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A Spearfisher's Secret Super Power

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New Member
Sep 8, 2023
*I'm not sure if people on here enjoy these types of educational posts, but this is a write-up I wrote for my latest newsletter and I thought it would be good to share it. You can check out other ones here: https://thestoneshot.beehiiv.com/

The Mammalian Dive Reflex (MDR)

The mammalian dive reflex, found in all mammals, including humans, is an automatic response to being submerged in water. It triggers a slower heart rate (bradycardia) and redirects blood away from non-vital areas to conserve oxygen. This adaptation allows mammals to stay submerged longer while minimizing oxygen use.

It is the reason why, before 6 months of age, when a newborn baby is underwater, the baby’s windpipe automatically closes to prevent water from entering the lungs, and why whales and dolphins can survive such great depths.

The 3 Main Benefits Of MDR
  • Slower Heart Rate: Bradycardia occurs which is the slowing of the heart rate. This conserves oxygen. When your face is immersed in water, the heart rate slows by 10% to 30%. Freedivers who train their MDR can achieve up to a 50% heart rate reduction.
  • Reduced Blood Flow: Peripheral vasoconstriction. This term refers to the narrowing of blood vessels in the extremities (arms, hands, legs, and feet). During the MDR, it happens to conserve blood and oxygen for vital organs like the heart, lungs, and brain. Reduced blood flow to the limbs means more oxygen for critical body parts.
  • Blood Shift: When peripheral vasoconstriction occurs and the diver descends, blood from the limbs moves toward vital organs and fills the space left as the lungs compress. This process, known as the blood shift, prevents the chest and lungs from crushing under water pressure. Blood doesn't compress, maintaining volume as the diver goes deeper.The Not So Fun Part of MDR
The Not So Fun Part of MDR

It makes you have to pee! The blood shift causes an increased blood volume in the body's trunk. In response, the kidneys detect this change and attempt to restore the body to its usual state by removing excess water from the blood through urine synthesis.

And what happens when you pee out all that extra water? You get dehydrated. That’s why it’s important to drink more fluids than normal while diving.

Another downfall of MDR is the build up of lactic acid. This is the burn you feel in your muscles, whether it is in your legs from finning or in your arms from free immersion, and what causes cramps or soreness.

How To Strengthen Your MDR

To enhance your freediving skills, you can take specific actions to bolster and condition the mammalian dive reflex:

  1. Stretch the intercostal muscles and perform Uddiyana Bandha
  2. Practice depth regularly
  3. Increase depth slowly

PS - If you like this type of stuff. I send out a weekly spearfishing email newsletter. It's free and comes out every Thursday. Subscribe here
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