The future is cramped.
After people started to collectively use their resources efficiently and clean up the environment, there was another population boom. Apartment complexes- which weren’t much more than stacked cubbyholes- are taller than modern day skyscrapers. There’s hardly any room to do anything.
There’s no room to run or play or even jump in place sometimes, but everyone is still really fit somehow. Some people even look like bodybuilders.
See, everyone wore these flexible bands on their wrists, hips, and ankles. These bands were connected to create axes on the body where, if the bands were moved out of alignment, the bands would exert force to pull themselves back into place. There were two axes that connected at the hip band, a belt really - a vertical one that connected the ankle bands and belt and a horizontal one that connected the wrist bands and belt. The belt was to be placed at the same height as the wrists’ resting height.
People were able to stay fit despite the lack of space and, unlike modern weight bands, the bands hardly took up any space and could be worn anywhere without causing awkward sliding or shifting.
Thus, despite the cramped world earth became, people were able to stay healthy. Humans didn’t become morbidly obese like how some movies depict the future to be. While sports weren’t as viable due to space, tasks like opening water bottles didn’t have to be delegated to robots.