with.  Some children seem to have an easier time with certain activities, whereas others have more trouble.  These differences may be related to body shape, muscle size or degree of coordination.  Sometimes it seems that natural ability runs in families.

Physical conditioning refers to the degree to which natural ability is improved through practice and exercise.  The more practice and exercise one engages in, the more skill and endurance one can expect in a given physical activity.

Personal interest refers to the degree to which a child wants to exercise or play sports.  Some children have little interest.  As a result the exercise less and are not well conditioned.  Their exercise capacity or stamina will be less than a child who enjoys exercise or sports, practices them a great deal and is therefore better conditioned.

Finally, medical problems can certainly affect the ability to exercise by limiting a child's strength, senses, coordination or heart or lung function.  A child with exercise-induced asthma, a muscle disorder or heart condition may be limited in how vigorously they can exercise, or for how long. 

When a family is concerned about a child who seems to tire easily it is helpful to consider these four factors.  An initial medical evaluation is often warranted to rule out medical problems; usually the primary care physician will check the history and examination, and then may or may not recommend further evaluation by specialists.  Once medical conditions have been satisfactorily ruled out the other three factors can be considered.

Often children who are less interested in exercise resist physical activities and become deconditioned.  Children may be disinterested for a variety of reasons:

• Children who do not feel that they have good natural ability may feel embarrassed to play sports with their peers, who seem better able to compete.  For these children non-competitive activities may be more inviting, such as bicycling, running or swimming.  Martial arts also provide a structured form of exercise in a non-competitive environment in most cases.
   
• Increasingly there are very interesting things for kids to do that do not include exercise.  Cartoons are on television 24 hours a day instead of just Saturday mornings as was the case 40 years ago.  Computer games and arcades provide entertainment and social interaction that once came with playing ball or riding bikes.  Limiting access to television, computers and arcades may help encourage alternative, more physical, activities.
   
• In many neighborhoods the opportunity for physical activity is limited by safety factors.  Parents keep a closer watch on kids, and may feel safer keeping them in the house during more of the day.  In this situation the family may need to design family activities that include exercise, such as trips to the park, running, swimming or bicycling together.
   
• Children who are overweight find exercise particularly difficult, which can lead to a spiral of deconditioning, exercise avoidance and further weight gain.  These children may benefit from a more structured exercise program that aims to gradually increase stamina.  For example, a daily walking route may be established with the introduction of gradually increasing stretches of jogging, with the aim to increase the running time (or to decrease the total course time) a little bit each week.