Longer breath-up period
After a 2 min dive, a diver needs to rest motionless on the surface for 4-10 minutes to expel carbon dioxide from the body and to resaturate tissues with oxygen. Any hyperventillation is dangerous, because it expels too much carbon dioxide from the body and delays the urge to breathe. Always breathe deeply but slowly.
Endurance fitness
Exercise teaches the body to consume oxygen more efficiently and work under conditions of hypoxia (lower oxygen), hypercapnia (high carbon dioxide), and excess lactic acid. It improves the brain's hypoxia tolerance and increases the myoglobin content of muscles.1 High myoglobin is a major reason many aqautic mammals can hold their breath so long.
Increased blood volume
Blood volume increases with athletic exercise. More blood means more hemoglobin to carry oxygen.
Lung capacity
Lung size can't be increased, but you can use your lungs more effectively by stretching the diaphragm and ribcage before breath-holding. Some professional freedivers use a technique called "packing" to push down up to 50% more air into the lungs, but excessive packing can lead to injury.
Economy of movement
Trained divers move gracefully through the water and make no unnecessary movements.
Tolerance of urge to breathe
Holding your breath becomes more comfortable with training, as any physical exercise. It can even be a missed sensation. Learn to relax and tune into the body's signals, like the contractions of the diaphragm and the psychological urge to breathe.
Mental and physical relaxation
Stress, fatigue, and fear tax the body's oxygen reserves. You should only dive when well rested, relaxed, and completely focused on the dive. Even economy of thought can conserve precious oxygen.
Cool water on the face
Water on the face induces slowed heart rate (bradycardia) and oxygen metabolism. You can hold your breath longer after the first "warm-up" breath-holds, as the diving reflexes kick in. You can hold your breath longer in water than on land and in cool water more than in warm water. Water at depth is cooler, so depth of the dives can help stimulate diving reflexes.
Water pressure
Redirected bloodflow from the extremities to the core (peripheral vasoconstriction) is one of the reflexes triggered by water pressure, which affects breath hold time.
Exercises
Never train in water alone.
Endurance
Jogging, swimming, row machine, biking, and high-rep/low-weight exercises are ideal for divers.
Interval training
Alternate work at full power with short periods of rest. Run as fast for 2 minutes, rest for 30 sec, then repeat. Swim the length of the pool under water, rest, then repeat several times (make sure someone is watching you closely).
Underwater Rugby or Hockey
Team sports like underwater rugby or hockey help build breath-holding skills.
Stretching
Learn to stretch your ribcage, abdomen, and diaphragm. This will help you take a fuller breath and hold your breath more comfortably. Yoga is a great source of stretching exercises. Practice slow exhalations as well as forceful exhalations to empty the lungs as much as possible.
Apnea walking
Practice holding your breath while walking or jogging to force your body to adapt to low-oxygen, high carbon dioxide (hypercapnia), and high lactic acid conditions. You can start by taking every second breath, then every third breath. Professional divers can hold their breath for 1 min at rest and then walk up to 400 m all on one breath.1
Negatives
Practice holding your breath after a full exhalation to simulate the effect of water pressure as well as accelerate the onset of the breathing reflex.
