Cardio Training: Beyond Calories and Intervals

By Katie Mark, November 30, 2021


Sprint, rest, repeat? Or a 45-minute jog on the treadmill? Intense bursts of movement in high intensity interval training (HIIT) do more than give you bang for your workout-buck in terms of calorie burning. But which is the prescription for better health? The mantra that HIIT workouts are more effective for fat loss does not mean that we should throw steady state cardio (SSC) – working at a lower intensity for extended periods – at the bottom of our training schedule.

In this blog, we’ll investigate HIIT and SSC and their effect on biomarkers such as glucose, testosterone, and creatine kinase. We’ll also discuss the physiological adaptations that result from both. Ultimately, a blended approach of HIIT and SSC may be the winning combo.

The steady state vs HIIT debate

HIIT is exercise with short bursts of vigorous activity at “all-out” effort and interspersed by periods of low-intensity exercise for recovery or rest. HIIT reaches intensities close to maximum oxygen consumption (VO2 max) – the body’s upper limit for consuming, delivering, and using oxygen for energy production. HIIT has gained momentum because of its feasibility, lower time commitment and maximum exercise adaptations.

SSC requires you to perform an activity at a challenging, yet manageable pace (60-70% VO2 max) for at least 20 minutes. SSC can be referred to as low intensity steady state cardio, but SSC is not always low intensity because it can still be challenging.

If you aren’t doing some HIIT, are you wasting your time? The truth is that you might not need to jump only on the HIIT bandwagon. It’s not just about crushing calories and fat loss. Both workouts complement each other when generating physiological adaptations and positively impacting biomarkers.

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What is HIIT, and how does it relate to improving conditioning?

HIIT includes an array of interval exercises, such as sprints, timed lap swimming, and circuit training. HIIT can be performed on a treadmill, bike, elliptical, or by using equipment such as battle ropes, kettle bells, jump ropes, and barbells.

HIIT relies on duration, intensity, frequency of the high intensity interval, and length of recovery (rest) interval.1 Work intervals can range from 5 seconds to 8 minutes. Intensity of the work should range from 80-100% of VO2 max. Intensity of the rest interval should range from passive recovery to active recovery (50-70% of VO2 max). 


HIIT has demonstrated to improve VO2 max, which is a good indicator of exercise performance. A study found that an 8-week HIIT program using men and women increased VO2 max more following the HIIT program (15%) than in a continuous training program (9%).

Even intervals lasting 15 to 60 seconds yield improvements in VO2 max 2 Aside from training performance, improvements in VO2 max is associated with a decrease in all-cause mortality risk,3 or death resulting from anything.


HIIT and SSC: The effect on biomarkers

Repeat intervals and steady cardio have similar and unique influences to your biomarkers, so it's not just about calories burned. Trainees should think about their goals and about how their workouts can be sustainable from better planning and monitoring.

Glucose and Insulin Sensitivity

HIIT improves fasting blood glucose.4 Moderate-intensity exercise from SSC also improves blood glucose, but few people reach the required exercise amount.5 In a review of HIIT studies with 80% VO2 max lasting 15 minutes per session, HIIT improved blood glucose 1 to 3 days following exercise in those with and without diabetes. 

HIIT improves glucose tolerance,2 or how quickly glucose is removed from the blood. This is because it increases glucose transporters on your muscle cells.6


HIIT also increases insulin sensitivity (how well your cells respond to taking up glucose). About 66% of glucose uptake from the blood heads to skeletal muscle, and it is suggested to be the adaptations in skeletal muscle that account for the improvement in insulin sensitivity.2

A study using 45 young women found a decrease in fasting insulin levels (31%) following HIIT three times per week for 15 weeks. The SSC group also experienced a decrease in fasting insulin levels (9%), but not to the same extent.

A single bout of exercise, such as SSC, can increase insulin sensitivity for at least 16 hrs post-exercise.7 This is because exercise causes our muscles to be primed for glucose intake due to more GLUT-4 receptors (a protein that acts as a regulated glucose transporter) on the cell surface.

Free Testosterone

Perpetual steady-state endurance exercise – or SSC for long periods of time, such as cycling and marathon running – can reduce testosterone levels for several hours or days. 8 Prolonged reductions in testosterone can hurt skeletal muscle adaptations needed for improving training and performance.

The response of testosterone changes to HIIT has generated inconsistent results. However, many studies fail to measure free testosterone, the biologically active form (a biomarker measured by InsideTracker).


 A study used 15 endurance-trained males to explore the relationship between free testosterone (FT) and HIIT and steady-state endurance exercise (SSE). The HIIT involved intervals of 90-second treadmill running at 100-110% VO2max and 90-second active recovery at 40% VO2 max for 42 to 47 minutes. The SSE session was a continuous 45-minute run at 60-65% VO2 max.

