In theory, circuit training routines such as P90X and Insanity sound like the perfect workout: you’re doing resistance exercises with minimal rest while keeping your heart rate up. And if an exercise regimen keeps you gasping for air and drenched in sweat then it MUST be good for you, right? Well, not necessarily. While it certainly has its merits, circuit training isn’t always a great tool to improve your strength- in fact, it may do more harm than good. Below we distill the research on how you can maximize the benefits of circuit training. As always, the most objective way to know if circuit training works for you is to keep track of your performance data and key biomarkers using a tool grounded in science such as InsideTracker.
Origins of the Circuit
Before diving into the research, let's be clear on one thing: what is circuit training? Circuit training was designed by exercise physiologists R.E. Morgan and G.T. Anderson at the University of Leeds in 1953 as an alternative to traditional resistance exercise routines . In a conventional strength training regimen, you target a specific muscle group, rest for 2-5 minutes, and then repeat sets until that muscle group fatigues.
Let's contrast that appraoch with circuit training; you complete several carefully organized resistance exercises targeting different muscle groups with minimal rest between them; that is called a circuit. A sample circuit may include doing sets of pull ups, weighted squats, and push-ups with 15 seconds of rest between them. After completing the circuit, you take a 2-5 minute rest and then perform the circuit again until your muscles fatigue. Circuit training is based on the principle of efficiency- it allows you to shorten the length of your workout and maximize the amount of calories and muscles trained in a shorter period. And of course, because there is less rest between sets, it keeps your heart rate up and theoretically improves the intensity, cardiorespiratory fitness, and post-workout metabolism.
The original circuit training regimen by Morgan and Anderson consisted of 9 to 12 weight-lifting exercises where each participant moved from one station to the next with a rest period of no greater than 30 seconds. In each station, they performed 8-20 repetitions of a resistance exercise equivalent to about 40-60% of their one-repetition maximum . Morgan and Anderson designed circuit training to be very customizable allowing body-weight, explosive (i.e. jumping), and aerobic (i.e. running) exercises to be seamlessly weaved into a program of varying difficulty. Today, we can see current variations of these techniques ranging from Curves to CrossFit that integrate a variety of resistance and aerobic exercises that target men and women of varying fitness levels.
However are these lengthy, non-stop exercise regimens worth all the sweat? And how do circuit training methods really stack up against more traditional forms of aerobic (i.e. cardiovascular) and anaerobic (resistance-based) fitness? Below we look at the research assessing the pros and cons of circuit training for athletes and non-athletes and how you can use InsideTracker’s biomarker analysis to optimize your training regimen.
Better than Endurance Training for Non-Athletes
Research supports that circuit training alone confers sizable benefits in power and strength. In an intervention study of physical education students (mean age of 21.4 years) of moderate fitness, researchers divided the 48 subjects into five test groups. Nine were placed in a no-training control group, 10 in endurance training, 9 in circuit training only, 10 into endurance training before circuit training in the same session, and 10 with circuit training in the same session. Each subject engaged in their respective exercises twice per week for 12 weeks for the same amount of time. The circuit training method was divided into two distinct sessions: weeks 1-6 consisted of strength training while weeks 7-12 consisted of power and explosive strength training.
When the researchers assessed compared key metrics of power and strength such as 1-leg half squat and 5-jump test before and after the exercise interventions, they observed that those who had participated in circuit training experienced more significant gains than the endurance and control groups . This suggests that endurance training may interfere with muscle gains. Because of their findings, researchers concluded “circuit training alone induced strength and power improvements that were significantly greater than when resistance and endurance training were combined, irrespective of the intrasession sequencing.” .
Key Takeaway: If you want to gain power and strength, performing circuit training alone is more effective than combining resistance training and endurance training
Recommendation: Circuit training without endurance training might be a good method to simultaneously maintain cardiovascular fitness and increase power and strength
Beneficial to the Middle-Aged & Elderly
A 2013 study supports the claim that circuit training is an effective method for training the elderly when compared to traditional strength training. Researchers recruited 37 healthy men and women with an average age of 61.6 years and randomly assigned them to one of three groups for 12 weeks. The first group of 16 participated in a high-resistance circuit training, the second group of 14 participated in a traditional strength training routine, and 7 did not do partake in any exercise and served as the control group. The subjects that performed resistance training performed six repetitions approximately of 85-90% of their one repetition maximum. Both exercises were of similar anaerobic difficulty- meaning that they lifted the same amount of weight for the same number of repetitions. However, the circuit training group had shorter rest periods between sets.
Researchers then monitored key fitness indicators of the subjects over the intervention including muscle size, strength, body composition, and indicators of cardiovascular fitness. They observed significant and equal increases in lean mass and bone mineral density in both of the experimental groups. However, only subjects in the high-resistance circuit showed three times as much of a decrease in body fat percentage (1.8%) compared to the participants in the traditional resistance circuit . These subjects also experienced greater improvements in metrics of cardiovascular fitness such as VO2 max- the maximum volume of oxygen that an athlete can use. Because of these findings, researchers concluded that “…HRC training was as effective as TS for improving isokinetic strength, bone mineral density, and lean mass. Only HRC training elicited adaptations in the cardiovascular system and a decrease in fat mass.” 
