A lot of people do concurrent training— cardio and strength training within the same session—because it seems to achieve multiple goals at the same time. It’s also a proven fat-burner, making it a popular choice for general fitness.
But if you’re training athletes or advanced fitness enthusiasts, you need to consider the risks of concurrent training. Skeletal muscles know the difference between cardio and strength training, and they become confused by multiple activation patterns within a short time span. The two types of exercise use different physiological adaptive mechanisms, which can compete with one another and potentially eradicate gains.
Consequently, new research tells us it can be counterproductive to train for both endurance and strength within the same session or day, if we look at results over a longer period versus only one session. It is imperative to note that in the studies we will explore here, strength and endurance training occurred over days, not hours. This difference is important, because previous data on concurrent training focused on the effects of just one session.
In addition to exploring concurrent strength and endurance training, we will examine the effects of complex
training, which combines strength and power exercises within the same session. Complex training is an advanced form of concurrent training most commonly seen in athletic settings, such as football training rooms where players have different strength and power goals depending on the skills required to excel at their positions.
Muscle Force Generation Capacity
Muscle force generation capacity, or MFGC, is the most important concept to understand when exploring the research on the effectiveness of concurrent training. MFGC is a direct measurement of muscular strength that can tell us if concurrent training is indeed undermining strength gains. Results have shown that MFGC is compromised from concurrent training over a 4-day test period with combined high-intensity lower-body strength and endurance training (Doma & Deakin 2013). This data is critical because it shows that when we compare the effects of concurrent training over time with those of one-session strength training, we find that gains diminish when strength work is paired with endurance training.
The potential mechanisms of conflict between strength and endurance training at the muscle level are typically called interference phenomena, which simply means that adaptation mechanisms compete and interfere with one another during concurrent training. The established direction of interference is that endurance training reduces the quality of strength training sessions. However, research has failed to specifically identify these mechanisms and the exact training conditions that cause interference. Scientists suspect that both morphological and metabolic processes may be the culprits.
The first and most obvious morphological theory is that endurance training changes muscle contractility in ways that counteract strength gains (Docherty & Sporer 2000). But as we will see later, contractility can also be affected by the length of recovery, the intensity of each mode of exercise, and the frequency and volume of endurance training.
Another proposed morphological reason why concurrent training may not be effective is that high-volume exercises like endurance training increase the risk of delayed-onset muscle soreness (DOMS). The mechanisms of DOMS cause a series of physiological events that prevent strength gains. One study has shown that when DOMS was present, strength decreased over the same time in novice exercisers (Beck, DeFreitas & Stock 2011).
Metabolic mechanisms for interfer- ence phenomena may include substrate depletion and increased protein breakdown (Fyfe, Bishop & Stepto 2014). These processes—which occur during endurance training and reduce available substrates and protein for muscle function and growth—would decrease strength gains. To date, however, the data is unclear on exactly how these mechanisms might work, owing to the many concurrent modes of training examined and different study methods.
Changes in testosterone levels are another potential metabolic mechanism. When testosterone levels were measured across different modes of training that included strength alone, endurance alone, and concurrent, testosterone increased only in the strength alone group (Mirghani et al. 2014). Conversely, testosterone levels decreased in both the endurance alone and concurrent groups. The point here is that testosterone levels are directly related to strength gains. The greater the testosterone level, the greater the strength gains—therefore, if concurrent training reduces testosterone levels, it can decrease the efficacy of strength training.
Concurrent Training Program Design Recommendations
A carefully designed concurrent training program can minimize the negative impact on strength. Primary design factors to consider are length of recovery, frequency of each mode of training, intensity and volume of endurance training, and endurance mode.
RECOVERY AND FREQUENCY
Doma & Deakin (2013) found that muscles need 48 hours for baseline strength to recover from a high-intensity strength training session, as measured by produced knee extensor torque (KET). In this study, KET was compromised for up to 2 days after high-intensity strength training on alternating days. The data shows us that if we strength train at a high intensity (normally defined as 85% or more of one-repetition maximum), full recovery requires 48 hours of rest. Therefore, if a program includes strength and endurance goals, it is best to schedule each mode of exercise on alternate days (Doma & Deakin 2013). Finally, endurance training frequency should be limited to less than 3 days per week to minimize the effects on strength (Wilson et al. 2012).
