Muscles and tendons develop and you become stronger by healing them beyond their initial state. By raising gravity, you increase the weight of your equipment, i.e., resistance in phase 1. However, your body would get stronger in general in order to cope with the increasing body weight. This is because muscle growth depends on stress, and stress increases as gravity does. The most obvious effect of increased gravity is that it makes you heavier. This comes with many other consequences though, such as: Larger muscles require more energy to move (oxygen & glucose). So, in order to keep moving you need to stop eating enough quality food and start using protein powders instead. Larger bones need more energy to heal. So, in order to stay active you need to keep injuring yourself (or use steroids), which are both bad for your health. Larger lungs require more energy to breathe out pressure created by large muscles. So, in order to stay alive you need to exercise intensely for a long time, which leads to fatigue and quitting before your body has had enough time to repair itself. Larger heart requires more energy to pump blood around the body. So, in order to stay alive you need to keep working out even when you're not feeling like it because exhaustion brings its own form of relief from pain.