Your body does most of its repairs while you sleep. Not enough sleep, not enough recovery. Not enough recovery, reduced or no gains.
Using HIIT (high intensity interval training) is an excellent way to reduce your training time, especially for cardio. Depending on your current condition you can get the benefits of a 45 minute cardio workout in 15 minutes or less. That's according to a bunch of recent studies anyway.
If you choose HIIT I suggest incorporating movements that are very similar or the same as movements you use when performing. That triples the duty of the session. You get the cardio workout, you train the muscles you're going to use, and you get training on executing the movements that you're going to use.
Keep in mind, this study is an extreme example. It is the one that made the most headlines. There are many others. I just knew where to find this one quick.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0154075
This is a quick summation of the PLOS one study:
http://dailynews.mcmaster.ca/article/no-time-to-get-fit-think-again/
Here's a good article:
http://www.simplyshredded.com/fit-w...-on-endless-bouts-of-steady-state-cardio.html
I like HIIT training for stand-up fighting, especially if you use speed more than power like I do, because it replicates a fight. Lulls where you recover followed by a flurry of action followed by another lull.
Solid study the PLOS one. Also, I think it's the longest duration wise I've seen of HIT protocol training. There is definitely no doubt that HIT, at least to some degree, works very well for many things with less time investments. However I think it's important to acknowledge that they are not the same, and do have different adaptions. There are some caveats with the HIT studies out there which a lot of people neglect. First of all are the parameters. Yes, HIT increases Vo2max equally with less investment AND is even better for transporting glucose away from the bloodstream. It's also an excellent way to generally combat metabolic syndrome, and excellent, in accordance with strength training, for most diabetics. However when it comes to the amount of oxidative enzymes and ability to transfer FFAs away from the bloodstream it is debateable. The creation of additional myoglobin in muscles and the capillary network is not going to be as great with HIT, compared to LISS, and the fibertype characteristics go towards either ST or FT depending on the modality.
The arterial adaptions are also different between the two, highlighted in this study with obese adults:
https://www.ncbi.nlm.nih.gov/pubmed/27255523
This study:
https://www.ncbi.nlm.nih.gov/pubmed/27500951 showed that myocardial vascular hyperaemic reactivity was impaired after HIT training, meaning that the bloodflow back to the coronary arteries after HIT was impaired, which is not optimal for someone in poor shape or with heart problems.
Most importantly is the central adaptions of the heart. The majority of these HIT studies are simply too short to gauge the proper cardiac hypertrophy effects with HIT training. That or they do not measure them. There is preliminary evidence showing that HIT is certainly not a good training modality when it comes to subjects who are already hypertensive. This study, also from PLOS, highlights some of that:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4372563/ Disclaimer, it was done in rats, but it did manage to test for some of the markers that other studies have not. Hypertensive rats showed a marked reduction in the fibrous tissue of the left ventricle (fibrous tissue is scar tissue which can form in the heart muscle over time with high bloodpressure) with LISS training. It also improved the capillary/fibre ratio markedly. HIT did not improve these factors, actually, it worsened some of them. Here is another simular one in hypertensive rats:
https://www.ncbi.nlm.nih.gov/pubmed/25855305
Obviously in healthy subjects the story is different, but what it really comes down to is eccentric vs concentric cardiac hypertrophy. The differences has been somewhat well established in endurance trained athletes vs power or strenght athletes:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2300466/
http://www.scielo.br/scielo.php?scr...0100-879X2011007500112&lng=en&nrm=iso&tlng=en
Eccentric cardiac hypertrophy primarily increases stroke volume and creates a larger and more flexible ventricle (heart chamber), while concentric cardiac hypertrophy increases left ventricle thickness and rigidity. Both have their uses and obviously it's roughly sketched because they overlap somewhat. Still, what drives concentric cardiac hypertrophy is the increase in blood pressure and less diastolic filling (when the heart fills with blood). During HIT some of the same principles are in play, like high blood pressure. The heart also pumps so fast that the diastolic filling is cut short, leading to a more concentric oriented hypertrophy adaption. It is not pathological, like it is when you have chronic high blood pressure, but long term you simply do not reap the same endurance benefits as you do with LISS. The adaptions are different.
There are more, but I wont keep harping on. Like different energy systems, endothel oxidation, plasma levels in the blood and blood thickness (hematocrit value) and more. I wrote about som in a more reader friendly way some time ago here:
http://forums.sherdog.com/posts/128248557/
Last thing I want to mention is parasympathetic vs sympathetic nervous activity and recovery. HIT training is predominantly sympathetic. It's high fueled, adrenaline and action. That will eat into your recovery time and can lead to overworking if not done at the right. That is just keeping it brief, because recovery very much benefits from LISS training.
So while it cuts training time, sympathetic activity can impact sleep negatively, especially if done before bedtime. It also mean you might need more sleep to recover, than you would doing LISS.
Both modalities are great, both are needed for an athlete, but they are not the same. IMO, you build capacity and base first (LISS) and then work towards HIT, so it depends on the person.
Anyway, sorry for the long winded article, but I feel like it's important to contextualise the HIT craze.
