I've been gradually adding these things called "intervals" to my workouts, particularly for my strength training sessions and my runs. Just what is or is not an "interval" seems to have lots of different interpretations.
At the softer end of the spectrum is what I'd call "surges", such as is done during fartleks, adding push and recovery segments to an otherwise ordinary run. At the softest end is switching back and forth between "race pace" and "cruising" on a longer run.
At the harder end is doing sequential 50 meter all-out sprints separated by brief pauses. The hardest of the hard end is doing them on hills or stairs, or while carrying weights.
I choose to be somewhere in between: Low enough to avoid damage to joints, tendons and muscles. High enough to be well beyond my endurance capabilities, to challenge my body's ability to mobilize energy in the muscles, to just shy of the point of being unable to continue with good form.
In strength training, my current practice is to do a sequence of exercises without breaks, each affecting different muscle groups, resting only briefly (under 2 minutes) when the entire sequence has been completed. Individual sets of muscles get challenged in each exercise, but not for long, typically 30-60 seconds. The cardio-respiratory system is challenged throughout the sequence.
The phrase used most in the literature is "High-Intensity Interval Training" or HIIT. A great term that also has lots of definitions and perspectives. Until recently there has been relatively little "hard" science done to quantify the effects of HIIT on various slices of the general population, with much of the prior work focused on elite athletes.
And as any age-grouper will know, what works for elites can easily fail for the rest of us.
However, it seems HIIT is special: Done right, HIIT seems to benefit just about everyone!
Let's start with some definitions and limits:
I. There are no absolutes in HIIT. What is normal activity for one person can easily be HIIT for another.
This is true even for the weakest of us: There is unambiguous medical evidence that doing limited HIIT immediately after surgery is of clear benefit to both cardiac and hip replacement patients. In the past it was thought that just getting these folks mobile was all that was needed, and in a general sense that's true, since for some even basic post-surgery movement is HIIT. But the bigger picture for these patients now is to get them up, work them hard (yet safely) for a moment, then get them back in bed.
For the rest of us, it is important for us not to all do the same workout. Each of us needs to tailor our HIIT to meet our abilities and goals. So doing HIIT as a group is not a good idea, though it is always OK to do your own HIIT workout while others are doing their own HIIT workouts.
II. HIIT is adaptive: What starts out as nearly impossible can soon become much easier.
How can this be? What's going on?
There are many components that go into "fitness", "strength", "endurance", "power" and related terms. Collectively they describe how hard we can use our bodies and for how long. Intensity versus time. There are many steps in the overall process that involve everything from eating, breathing, carbohydrate (glycogen) transport and utiliztion, fat catabolism, fatty acid transport and utilization, muscle mass, composition and density, ATP metabolism (mitochondria), and so on.
The primary (most obvious) measurable physiological effect of HIIT is increasing the number of mitochondria in muscle cells. We are still largely ignorant of much of what goes on in mitochondra. We can't tell all that much about the mitochrondrial activity going on within a muscle by examining the blood. It takes a muscle biopsy, then putting the cells under a microscope and simply counting the mitochondria.
What we do know is that more mitochondria means better energy production in the muscle, both for the long (endurance) and short (sprint) terms. There are certainly other factors involved, but the change in the mitochondrial count is a dominant factor.
Recent HIIT research has shown a simple result: Muscles utilized during HIIT exhibit an increase in the number of mitochondria present.
I want to make this point very clear: HIIT directly helps endurance performance.
There's a favorite triathlon truism: "Train like you race; race like you train." That is broken where HIIT is concerned. Train with HIIT, but never do an all-out 30 second sprint in a race. Other than to beat your buddy at the finish line, of course.
III. HIIT must be performed well within the body's limits.
Injuries are very common among folks "suddenly" adding HIIT to their workout regimen. It is important to start out gently, to prepare the body for the increased stress.
The most common HIIT injuries are muscle and tendon pulls, with severities going all the way up to detachments and tears. Taking any HIIT interval too far can also lead to joint injuries as form falls apart, and other injuries if falls occur.
