Count slowly to 30. When you have reached 30, step out of the doorway and breath in. For 30 seconds, you contracted the muscles that raised your arms. Your muscles were under tension, which means they were in use. When you stepped out of the doorframe, your muscles had adapted to pushing out.
Muscles adapt very quickly.
Scientists are studying how the muscles of astronauts adapt to the low gravity of space. They want to know what kind of effects long space trips have on muscles and what happens when astronauts return to Earth or land on a planet or a moon.