The question it simple yet hard to answer. To answer we have to look at how a generator works and how they can connect to work out machines. Let’s start with the generators. A generator has a stator, a set of electrical conductors wound in coils over an iron core. Then a moving rotor generates a rotating magnetic field around the stator and produce the alternating current (AC) output of the generator. The connection between a generator with a cross trainer for an example is that the movement from the cross trainer starts the rotor in the generator. This is then the exact same thing in all of the work out machines, the movement starts the different generators. 

The technique is clever and can solve more problems other than just taking care of the kinetic energy. Gyms with machines driven by kinetic energy could both operate by themselves and if the electricity network goes down the produce energy from the machines could operate parts of the gym. It’s unfortunately not the case the energy could operate the whole gym because the machines and persons couldn’t produce so much energy. Another problem kinetic energy driven machines could solve is heating. It’s common in larger gyms where a lot of machines are in use almost the whole day, extra heat is produced from the machines and persons. This extra heat will be cooled by the gyms cooling system to contain a good temperature indoor, and to do that extra energy is used. A bad spiral is formed where the kinetic energy is not taking care of and extra energy is used. On top of all that the used energy isn’t green.  

Model made by Anton Goman