IE (high-intensity interval exercise) and SSE both increased free testosterone, but IE increased free testosterone more than SSE. This response is suggested to be due to the higher level of stress-strain during muscle contraction in HIIT, which requires a greater anabolic (or growth) response.

Creatine Kinase

Creatine kinase (CK) is an enzyme mostly found in muscle. Normally, a small amount of CK circulates in our blood, but its level rises when there is muscle damage, which can suggest that your muscles are overexerted.

A study used eight physically active males to complete three HIIT sessions per week over 3 weeks. All sessions included four to six 30-second bursts of high-intensity cycling separated by 4 minutes of recovery. The study found a decrease in CK even though the total work completed was greater in the post-training test compared to the pre-training test. Simultaneously, there was an increase in antioxidant activity. Essentially, there was little to no muscle damage. However, this HIIT used cycling, which may not be the case for other forms of HIIT.


It is suggested that a person’s CK response to exercise depends more on the physical characteristics or training background of the individual.9 Therefore, it is important to test with InsideTracker to evaluate how your training is impacting your muscles.

Summary: Both HIIT and SSC improve fasting glucose levels and insulin sensitivity. The novel finding that free testosterone responds differently to HIIT (increases free testosterone levels) and SSC is important to consider when designing a training regime to maximize muscle gain. Prolonged SSC can reduce testosterone levels, which impairs muscle growth. HIIT increases free testosterone level, which may be due to a greater need for muscle growth. HIIT may not generate an increase in CK levels, but the CK response following exercise depends on the individual’s physical characteristics and training experience. Therefore, it’s important to test.


HIIT charges your muscle batteries 

Increased number and size of mitochondria (the batteries of muscle that generate energy) in skeletal muscle increases endurance in recreationally active or untrained healthy people.10 The more mitochondria, the more energy available to muscle, which allows an athlete to work longer. This has been a major adaptation of HIIT.11 The more mitochondria you have then the more fat burned – helping fat loss.

So what stimulates these adaptations? A recent study, and first study to explore what happens in muscle cells during HIIT, identified the mechanism in the muscular adaptation and found that HIIT is less effective in well-trained people. The study used recreationally active men to perform intense HIIT consisting of 30-second intervals of cycling with 4-minutes rest in between intervals. There was less than 3 minutes of total exercise time.


Thigh skeletal muscle samples were taken from the men 24 hrs after the exercise. Results showed HIIT breaks down calcium channels in muscle cells. Calcium channels are stimulated by nerve impulses and help muscles contract and relax. HIIT stimulates a change in how these cells handle calcium. It also stimulates a signal to create more mitochondria. This then increases muscle endurance.

HIIT also increases the amount of free radicals in the body. The study found an extensive amount of free radical release that was hitting the calcium channels. Traditional exercise releases free radicals, but not to the same extent as HIIT. The study reported that the antioxidants vitamin C and E can prevent this calcium breakdown.

Summary: HIIT increases the density and size of mitochondria (powerhouse of the cell that generates energy) in skeletal muscle because free radicals released during HIIT breakdown muscle calcium channels. This improves muscle endurance.


Steady state cardio is important for the heart & recovery

The left ventricle of the heart stores oxygenated blood before it is pumped through the body. During SSC, the left ventricle fills completely before it contracts, which increases its ability to pump more blood during each contraction over time. This causes a decrease in blood pressure during exercise and rest.

Conversely, the heart beats so quickly during HIIT that the left ventricle is unable to fill completely between contractions. SSC can trigger important heart adaptations, such as thicker heart muscle, larger left ventricle and improved heart function during exercise.


SSC also increases capillary density, which improves the transfer of oxygen from the blood to your muscles.12

The lower heart rate generated by SSC workouts yields a healthy and better functioning cardiovascular system, which improves the ability to relax, focus, and recover from intense exercise like HIIT.

Summary: SSC improves heart muscle and function and decreases heart rate. All of which help you recover from intense exercise like HIIT.


HIIT & steady state cardio are better together

As we've established, both HIIT and SSC have their pros and cons. While there is much less data on the response of athletes to HIIT, the shorter duration of HIIT training results in muscle-sparing — meaning, your muscle is less likely to be broken down as compared to SSC. Yet, there is evidence suggesting that incorporating some HIIT into high training volumes can improve performance.13,14

Essentially, the huge benefits from HIIT mean that less is more. Continual HIIT can keep you in a hypervigilant state and hurt recovery. It can also increase your chance for overreaching or overtraining.


A study investigated the relationship between exercise intensity and emotion using 26 healthy, middle-aged men. They found an increase in negative emotion from HIIT than from moderate-intensity continuous training. This could hinder adherence to exercise and negatively impact biomarkers related to mood, such as cortisol (the stress hormone).

HIIT should be performed every other day, or 2-3 times per week, as research suggests that this is the best approach to yielding results and limiting injury15 because HIIT requires adequate recovery time between sessions.

