Fat is lost much faster with intervals which is why they are usually used by football and wrestling programs as well as fitness models.
I think oovertraining is MUCH more of an issue with upper body intervals… Including battling ropes, burpees, and hitting the heavy bag has much more of an effect on weight workouts…. the muscles are smaller, and therefore have to be worked closer to failure to get an anaerobic cardio workout.
When using legs, the sprints, stairs, jumping rope (usually isn’t intense enough unless u are very heavy), etc….
Usually with legs you can gain some strength in the higher rep ranges (20 reps sets etc) from hiit(as long as it uses the same muscles, and the way to mitigate overtraining is to reduce # of sets or exercises on your lower body lifting day, and also go heavier, because you are working on coniditioning of the glutes, quads, and hams on a separate day (hiit)).
If you are worried about going too catabolic during workout, there are very effective supplements to fix this.
Hiit works via increases in igf-1 so any supplements that claim to “boost hgh” will be particularly effective while employing hiit.
Example: niacin, arginine, ornithine, bcaas, l-dopa, gaba (at night), alpha gpc etc etc….