Acceleration and de-acceleration are expressed in terms of "G", with 1-G describing the effects of gravity we experience under normal conditions while at rest on the surface of the Earth. Therefore, when accelerating to 2-G you would feel the force of gravity doubled. As G-forces increase due to acceleration, you begin to feel heavier and experience difficulty in moving your arms and legs. As G-forces continue to increase, you may experience difficulties in moving your hands and feet, and are not able to lift your head to look around. At 5-Gs, your ten pound head would weight 50 pounds, a load that your neck muscles are not accustomed to handling.
There are other effects of increasing G-forces as well. Blood circulation becomes impaired as the heart must work harder to pump blood through the body. This impaired circulation can lead to a dimming of consciousness as the heart can no longer pump blood to the brain. You will start to experience tunnel vision, then colors will fade away and everything will appear white, then fade to black. You have just experienced a gravity induced loss of consciousness, or blackout. In general, anything more than 4 or 5 Gs will cause the average person who has not been trained to deal with these effects to black out. Extremely high G-levels can lead to more serious complications such as compacted bladders, bursting red blood cells, subdural hematomas, the inability to breathe and the cessation of circulation.