Research has indicated that combined aerobic and anaerobic training (concurrent training) may improve aerobic performance more than aerobic training alone. High-intensity interval training (HIIT) is an example of concurrent training.

“Combining anaerobic and aerobic training in HIIT provides both cardiovascular and muscular benefits in less time,” says Nicola. “It can be more engaging and diverse than steady-state workouts and may lead to significant improvements in endurance, strength, and weight loss.” 

On the flip side, Nicola adds that concurrent training comes with a higher risk of injury due to the intense nature and, therefore, might not be suitable for everyone, particularly those with certain health conditions. HIIT can also lead to overtraining or burnout without proper rest and recovery. 

High-intensity interval training has become so colloquialized that many people don’t truly understand what it actually is and isn’t. The ACSM defines HIIT as alternating between periods of very high-intensity work (performed at 80% maximum heart rate or higher), followed by varying periods of recovery performed at 40-50% of maximum heart rate—or total rest. 

“True HIIT is very difficult and uncomfortable and is meant to stress and train your anaerobic energy pathways. What we tend to see more often in the group fitness or digital fitness areas are better described as interval training,” explains Reed. “Rather, they’re staying in heart rate training zones 3, 4, and 5, alternating between higher intensity work and steady state moderate intensity work—never returning to the 40 to 50% actual recovery between ‘all out’ efforts.” 

That’s not to say that the moderate-vigorous group exercise classes aren’t beneficial, but there is something to be said about returning to lower- to moderate-intensity zone 2 work for building your aerobic base.