In my own case, I had been doing lots of lunges, squats and jump squats, so I thought I was ready to add HIIT run sessions. On my second HIIT run session I got a moderate hip flexor pull: None of my preparation had included increased rearward leg swing. It is best to ramp up gradually, and let the body adapt to the actual exercise being attempted.
Vital point: Always stop at the first sign of pain! The old adage "No pain, no gain" is total bullshit. Fatigue and discomfort, even "the burn", are OK; the pain of damage never is.
IV. HIIT needs far more recovery compared to conventional workouts.
While many of us have no problem doing daily runs, daily HIIT run sessions are not a good idea!
The medical studies have various observations of the muscle's condition after HIIT, as well as that of the body in general. The overall picture is that HIIT depletes not only the energy stored locally in the muscle (needed for the mitochondria to convert ADP to ATP), but also depletes the muscle's ability to use energy.
I started out with one HIIT session per week, on the day before my day off. Even doing a light run on the day after a HIIT run session could be surprisingly difficult. I think the best plan is to not run at all on the day after a HIIT run session, having either a day off or just a swim and/or light bike day. And the next run should not be a hard one.
Pardon me while I climb back on to one of my favorite soapboxes: Foam rolling after HIIT greatly aids the recovery process. While you shouldn't do HIIT daily, foam rolling daily is a massive plus.
OK, enough about HIIT in general. What about the specifics? How am I, a perma-newbie in triathlon, using HIIT?
In my strength training workouts I use only my own body weight, and my only piece of equipment is a TRX. The specific exercises included in each workout cycle vary every time, and include random selections to push and pull with each skeletal muscle group (push-ups + pull-ups, squats + hamstring curls, kicks face-up + face-down, etc.), static and dynamic exercises for the core (planks, side planks, sit-ups, crunches, crab-crawls, etc.), integrated exercises to work secondary muscle groups (walking lunges, twisting lunges, kneel-to-stand, etc.), and finally runs with varying gaits (regular run, skipping, side-to-side, ice skaters, backwards run, etc.).
The only exercise type not presently included in any of my sequences are plyometrics, such as jump squats or clap push-ups. Doing these with perfect form as fatigue grows is almost impossible, and they simply add injury risk for minimal gains. I'm not saying I'll never include plyometrics, but only that they're not at all a priority.
Other than the run, each exercise is done for only 30 seconds, with no pauses between exercises within the sequence (not quite a scramble between exercises). After each sequence I take a sip of water and a brief rest that never exceeds 2 minutes. The goal is to have good sets with minimal rest. But always rest enough to ensure the next round is a good one! If two minutes isn't enough rest, then you're done, and no more sequences should be attempted.
It is important to note that I said nothing about the number of reps or the duration of each rep within each exercise. It is most important to always maintain perfect form. After that, it is important to maintain smooth flow, without jerking or tugging or bouncing. Finally, it is important to just keep moving, never stopping, but always permitting yourself to back off the level of effort when and if needed. It's the contact time that's important, done while always maintaining perfect form.
That's quite a bit of description for the strength HIIT sessions. My runs are much simpler.
First, it is important to never do a full-intensity run interval on a cold muscle (much like stretching), and it is also important not to stop cold after HIIT. So every HIIT run session should start with a warm-up run of a mile or so, then the intervals, then a cool-down run and/or a brisk walk for another mile or so.
Each of my intervals starts at a jogging pace then smoothly builds over about 5 seconds to a flat-out sprint. When I start to feel fatigued, I smoothly back the sprint down to a brisk walk. Wait up to two minutes and repeat. I'm done with intervals if I'm not recovered in two minutes. The goal is to start the next interval as soon as I feel I'm recovered enough for it, and not automatically wait a full two minutes.
That's where I am now. As I improve, I expect to eliminate the walk between intervals and use a jog instead. At this point, I see no reason to ever do more than a dozen intervals. Starting out, three can be plenty!
Again, it's not about the number of intervals or the amount of time spent at top speed. It's about always having perfect form, and letting the rest period dictate when or if the next interval happens.
I lost two months of winter training due to injuries, illness, weather and holidays. I need to get ready for SuperSeal and O'side 70.3 (my first half!) with the time I have left. HIIT has taken on a higher priority for me simply because of the large payoff for a small time investment.