High-intensity interval training (HIIT) has made its name as a fitness game-changer, and there are plenty of studies highlighting its remarkably transformative effects. The results come from going as hard as you can for a short period of time, resting, and then repeating. It’s a formula that allows you to keep reaching your maximum training zone again and again, shocking your body each and every workout.
For the most part, the stress this puts your body under is a good thing – as it’s this stress that drives change. But your body is only able to handle a certain amount of stress at once, explains Bryce Hastings, Physiotherapist and Les Mills Head of Research.
Jinger Gottschall, Associate Professor at Pennsylvania State University, elaborates, “When your body is introduced to a stressful scenario, such as high-intensity exercise, stress hormones, like cortisol are released. The effects of cortisol can be positive; enhanced fat loss, improved immunity, and reduced inflammation. But too much high-intensity exercise can cause greater cortisol release than necessary which can reverse these effects.”
While this risk is commonly associated with HIIT, says Hastings, it applies to any form of overtraining.
There are many factors that make HIIT so highly effective, including pushing your heart rate to greater than 85 percent of your maximum capacity, impact forces greater than ten times your body weight, and performing exercises until failure. Because of these extreme characteristics, says Gottschall, it is imperative to adequately recover between sessions.
“Too much HIIT without proper recovery will prevent you from reaching the intended maximal heart rate training zones during your high-intensity training session,” she says. “This means the workouts simply become vigorous training but without the same results.”
Not only does this reduce your training’s effectiveness, says Gottschall, it adds to the risk of injury – typically because good technique may be compromised. Depending on your fitness level and other training session intensities, if you spend more than 40-60 minutes a week with your heart rate above 85 percent max (as it is during the majority of a HIIT workout), injury incidence and over-training symptoms increase. “For example,” says Gottschall, “when you are fatigued, the core cannot properly support the trunk, increasing the likelihood of injuries to the shoulder or lower back.”
How much HIIT should you be doing?
After evaluating the heart rate profiles from thousands of athletes in the last 20 years and completing three different studies using Les Mills’ HIIT programming, Gottschall has concluded that two 30-minute HIIT sessions a week appears to be optimal to allow for proper recovery and ideal performance. This equates to approximately 40 minutes of intensity above 85 percent max per week – and she says it is possible to divide these 40 minutes into three shorter sessions.
"Two 30-minute HIIT sessions a week appears to be optimal for ideal performance."
So what happens if you do HIIT workouts more often but simply lower the intensity?
Dropping the intensity can mean your training shifts into the vigorous-intensity interval based training category (which is often defined as 70-85 percent max heart rate). While this can be completed for longer periods of time per week, Gottschall says there are lower impact forces, and a focus on different physiological systems. Consequently, you don’t get the same benefits of a true HIIT workout above 85 percent max heart rate.
So, rather than dropping the intensity of your interval training, embracing moderate-intensity cardio can be a wise move. Furthermore, the value of moderate-intensity cardio should not be overlooked. “Moderate-intensity cardio can be performed for longer periods of time and thereby train endurance systems,” Gottschall stresses. “This intensity has been tested in countless studies and provides multiple benefits in terms of cardiovascular health, cholesterol levels, triglycerides, and fasting glucose.”
Basically, if you’re not doing some moderate-intensity cardio your body is missing out, says Hastings.
”While short, sharp HIIT workouts are great, you need to ask yourself what your body is doing in the long periods between workouts. It’s no good exhausting yourself with HIIT and then sitting on the sofa – you still need to be active to be transporting glucose around your body, you can’t just do this for a few minutes a day.
The winning formula: what’s the best way to feature HIIT in your exercise regime?
If you’re keen to embrace HIIT and take your fitness to an elite level, Gottschall recommends working out for a total of six to seven hours per week, and making sure you have one full rest day. Your weekly workout regime should feature the following:
- 2x HIIT workouts (30 minutes each)
- 2x strength training sessions (45-60 minutes each)
- 2x core workouts or yoga sessions (30-45 minutes each)
- 2-3x moderate cardio sessions (45-60 minutes each) – these may include intervals up to 80 percent max
By combining moderate-intensity cardio with HIIT, strength and core training, she says, you will have a combination of stimuli that will train all the varying energy systems, motor units, and muscle fiber types – and that’s what drives real results.
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