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Training for freediving can take many forms and be done on the land.
One example is the apnea walk. This consists of a preparation "breathe-up", followed by a short (typically 1 minute) breath hold taken at rest. Without breaking the hold, the participant then initiates a walk for as far as they can, until it becomes necessary to breathe again. Athletes can do close to 400 meters in training this way.
This form of training is good for accustoming muscles to work under anaerobic conditions, and for tolerance to CO2 build-up in the circulation. It is also easy to gauge progress, as increasing distance can be measured.
Before diving, inexperienced freedivers hyperventilate to a certain degree, resulting in a lower level of CO2 in their lungs and bloodstream. This postpones the start of stimulation to the breathing centre of the brain, and thus delays the warning signals of running out of air. As the oxygen level of the blood is not increased by hyperventilation, this is very dangerous and may contribute to shallow water blackout and deep water blackout. Trained freedivers are well aware of this and will only dive under strict and first aid competent supervision. However this does not, of itself, eliminate the risk of deep or shallow water blackout. All safe freedivers have a 'buddy' who accompanies them, observing from within the water at the surface. Due to the nature of the sport, safety is an integral part of free-diving, requiring participants to be adept in rescue and resuscitation. Without proper training and supervision, free-diving/apnea/breath-hold diving is extremely dangerous.