Karting is a sport that can be very demanding on the driver's physique.
Different race types can require different physical abilities. Indoor karting involves a lot of steering, high-speed outdoor tracks can produce g-forces, endurance racing requires driver's endurance.
Involved muscle groups Edit
The muscles involved in steering are shoulder muscles, specifically front deltoids.
To control the body position in cornering and overcome g-forces, abdominal muscles (abs) and lower back muscles are important.
Other important muscles are arms biceps and triceps, neck muscles.
Because of the low weight of a kart, driver's weight plays an important role in the system. Generally, lower driver's weight is preferable. Although higher weight will produce more traction while cornering, lower weight produces less centrifugal force, allows better acceleration and braking.
Don't sacrifice proper hydration pursuing lower body weight before a race. Even slight dehydration affects attention, reaction speed and proper judgement, which can be more important to overall success than insignificantly lower weight. This can be easily tested by timing the same driver without and with a ballast weight.