To “Cardio” or Not to “Cardio?” That Is the Question: A Brief Review of Concurrent Training

Within the health industry, many fitness programs include components designed to improve both the neuromuscular system (e.g., resistance training) and cardiovascular system (e.g., aerobic exercise). Combining resistance and aerobic training into a single exercise program is known as concurrent training (CT), simultaneous training, or cross-training. CT is often used as a time-saving method to improve neuromuscular function, decrease adipose tissue, increase endurance, and mitigate the myriad diseases and disorders associated with physical inactivity (20,23,25).

However, there is a long-perpetuated belief within the fitness industry insinuating that performing aerobic exercise (also known as “cardio”) alongside strength training interferes with adaptations to muscular strength, power, and hypertrophy, aptly named the “interference effect.” The interference effect is often thought to be a result of excessive volume or frequency of exercise, leading to the potential need for an extended recovery period.

For the purpose of this brief review, the term “cardio” refers to any aerobic-based exercise mode designed to improve a given outcome of the cardiovascular system, such as endurance, aerobic capacity (VO₂max), or other outcome variables. CT is often recommended for the general population and athletes despite the somewhat inconclusive scientific consensus on the interference effect. Therefore, this article aims to provide insights into some of the most common questions personal trainers and clients may have regarding CT and its effects on neuromuscular adaptations.

DOES CT HAVE BENEFITS?

Yes, CT has been shown to have health and performance benefits within various populations including athletes, general population, and clinical patients (3,6,21). For instance, patients with metabolic syndrome who performed CT exhibited reductions in waist circumference, triglyceride levels, fasting blood glucose, and low-density lipoproteins (6). CT is also beneficial for improvements in aerobic capacity, anaerobic power, speed, vertical jump height, and body fat reduction in sedentary and active populations (1,2,10,11,12,24).

For example, increases in maximal oxygen consumption (VO₂max) of about eight percent were observed in college basketball players after seven weeks of CT compared to a four percent increase following endurance training alone (2). The CT included four days per week of full-body strength training (four exercises), plyometrics (e.g., jumps, cone drills), and an aerobic endurance regimen (e.g., steady-state running and intervals) performed during separate sessions. While the previous study was only seven weeks, 21 weeks of CT in recreationally-active men elicited increases in VO₂max up to 18.5% (10). Collectively, CT has been shown to improve skeletal muscle endurance to a greater extent compared to strength or endurance training alone (19).

WHAT ARE THE EFFECTS OF CT ON POWER AND PERFORMANCE?

While most athletes attempting to improve muscular power rely on resistance training and plyometrics, the effect of adding cardiovascular exercise has shown conflicting results regarding the rate of force development (RFD), sprint speeds, and bar velocity (4,10,17,24). For instance, following 12 weeks of lower limb strength training (two days per week), squat jump height increased by 13% in untrained males compared to a 6.2% increase in a CT group consisting of trained cyclists performing cardiovascular exercise about 10 hr per week plus two days of strength training (17). Additionally, after performing CT for eight weeks, male soccer athletes improved 10-m and 30-m sprint times by 0.11 s (-11%) and 0.12 s (-12%), respectively, compared to no change in a control group completing traditional soccer training (24). Along with sprint speed, CT also showed improvements in back squat bar velocity by about 15% after seven weeks of CT versus a 6.6% increase after resistance training alone (8). While results are conflicting, a meta-analysis determined only a small interference effect (d=0.28) of performing CT on muscular power outcomes (20). Practically speaking, interference with power and performance is more likely to occur when long-duration cardio activities are frequently performed immediately prior to a strength session. When possible, it is suggested to allow recovery between resistance and cardio sessions.

WILL CT INTERFERE WITH SKELETAL MUSCLE HYPERTROPHY?