Key Takeaways: For healthy non-athletes, high-resistance circuit training confers additional cardiovascular and body composition benefits
Recommendation: If you’re a healthy man or woman in his 40s or older, consider circuit training to get a more efficient workout
Circuit Training: Better Than your Traditional Military Routine
In a 2012 study, military scientists compared the effects of the “mission essential fitness” circuit-style training (MEF) program with standard the army physical readiness training (APRT) on fitness, physiological, and body composition changes in active military personnel. The MEF program consisted of fifteen resistance exercises (i.e. Olympic weightlifting movements, plyometrics, pullups) performed continuously for 60 to 90 seconds for 45 minutes with short rest periods between sets. In contrast, the APRT program consisted of a brief warm-up, 50 minutes of running and standard strength-training exercises such as push-ups and sit-ups with longer rest periods, and a cool-down. Researchers assigned 34 subjects to the circuit training program, 33 to the Army training program and had both groups participate in 15 sessions over eight weeks (an average of two sessions per week). About 83.5% of subjects were male, and they had an average age of 28.
Researchers measured baseline and post-test measures including the army physical fitness test, physiological indicators, body composition, and additional fitness indicators. A one-way analysis of covariance models indicated that MEF participants significantly increased their push-ups, bench press, and flexibility, and significantly decreased their 2-mile run . Additionally, they maintained body composition and reported no injuries. The MEF training program safely improved muscular strength and endurance, cardiovascular endurance, and flexibility, supporting functional fitness circuit-style exercise training for military personnel.
Key Takeaways: The shorter resting periods and variety of resistance exercises in a circuit training routine provide greater fitness benefits
Recommendation: If you’re young and fit, consider using a greater variety of resistance exercises with shorter resting periods between sets
Low-Intensity Circuit-Training: Not Intense Enough
Now we’re going to delve into some of the shortcomings of circuit training. The benefits of low-intensity circuit training are less pronounced in fit, athletic populations. In a 2011 study, researchers recruited 20 experienced firefighters and had them each perform a 12-station circuit composed of resistance and cardiovascular exercises that targeted all major muscle groups. All firefighters were asked to perform as many repetitions as they could for bodyweight exercises and take 30 seconds of recovery before moving on to the next station. The total circuit lasted about 29 minutes.
The researchers measured the average heart rate of the firefighters to assess the aerobic intensity of their circuit training and lactic acid blood levels after the workout to assess anaerobic intensity. When comparing this data with previous data in which firefighters practiced firefighting-related tasks, they observed similar lactic acid blood levels but a lower resting heart rate . These data suggests that circuit training as similarly anaerobically demanding but not as aerobically demanding.
As a result of their findings, the researchers recommended that firefighters “should supplement low-intensity circuit-training programs with high-intensity cardiovascular and resistance training (> 85% 1-reptition maximum)” . Thus, low-intensity circuit training confers similar benefits in muscle growth but not in aerobic fitness for a physically fit population such as firefighters.
Key Takeaways: Low-resistance circuit training is less anaerobically challenging than other higher-resistance, explosive exercises
Recommendation: If you’re already fit and trying to get stronger, skip low-resistance cross-training
Circuit Training: Effective but not the Most Effective for Runners
Circuit training may not be as beneficial to endurance athletes when compared to other strength-training methods. In 2010, researchers recruited 28 endurance athletes and had them each complete six weeks of preparatory strength training of standard resistance training of 3 sets of 10-15 repetitions. Then, they divided the subjects into three different strength interventions for eight weeks. The first 11 runners performed three sets of 4-6 repetitions of squats and leg presses equivalent to 85% of their one repetition maximum. The second group of 10 runners participated in a set of explosive strength training exercises such as explosive squats and maximal squat jumps. The final group of 7 runners performed a standard body weight circuit training of 3 sets for 40-50 seconds of exercises such as squats, push-ups, and sit-ups. After the conclusion of each 8-week regimen, the subjects performed a reduced volume strength period of their respective programs.
The researchers assessed key fitness metrics such as maximal strength (1 repetition maximum), muscle activation of leg extensors, and maximal energy uptake. Metrics such as one repetition maximum and VO2 max increased in all three groups. However, the improvements were greater in the maximum and explosive strength training groups. Additionally, certain indicators such as muscle activation improved in the maximum and explosive groups. Furthermore, at the end of the 8-week strength training intervention, the circuit training group was the only group that experienced reduced levels of testosterone. In contrast, the other two groups experienced noticeable increases in testosterone. Because of these findings, the researchers concluded that “maximal or explosive strength training performed concurrently with endurance training was more effective in improving strength and neuromuscular performance and in enhancing VO2 max and RE in recreational endurance runner than concurrent circuit and endurance training” . This assertion suggests that while circuit training is beneficial to recreational athletes, it might not be as effective as exercises that have higher resistance and explosiveness.
