Is it healthy to combine Strength training with HIIT cardio?
Why wouldn't it be?
Name a professional football, hockey, basketball, baseball or soccer athlete that doesn't strength train in this day in age to some degree?
Now think about what those sports are, if not basically HIIT? If not basically very very intense, very long HIIT.
See this word ‘healthy' it doesn't mean what everyone thinks it means or at least how we use it colloquially.
The issue isn't combining HIIT and strength training, it's combining them poorly. Plenty of people combine them with no problems, but they aren't killing themselves with it.
The issue really comes down to recovery and a lack of recovery can lead to poor health outcomes over time (rarely immediately).
Take for instance how most sprinters actually train. It isn't how most people think. You don't show up for 20 minutes every day do a few sprints and leave. Warm ups are typically longer than most people's workouts and workouts can last 2–3 hours.
Twice a week is real HIIT. That is, twice a week the intensity is about as high as it can be and on those days you'll typically finish in the weight room after you run, not before. So you're putting the stressor all on the same day so that you can use the other days for recovery runs, done at far lower intensities (like just at or above anaerobic threshold).
Typically this is referred to as the high-low method. It takes about 72 hours to recover from the very intense sprinting and the resistance training and to encourage recovery you need some sub-maximal technical work on the days in between. Those days also give you an opportunity to work on muscular endurance which doesn't take as much time to recover from as speed/power related stuff.
You can't redline your body at all times and expect a good outcome. HIIT is a jargon term most people don't comprehend fully to begin with. Take one of Mindy's Tabata HIIT cardio workouts for instance. I'm not hating on her, but that's not HIIT. It's calisthenics using a HIIT protocol, but Tabata's are done on a erg bike not with pushups.
Why? Intensities with pushups just can't get as high. It's an exercise for muscular endurance, not for improving energy system development. HIIT improves energy system output.
*Disclaimer: my blog...
Typically what happens in the real world is that most people pawn off interval training or bodyweight circuit training as HIIT and what they really end up doing is a workout that is practically and technically speaking aerobic. Meaning it may have an interval protocol (like 20 seconds on, 10 seconds off) in there but that doesn't make something HIIT, the intensity does. The overall intensity of a Tabata bodyweight circuit is a far lower intensity than true HIIT.
It's still generally in the aerobic training zone in fact.
Which is cool, again I'm not hating on that, the point is that's probably a good thing for most people.
Provided you don't mix and match too many muscle groups together too frequently that is. Doing bodyweight circuits to fatigue on days between lifting is a bad idea because they are more similar than they are dissimilar. However, light bodyweight training never to fatigue can promote recovery; There is that nuance again. You're going to get more of an interference effect than if you did deliberate erg biking on off-days between moderate rep range lifting. The same reason you'd over train if you did intense isolation resistance training on back to back to back days over too long a period of time.
You can run yourself into the ground doing nothing but strength training and real HIIT. Very quickly.
The bodyweight circuit training that people try to pass off as HIIT combined with strength training means that most people can actually recover from the training and thus it is generally not an issue. If it's planned well enough.
It's all in how you plan the training and spread it out over the week/month.
If you did strength training on Monday, HIIT on Wednesday, Strength training on Friday HIIT wouldn't be an issue. If you did Lower body strength with HIIT on Mon/Fri, and upper body strength training with aerobic work on Tues/Sat, again you most likely wouldn't experience an issue.
HIIT typically taxes the lower body more heavily, especially running based protocols but non-weight bearing protocols like those on the bike or in a pool are easier to recover from too so modality needs to be a consideration.
You'd have to work up to it and plan a way to increase your intensity over time, while also planning deliberate down periods into your routine so that you can recover but there are a lot of ways to integrate HIIT with strength training successfully.
Most people just plan it out and execute their plan all wrong. They end up hurting themselves in the process or at least limiting the result they can get.
You shouldn't feel bad about that either, I spent a lot of time in school and in practice learning how to do it. It's my full time job, few people have fitness as their full time job, so it's unreasonable to expect you to know all of this.