The Games
The Games began back in 2061, marked by the return of Halley's comet, as seen from the home planet, Earth. The other planets hadn't been populated by the time of that first session of The Games, so the contenders back then had all exclusively lived on Earth. That first experience of The Games, however, set the tone for what was to follow.
On each return of the comet (approximately every 75 years or so) a new session of The Games was held in order to see which planet would win in gladiator-style combat--then ruling over the others for the next half of what was now the average life expectancy of 150 years.
The four contenders were Earth (diameter: 7900 miles), Mars (diameter: 4200 miles), Jupiter (diameter: 87,000 miles), and Saturn (diameter 75,000 miles):
Earth
The Earth colony utilized DNA from the plant and animal kingdom of Earth in order to create hybrid-fighters who often had suprising capabilities which they only fully revealed in the midst of the combat of The Games. The other planets had spies on Earth to try to uncover which sets of DNA were currently being used in order to create the "hybridized" humans--or "double H's", as they were called by regular people.
The first hybrids were the simple ones such as a 40%/60% cross with a gorilla. It had been determined to be against the law to make a human "mostly" an animal, and so the industry standard was set, somewhat arbitrarily, at a maximum of 40% genetic enhancement. The gorilla hybrids performed well in the first Games, but were very easy to identify and eventually to adapt to.
Future hybrids had more subtle and complex powers, such as venom capabilities, and even shark skin which lacerates you, if rubbed against you in one direction. The most dangerous hybrids, however, were found after trial and error--and they were the ones whose DNA had been supplemented by the DNA of insects, such as the Bombardier beetle (capable of shooting a jet stream of boiling-hot acid).
Mars
Being smaller than Earth in all respects (size, mass, density), the gravity on Mars is only 38% of that of Earth. The Mars colony trained in special gymnasiums called "gravitrons" which mirrored the gravity of the Earth or actually just slightly above it (to induce supercompensation in their muscles).
Their training focused mainly on martial arts and a dedication to perfection in fighting styles to the point that skills were honed to their biological limits with the use of reactobot automatons (RAs), which were sparring partners programmed to defend at exactly 99.9% of the level of speed and skill which was being used against them.
You could never be much faster or better than the reactobots, so there was never a time when you could dominate them in a fight--every victory was obtained by operating within that last tenth of a percent of a difference in both speed and skill. Every fighting style was matched up to its perfect counterpart--so that the fighter would instinctively adjust his style to an opponent, whether during an attack or a defense.
Jupiter
Jupiter is the largest planet but, back in its natural state, it was a gas giant instead of a terrestrial planet, so it had lower density than Earth before the terraforming of its gasosphere began. After terraforming the gasosphere with a reactant which transforms particles from the gas phase into their respective liquids and solids, the gravity on Jupiter ended up at a level that is 250% that of Earth.
The Jupiter colony was hand-picked for genetic superiority with regard to physical stature and, even then, many infants could not survive the gravity they were born into. The infant mortality rate decreased every generation, as the microevolution of natural selection took place--just as when race horses are intentionally bred for speed and stamina.
The average bone density of a member of the Jupiter colony was now up to about twice the bone density of someone from Earth, so an elbow strike from a Jupiter colonist could shatter your skull. They also had the muscle to move those heavy bones with great force and, when brought to Earth for The Games, the relative microgravity of Earth allowed them to leap at you from great distances.
Saturn
Because Saturn was like Jupiter but even less dense in its natural state, after terraforming its gasosphere, the gravity ended up at a level that is 150% that of Earth. Also, because it was the last planet colonized, the colony on Saturn had developed technology to the highest level on Earth before departing.
The contenders were fitted with microprocessors linked to, and supplementing, their own endocrine systems--including systems governing reaction time and even felt pain. The normal human brain has a "refresh rate" of about 30 cycles/second so that--within each second--there are approximately 30 "choice points" (instances when a choice could be made to move left, move right, etc).
However, it was discovered that in emergencies such as car crashes, the appearance of time slows down. It was found that this is because, during the emergency, there is a precise mix of "fight-or-flight" chemical messengers which effectively double the "refresh rate" of our brains, allowing us to have 60 "choice points" per second.
The Saturn colony discovered how to program a stored-up release of this mix of chemical messengers so that their fighters could remain in a heightened state of awareness and reactivity--where everyone else would seem to be moving only at half-speed during the fight (giving them twice the time to react to any attack).
Experiments uncovered a time limit of exposure, however, as the human brain would overheat after approximately 20 constant minutes running at double velocity. The microprocessors were programmed to turn off this high throttle when brain temperature approached 109 degrees, where permanent brain damage and sudden death become likely.
They trained under the influence of anabolic agents such as steroids, so their bodies were easy to identify but, by that time, it was often too late (because they were reacting twice as fast as you). Even when they were heavily-injured, they did not seem to slow down--due to the programmed chemical blocking of their pain receptors. They were known to continue fighting even after mutliple broken bones and, in a few recorded cases, severed limbs.
The Comet Approaches
The year was 2284, and Halley's comet had already passed by Saturn on its path toward Earth, so people from the 3 non-Earth colonies began gearing up for the optimal launch windows with regard to their space shuttles, which used a combination of ion drives along with photonic propulsion.
While contenders, trainers, and sponsors all had permanent passes to travel, lotteries were held in each colony to determine who would be lucky enough to witness The Games first hand, on Earth. Only 100,000 people from each colony would be lucky enough to be able to go and witness The Games, first hand, in the colliseum, with a capacity of a half-million.
The people at home would still be able to watch the live event in "real time" with only just a few minutes of delay, depending on broadcast distance. Everyone was excited for the lottery, especially the young ones--who each had picked out their own favorite fighters.
Image Attribution
Page URL: https://commons.wikimedia.org/wiki/File:Gladiatorer_utst%C3%A4llningen.jpg
Attribution: Marknaddjab [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]