Key Takeaways: Circuit training with body weight exercises challenges your body less than high-resistance and explosive exercises
Recommendation: If you’re an endurance athlete, use higher resistance and more explosive exercises to gain greater fitness and health benefits
Circuit Training vs. Resistance: Putting it All Together
Provided that circuit training is highly variable, are certain forms better for improving one’s fitness than others? A research study in 2012 study suggests that adding a brief burst of explosive exercise leads to greater enhancement of fitness metrics. In it researchers recruited 11 recreationally active females and had them participate in three different forms of circuit training. The first, designated “traditional circuit weight training”, consisted of three stations of exercises- station A, station B, and station C- separated by a brief resting period. Each station consisted of three exercises each performed for three sets of 13 repetitions each with a 30-second lift period and 30-second rest period. The second, designated “aerobic circuit weight training”, consisted of the same three stations of resistance exercises but with a shorter period (15 seconds) of rest between sets. Additionally, the subjects performed a moderately intense 2 minute and 30 seconds bicycle session between sets. Finally, the third protocol, called the “circuit weight interval training”, consisted of the same blocks of resistance exercise as the other two protocols with the same rest periods as the aerobic circuit. However, instead of biking moderately for 2 minutes and 30 seconds, they biked with maximal effort for 30 seconds then slowly pedaled for three minutes. Each of these protocols was about 40 minutes long.
As expected, the aerobic and circuit weight regimens produced a greater intensity workout. However, the circuit weight regimen resulted in a considerably higher blood lactate level- an indicator of the anaerobic intensity of the exercise- by more than 60% . Additionally, the circuit weight group had a considerably higher elevated average heart rate compared to the other two groups. While the authors acknowledged that the third group had the greatest intensity of exercise, they concluded that by integrating the maximal exertion exercise “…may enhance fitness benefits and maximize time-efficiency more so than traditional circuit training methods” . Thus, circuit training is more effective when it includes an intense sprint and shorter rest periods.
Key Takeaways: Shorter rest periods and more intense bursts of exercise challenge the body more than steady circuit training
Recommendations: Integrate brief, all-out bursts of cardiovascular exercise between your weight-lifting sessions. Of course- be careful to not to over train!
Completing the Circuit: What Works and What Doesn’t?
In theory, circuit training regimens such as P90X and CrossFit sound like a short circuit to greater strength, fitness, and health. However, the research paints a more nuanced picture. Circuit training is an excellent method compared to traditional strength and endurance programs for efficiently increasing cardiovascular and muscular fitness in beginners (with supervision) and people of intermediate fitness. However, the benefits of circuit training taper off when applied to more athletic populations such as recreational athletes and firefighters. Furthermore, the type of circuit training program makes a big difference: higher-resistance circuit training with fewer repetitions is more effective than lower-intensity circuit training for long-term gains. Similarly, shorter rest periods and intense, explosive exercises make a bigger impact than traditional circuit training methods.
Finally, research indicates that circuit training might induce overtraining because it lowers testosterone and increases levels of inflammation indicator creatine kinase. And no research has been done to show the effectiveness of circuit training on other biomarkers monitored by InsideTracker such as cholesterol and sex hormone-binding globulin.
Is circuit training right for you? The best way to know is to keep track of your fitness and biomarker data. We can’t keep track of your VO2 max or 1 repetition maximum- but we can evaluate fitness and health-related biomarkers from the inside. Click below to schedule your test today and learn where you stand.
List of References
 Kravitz, L. (1996). The fitness professional's complete guide to circuits and intervals. IDEA Today, 14(1), 32-43.
 Chtara, Moktar, et al. "Effect of concurrent endurance and circuit resistance training sequence on muscular strength and power development." The Journal of Strength & Conditioning Research 22.4 (2008): 1037-1045.
 Romero-Arenas, Salvador, et al. "Effects of high-resistance circuit training in an elderly population." Experimental Gerontology 48.3 (2013): 334-340.
 Heinrich, Katie M., et al. "Mission essential fitness: comparison of functional circuit training to traditional Army physical training for active duty military." Military Medicine 177.10 (2012): 1125.
 Abel, Mark G., Anthony J. Mortara, and Robert W. Pettitt. "Evaluation of circuit-training intensity for firefighters." The Journal of Strength & Conditioning Research 25.10 (2011): 2895-2901.
 Taipale, R. S., et al. "Strength training in endurance runners." Int J Sports Med31.7 (2010): 468-476.
 Bizheh, Nahid, and Mohsen Jaafari. "The effect of a single bout circuit resistance exercise on homocysteine, hs-CRP and fibrinogen in sedentary middle aged men." Iranian journal of basic medical sciences 14.6 (2011): 568.
 Skidmore, Brook L., et al. "Acute effects of three different circuit weight training protocols on blood lactate, heart rate, and rating of perceived exertion in recreationally active women." Journal of sports science & medicine 11.4 (2012): 660.