Does HIIT preserve muscle better than endurance training?
Studies show that HIIT combined with strength training preserves muscle mass well and is at least as effective as adding endurance training. The one thing worth bearing in mind is that bike-based HIIT may affect lower-body strength slightly more than other forms.
HIIT keeps your muscles in good shape when combined with strength training, but whether it works better than endurance training depends on exactly what you measure. At the level of the whole muscle, there is barely any difference: combined HIIT-plus-strength training produces similar muscle growth to strength training without a cardio component. This holds both for muscle size measured with imaging and for the growth of individual muscle fibres in studies using longer-interval HIIT.
There are nuances, however. When HIIT with shorter, more intense intervals is combined with strength training, muscle fibre growth at the individual level lags slightly behind pure strength training on its own. The measured difference is small but real. Lower-body strength also grows a little more slowly with combined training, particularly when the HIIT is done on a bike at high intensity rather than by running. Bike-based HIIT appears to 'compete' more with leg muscle training than running-based HIIT does. Resting longer between the two types of session reduces this drawback.
Notably more favourable are sprint interval protocols: short sprints of 10 seconds or less, combined with strength training, have no negative effect on strength or muscle growth and improve fitness (measured as maximum oxygen uptake) considerably more than strength training alone. Short sprint protocols even show an improvement in jumping ability.
For a direct comparison of HIIT with regular endurance training the picture is less clear. What the studies show is that combined training with continuous endurance work interferes with muscle fibre growth less than combined training with HIIT does. A small study in trained women found that high-intensity circuit training in less time produces comparable muscle gains to traditional strength training, although the study is too small to draw firm conclusions from. The practical summary: HIIT is not the enemy of your muscles, but if building muscle strength is your primary goal, it pays to schedule HIIT and strength training separately, to choose running over cycling, and to consider short sprint protocols.
This answer draws on three reviews that combine findings from multiple studies and two further studies in people. The reviews focus on combined training and look at muscle growth and strength; the other studies measure muscle protein building and fibre size directly. The findings on HIIT versus endurance training are less direct: the studies compare both approaches mainly with pure strength training, and rarely with each other head to head. The researchers themselves point to small sample sizes in several of the individual studies and variation in training protocols as limitations.