Sir Chris Hoy, the British sprint phenomenon doing a test in which he must deliver the highest possible output for two minutes. What a muscular body. Looking at his facial expression, he really goes to extremes. Then he suddenly stops and he falls off the bike. He breaks his fall with his left hand, but his feet are still stuck in the pedals. The test leader does not seem to panic and disconnects him to put him down on a proper way. On his back with legs up. On a mattress, on the floor, next to the bike. Apparently, this happens more often. Hoy is visibly not well. As he lies down on his stomach he feels better. He just wants to sleep.
What is happening?
During maximal exercise a maximum amount of blood flows to the legs. Because of the muscle contractions of the legs during exercise blood is pumped back towards to the heart, and the rest of the body. This phenomenon is known under the name of the muscle pump of the legs. When Hoy suddenly stops his effort the muscle pump stops working.
At the moment that Hoy stops his effort this muscle pump also stops. The consequence is that the blood still massively flows to the legs, but the legs do not send the blood back to the heart and to the rest of the body. This creates an acute lack of oxygen in the brains and collapse-like symptoms. Like an extremely fast heart rate and low blood pressure or hypotension. Beyond the pain barrier, the only way to win an Olympic gold medal.