Sports
Defaults: DX-5 and others.
This is the ability to play a particular sport well – perhaps well enough to earn a living. Each sport is a separate Sports skill. Most Sports skills are DX/Average and default to DX-5, but those that put a premium on strength (e.g., rugby) might default to ST-5. Some Sports skills might default to one another or to other skills as well.
Make an IQ-based roll to recall the basic rules of your sport. Detailed knowledge of the full rules governing leagues and tournaments – as would be expected of a coach or referee – is covered by the relevant Games skill.
The GM may rule that certain Sports are useful in combat situations. For instance, Sports (Baseball) might let you use a bat to parry hurled rocks and grenades at (skill/2) + 3, Sports (Bullfighting) might give a Parry equal to (skill/2) + 3 against a slam by a beast that uses a "running head butt," and you might be able to roll against Sports (Rugby) to hit with a slam.
Martial Arts
Competitive martial arts use Combat Sport skills, not Sports. A lot of sports are essentially "bloodless" gladiatorial contests, though. Team sports in particular often involve violent contact. Sometimes, it's even legal.
Optionally, the GM may permit Sports rolls in combat; e.g., Sports (Rugby) to connect with a slam or Sports (Hockey) to attack with a two-handed stick. He might even allow techniques to default to Sports, where logical: Evade might default to Sports (Rugby), Hook might default to Sports (Hockey), and so on. Sports and techniques based on them are at -3 in a real fight – just as Combat Sport skills default to combat skills at -3. For a related topic, see Techniques and Combat Art/Sport Skills.