In order to increase muscle mass, your body must be forced to adapt. A state of balance is called homeostasis. The essence for inducing a strength and/or hypertrophy increase requires disrupting homeostasis. The way we do this with strength training is by lifting weights in a manner that exceeds what our body is adapted to. There's a number of ways to do this. You can increase the weight on the bar, increase the number of repetitions, increase the number of sets, or a combination thereof. If you do not in some way alter the variables to place a stress on the body that disrupts homeostasis, no impulse is provided for it to adapt.
After this disruption, the body will prepare for a repetition of this stress in the form of altering muscle tissue, bone density, vascularity and more. This occurs in the period of 48 to 72 hours after the workout, for a novice lifter. This means that any muscle group is ideally trained every 48 to 72 hours to maximize efficiency as a novice. After the adaptation, there is a period where one's performance is somewhat increased, making it possible to increase the workload by altering one or more of the parameters (weight, reps, sets, rest time). Doing this regularly in a way that doesn't exceed your capacity for recovery is what increases the baseline over time. After a workout, however, if no new stress is induced, you will gradually move back towards your previous baseline. When you stop working out, the body will no longer feel the need to maintain the metabolically more expensive muscle tissue and return to its old homeostasis.
If you are working out only once per week, you'll have started back on that downwards slope before the next workout is done. Even if you do an extensive full-body workout on that one day, this is a frequency that is very sub-optimal for a natural trainee (read: someone who doesn't use steroids). Perhaps you'll still be able to make progress, but it will be much slower compared to working out each muscle group twice per week. In the worst case you will see no progress.
Furthermore, trying to provide a stress that is enough to spur adaptation to avoid returning to baseline before the week is over will be difficult with just one workout. Total volume over a week is also important. You can't take the volume that you'd normally do in two workouts and try to cram it into one workout, because you would go well over your capacity for recovery, which might lead to injuries, or even a negative "return on investment".
So I'd strongly advise you to stick to two workouts per week. This leaves us with the interval training. HIIT is cardio training and will mostly benefit endurance, fat burning, adaptations for oxidative stress etc. It is not a substitute for weightlifting to induce strength gain and/or hypertrophy. That said, just doing cardio alone is not enough to negate muscle gain. As long as you maintain a caloric surplus, with sufficient protein intake, muscle gain can occur. However, unless you are a beginner, or very overweight, gaining muscle while burning fat could prove challenging or impossible. Hypertrophy requires a caloric surplus while weight loss is only possible using a caloric deficit. Trying to do a "recomp", slowly reducing body fat while gaining some muscle, requires very precise calorie intake and the progress will likely be slow.
It sounds like you want to make your workouts a bit more varied and include some HIIT for fat burn, or maybe save some time on your workouts. So maybe try this: do weightlifting twice a week, and do the HIIT training immediately after that. It will have some benefits:
- You will already be warmed up for the HIIT.
- The weightlifting will have depleted some glycogen and glucose from your body, shifting more quickly into the use of fat storage once you get to the HIIT.
- You'll already be in sportswear and will only have to shower once after the workout. Maybe details, but silly stuff like that can end up taking quite a bit of time and it's easy to forget to take it into account.
Sticking to a program for your weightlifting that has some variation between the two days can also keep things interesting. When you try to improve on your previous workout every week, or have to work on periodization (if you're past beginner stage) the whole thing becomes a lot more interesting than just getting to the gym every week and just lifting "whatever". It becomes a game, where your opponent is you from the past week, every week.
Good luck!
