19/03/2026
Zone 2 cardio has become one of the most talked-about training strategies in recent years.
It’s often promoted as the key to building mitochondria, the structures inside our cells that produce energy.
But recent research reviews suggest the story may be more complicated.
A large analysis of studies examining Zone 2 training concluded that there isn’t strong evidence that this intensity is uniquely effective for increasing mitochondrial capacity.
In fact, the available research suggests mitochondrial adaptations tend to increase with exercise intensity, meaning harder efforts may produce larger changes.
That doesn’t mean Zone 2 has no value.
It allows people to accumulate longer training sessions with less fatigue and plays an important role in endurance training systems.
The more useful takeaway is probably this:
No single intensity appears to deliver every adaptation.
Low-intensity work builds endurance and recovery capacity.
Higher intensities stimulate stronger physiological signals.
Most successful training systems quietly include both.
If you think about your own exercise routine, does it expose your body to a range of intensities — or mostly the same one?