MODE, INTENSITY AND VOLUME
Wilson et al. (2012) found that running paired with strength training results in greater loss of strength than cycling paired with strength training. For concurrent training programs that have strength and/or hypertrophy as goals, cycling is therefore recommended over running. The degree of strength impairment is related to the intensity of the endurance training (Doma & Deakin 2013). Moderate-to high-intensity endurance training decreases the efficacy of strength training, so the intensity of endurance sessions should be reduced to limit the decrease of strength. However, practical considerations and training goals will warrant a close investigation of the utility of low-intensity endurance training sessions and the periodic frequency of moderate to high sessions. Finally, endurance volume should remain between 20 and 30 minutes to mitigate the negative effects on strength gains (Wilson et al. 2012).
Whether to use concurrent training or not depends on the trainee’s goals. For general fitness, concurrent training is desirable because it is the most effective way to lose fat mass (Wilson et al. 2012). However, athletic training programs shift the design focus from weight loss to performance. Power—the ability to produce force quickly—is a common training goal for sports and unfortunately it is most susceptible to the risks of concurrent training (Wilson et al. 2012).
Complex training aims to build power and strength in the same session, while leaving endurance training out of the program design equation. Complex training is common in athletic training settings where the goal is power or strength, or some combination of the two.
The key to good complex programming is to pair biomechanically similar high-load Olympic lifts with lower-load explosive plyometric drills. For example, a back squat (Olympic lift) can be combined with a low-intensity explosive drill like a squat jump (Carter & Greenwood 2014). Complex training is best suited for athletes and recreational exercisers who have already achieved a baseline of strength (Carter & Greenwood 2014), because their muscles are more responsive to the priming effects of the Olympic lift on the explosive drill. As with concurrent training, complex training requires a 48-hour rest for a complete return to baseline strength. However, any rest longer than 96 hours can result in detraining (Carter & Greenwood 2014).
Although complex training has proved effective for athletes and highly trained recreational exercisers, it has not been shown to have an advantage over conventional training for novice exercisers (Juarez, Gonzalez-Rave & Navarro 2009). Therefore, fitness professionals should carefully consider the utility versus the risks of high-intensity complex strength training programs for novice exercisers.
Whether concurrent or complex training belongs in a fitness or athletics program will depend upon the client’s goals and fitness level. Other factors— such as frequency, volume and intensity of each mode of exercise—should be considered as well, along with the careful manipulation of each variable over the duration of the program.
Beck, T.W., DeFreitas, J.M., & Stock, M.S. 2011. The effects of a resistance training program on average motor unit firing rates. Clinical Kinesiology, 65 (1), 1-8.
Carter, J., & Greenwood, M. 2014. Complex training reexamined: Review and recommendations to improve strength and power. Strength and Conditioning Journal, 36 (2), 11-19.
Docherty, D., & Sporer, B. 2000. A proposed model for examining the interference phenomenon between concurrent aerobic and strength training. Sports Medicine, 30 (6), 385-94.
Doma, K., & Deakin, G. 2013. The cumulative effects of strength and endurance training sessions on muscle force generation capacity over four days. Journal of Australian Strength and Conditioning, 21 (Suppl. 1), 34-38.
Fyfe, J.J., Bishop, D.J., & Stepto, N.K. 2014. Interference between concurrent resistance and endurance exercise: Molecular bases and the role of individual training variables. Sports Medicine, 44 (6), 743-62.
Juarez, D., Gonzalez-Rave, J.M., & Navarro, F. 2009. Effects of complex vs non complex training programs on lower body maximum strength and power. Isokinetics and Exercise Science, 17 (14), 233-41.
Mirghani, S.J., et al. 2014. Influence of strength, endurance and concurrent training on the lipid profile and blood testosterone and cortisol response in young male wrestlers. Baltic Journal of Health and Physical Activity, 6 (1), 7-16.
Wilson, J.M., et al. 2012. Concurrent training: A meta-analysis examining interference of aerobic and resistance exercises. Journal of Strength and Conditioning Research, 26 (8), 2293-2307.