How to train lung capacity/endurance on leangains bulk?

[E_Scapegoat]

I am currently on a 3 day per week full body lifting routine, and have seen some great progress doing leangains (training an hour at 7am, and breaking the fast at 11:30) 2000-2500cal on rest days and 3500cal on lifting days. I look better than I ever have and feel really confident about my strength increases.

I am trying to work on becoming a better singer, and realize that my endurance and core is weak- so I want to incorporate steady state or HIIT cardio. I am okay if my gains are slow but steady- my bigger concern is nervous system burnout that comes with adding cardio on top of intense training. I could care less about cardio for fat loss- I already get plenty of that from diet and lifting. But my endurance sucks balls, and I don't use enough air when I talk, even though I've been trying to train myself to take deep breaths.

Which of these options do you think would help me focus on endurance and lung capacity without my nervous system imploding?:
1. 5-10 minutes of a few lower intensity HIIT every morning (with BCAA) (jogging between sets instead of resting)
2. 15 minutes of hiit 1-2x a week
3. 30 minutes of hiit 1-2x a week, progressively lower intensity and raising duration of each successive interval
4. 1 hour of swimming a week (a few laps, rest, repeat)
(all cardio would be on training days after training, except #1, if on a rest day)

Before you say it, I am already busting my ass on legs with low rest and have raised my core training volume. My main goal is to improve ease of breathing for my voice, over anything else. Thanks for your advice.

 

[scherbs]

#4 is the only option that will truly help your lung capacity (although you could combine it w/ your other options)

 

[E_Scapegoat]

do you feel swimming specifically, or the length of the workout is more important?

 

[scherbs]

Both

Swimming is a great exercise for building one's wind, as it were (I have no experience singing--but plenty w/ endurance training).
Moreover, HIIT is wonderful for many things, but short bursts of intense activity won't really help w/ lung capacity.
What you have described in option #1 really isn't HIIT, but rather aerobic intervals w/ active rest. That is another option, but the best one to build lung endurance is long and steady.