A drawback of SSC is that people may adapt to it. However, the simple solution is, if efficiency increases then increase the workload of your SSC.

Remember the different needs of recreational exercisers and competitive athletes. The number of sessions per week of HIIT and SSC are different. The key is the appropriate level of intensity and frequency of each, especially HIIT.

And if you’re just looking for a leaner body composition, both approaches will work as long as you stick with a schedule that’s convenient for you and your goals (and a healthy diet, which InsideTracker can help tailor to your needs).

Summary: HIIT is not always better than SSC. The ways HIIT and SSC improve internal health is just as important as getting lean. SSC serves an important role in the holistic approach to training. Two to three times of HIIT per week is sufficient to reap its benefits because HIIT is very demanding and requires adequate recovery. SSC following HIIT can help start muscle repair mechanisms following HIIT.


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An extremis workout approach may not be the best approach. There's no one-size-fits-all. Balance is key to reap the benefits of both HIIT and SSC.

When pursuing any new exercise regimen, it is critical to carefully monitor your health to make sure you avoid any complications. Find what’s best for you when adding SSC and HIIT to your workout routine. Then evaluate your approach to exercise and nutrition by adding InsideTracker as an evaluation tool to complete your package toward better health.

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Some other blog posts we think you'll love:



1. Kubukeli, Z.N., Noakes, T.D., Dennis, S.C. 2002. Training techniques to improve endurance exercise performances. Sports Medicine, 32 (8), 489-509.

2. Shepherd, S.O., Wilson, O.J., Taylor, A.S., Thogersen-Ntoumani, C., Adlan, A.M., Wagenmakers, A.J. & Shaw, C.S. Low-volume high-intensity interval training in a gym setting improves cardio-metabolic and psychological health. PLoS One, 2015; 10(9).

3. Lee, I.M., Sesso, H.D., Oguma, Y., Paffenbarger, R.S. Jr. Relative intensity of physical activity and risk of coro- nary heart disease. Circulation. 2003; 107(8):1110–6.

4. Jelleyman, C., Yates, T., O’Donovan, G., Gray, L.J., King, J.A., Khunti, K., Davies, M.J. The effects of high-intensity interval training on glucose regulation and insulin resistance: a meta-analysis. Obes Rev. 2015;16(11): 942-961.

5. Adams, O.P. The impact of brief high-intensity exercise on blood glucose levels. Diabetes Metab Syndr Obes 2013; 6:113–122.

6. Burgomaster, K.A., Cermak, N.M., Phillips, S.M., Benton, C.R., Bonen, A., Gibala, M.J. Divergent response of metabolite transport proteins in human skeletal muscle after sprint interval training and detraining. Am. J. Physiol. 2007;292, 1970– 1976.

8. Borghouts, L.B., Keizer, H.A. Exercise and insulin sensitivity: a review. Int J Sports Med. 2000; 21(1), 1–12.

9. McMurray, R.G., Hackney, A.C. The endocrine system and exercise. Exercise & Sports Science. Garrett W & Kirkendall D (eds). Philadelphia: Williams & Wilkins Publisher. 2000, 135-62. 

10. Totsuka, M., Nakaji, S., Suzuki, K., Sugawara, K., Sato, K. Break point of serum creatine kinase release after endurance exercise. J. Appl. Physiol. 1985; 93, 1280–1286 (2002).

11. Place, N., Ivarsson, N., Venckunas, T., Neyroud, D., Brazaitis, M., Cheng, A.J., Ochala, J., Kamandulis, S., Girard, S., Volungevicius, G., et al. Ryanodine receptor fragmentation and sarcoplasmic reticulum Ca2+ leak after one session of high-intensity interval exercise. PNAS. 2015. Published ahead of print November 2, 2015.

12. Gibala, M. Molecular responses to high-intensity interval exercise. Applied Physiology, Nutrition, and Metabolism, 2009; 34 (3), 428-32.

13 SSE #54: Muscle Adaptations to Aerobic Training. (n.d.). Retrieved November 13, 2015, from

14. Hawley, J.A., Myburgh, K.H., Noakes, T.D., Dennis, S.C. Training techniques to improve fatigue resistance and enhance endurance performance. J Sports Sci. 1997;15:325–333.

15.Iaia, F.M., Bangsbo, J. Speed endurance training is a powerful stimulus for physiological adaptations and performance improvements of athletes. Scand J Med Sci Sports. 2010. October;20 Suppl 2:11–23.

16. Daussin, F.N., Zoll, J., Dufour, S.P., Ponsot, E., Lonsdorfer-Wolf, E., Doutreleau, S., Mettauer, B., Piquard, F., Geny, B., Richard, R. Effect of interval versus continuous training on cardiorespiratory and mitochondrial functions: relationship to aerobic performance improvements in sedentary subjects. Am J Physiol Regul Integr Comp Physiol. 2008; 295, R264-72.

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