Just to play devil’s advocate and not because I’ve drank the HIIT Kool-Aid, so I won’t regurgitate any Wikipedia HIIT defenses, but try and express my own stance on the muscle sparing of HIIT vs traditional cardio from my own understanding of physiology. Much of what I say you’ll already know, so it’ll be easy to follow.
Our body’s number priority in any kind of stressor we undergo is to be proficient at such stressor, because efficacy ultimately leads to self preservation. The body perceives stress as something that must be overcome. For example, the biological purpose your chest grows in response to a heavy bench press is to preserve the heart & lungs. Cause and purpose are different, motor unit recruitment, muscle hypertrophy, sarcolemic hypertrophy are causes, but the purpose of this biological function is to guard the thoracic region from being crushed in by the perceived stress of a heavy bench press. The purpose of muscle growth is to protect the body in some fashion, whether it be joints or organs, it’s a self preservation issue and therefore, proficiency at responding to stressors.
In traditional cardio, or jogging, the way the body becomes proficient at handling the stress of jogging ie.. Protecting the joints of the lower extremities, relieving the efforts of the Heart & Lungs to compensate for the energy, oxygen, nutrient demands of such stress, is to relinquish weight. Muscle is the heaviest weights we carry around, so although jogging burns fat for fuel, our bodies also enjoy relieving the luggage that outweighs fat, which is muscle.
All that being said, this pertains primarily to physically jogging on the street or field where joints are stressed. Static cardio on a stairclimber, much, much less catabolic, or a spin bike, something less impacting on joints, therefore, less stress, therefore, less of a reason to relinquish heavy baggage for proficiency and self preservation.
HIIT are only a few seconds burst and the duration is generally much shorter, so heavy baggage is less of a factor. Now, THAT being said, I can’t quantify X amount of traditional cardio = to X amount of HIIT in terms of physical stress. I’m not really trying to sell you on the idea, as much as sharing where I can see how this muscle sparring business can be reasoned.
