(Updated March 2015) Weightlifting, training for a triathlon and chugging along on your mum’s treadmill will all result in your body adapting to the given training stimulus. You’ll create more blood vessels, develop more mitochondria in your cells, trigger gene expression and transform your body right down to a molecular level. You’ll become better at what you’re training to do and collect on all the corollary benefits: health, fitness, energy, longevity, intelligence, happiness, calmness, etc. You’ll also spend more of your life feeling awesome, since exercise affects your neurotransmitters and releases endorphins.
So exercise in general is great. However, not all exercise is equally great. Different types of exercises accomplish different goals. Cardio and endurance training is mostly an oxygen delivery thing—more blood vessels, more red blood cells, more blood, etc—whereas strength training is mostly a muscle thing—more muscle fibres, thicker muscles fibres, more fluid in your muscle fibres, etc.
Both types of adaptations are incredibly good for your health. Weightlifting keeps you young, spry, strong, resistant to injury, lean, intelligent, focused. Cardio keeps you energetic, lively, calm, happy and strong-hearted.
So for optimal health you need to be both strong and fit.
The trouble is that mixing strength training and cardio together means that your body will be trying to adapt in at least two separate ways. Some people think that’s good, some people think that’s bad.
Things get more confusing still when you’re an ectomorph—naturally thin and already burning a ton of calories just by reading this blog post. That last thing you want to do is burn calories by jogging around town, since that will mean you need to eat even more. Ain’t nobody got time for that. (Kidding, everyone in the world has time for that except for us.)
Anyway, that raises the age-old question: should ectomorphs do cardio?