Speeding up is acceleration, slowing down is acceleration and, rotation is acceleration. Traveling at a constant speed is NOT acceleration.
Even a broken clock is right twice a day.
So, when you jump up, your body accelerates due to stored energy in your muscles, right? Energy is also required to stop you from moving up and accelerate you down.
What causes you to accelerate downwards towards the earth? Why do you simply not fly into space?