As the research on power adaptations with CT is inconsistent, the addition of aerobic exercise to a resistance training regimen has also shown conflicting results for skeletal muscle hypertrophy (11,14,16). For example, recreationally-active males and females undergoing CT for 10 weeks experienced increases in thigh girth of 1.7 cm compared to 2.3 cm in a strength-only group (11). Additionally, equivalent increases in the cross-sectional area (CSA) of thigh flexor muscles (6% versus 7%) and thigh extensors (12% versus 14%) were observed when whole-body strength training was compared with CT (strength training plus cycling) (16). These percentage differences are consistent with research examining another group of recreationally-trained men showing that strength training alone increased lower limb CSA to a 2.3% greater extent than a CT group (13). Despite these trivial to small differences, meta-analytic data confirms that CT may have no effect (effect size: -0.01) up to a small negative overall effect (effect size: -0.23) on skeletal muscle hypertrophy (14,20).

Lastly, performing CT may possess varying effects on the distinct muscle fiber types (i.e., type I versus type II). For example, type I fibers may have a greater interference effect on hypertrophy, particularly when the CT involved running as opposed to cycling (14). It is important to note that while CT results in skeletal muscle hypertrophy, gains may occur to a lesser degree. However, these differences in hypertrophy are typically small and may have little practical value for general population clients. Therefore, it is prudent to consider the individual goals of the client, as well as the return on investment of both inclusion and exclusion of cardio training in a program. 

WHAT ABOUT INTERFERENCE WITH MUSCULAR STRENGTH?

Like muscular hypertrophy, meta-analyses demonstrate trivial to small effects of CT on muscular strength (effect size: -0.06) and explosive strength (effect size: -0.28) (20). These results are consistent when examining the differences in one-repetition maximum (1RM) leg extension in recreationally-active males (mean age 38 years) (10). For this study, two groups (CT versus strength only) completed 22 weeks of full body strength exercises along with 30 min of cycling twice per week and progressing to 60 – 90 min of cycling by the end of the program. Once training was completed, results indicated trivial differences in 1RM leg extension between the strength only (21%) and CT (22%) group. However, conflicting results may demonstrate an interference effect of CT on muscular strength (12,17,18,19). For instance, after 12 weeks of CT (leg strength exercises and cycling), trained and untrained adults showed a 25% increase in 1RM half squat and leg press. Although, when leg strength training was completed without the additional aerobic activity, increases in strength were about 35% (17). Similarly, when older adults (aged 65 – 74 years) performed eight weeks of resistance training, 1RM half squat strength increased by 27% compared to 22% in a matched CT group (resistance training plus 30 – 40 min of cycling once per week) (12). Overall, the effects of CT on muscular and explosive strength may be negligible for the general population. However, for higher level athletes where small differences could play a role in competitive success, personal trainers may need to monitor training variables more closely to determine if an interference may be present.

WHICH SHOULD BE PERFORMED FIRST: CARDIO OR RESISTANCE TRAINING?

The quick answer is: it depends on the goal. If the client’s goal is to compete in a 10k, aerobic training would be considered the priority and should be completed first within the training session or receive the most attention during that training block. Whereas, if muscular strength is the goal, resistance training should be programmed towards the beginning of the session. If the target is general health and wellness, personal trainers may still opt to place resistance training first as performing aerobic exercise beforehand may have a potential interference effect on neuromuscular outcomes (4,7,18). Yet, this is highly dependent on the goal or variable that requires the most attention.

While client goals are the main importance, research indicates that altering the order of exercise modes (i.e., resistance training versus cardio) within a given workout can impact certain outcomes over time. For example, when aerobic exercise (high-intensity intervals at 100% VO₂max workload) was performed prior to muscular strength testing, physically-active males experienced an acute 20% reduction in lower-body muscular endurance and about a 20-kg decrease in leg press 1RM (7). Despite this detriment, no acute interference in 1RM bench press or upper-body muscular endurance was experienced. Similarly, when elderly males performed resistance training before cardio, knee extensor 1RM increased by 35.1 ± 12.8% compared to an increase of 21.9 ± 10.6% when 20 – 30 min of moderate-intensity cycling was completed prior to strength training (4).

However, conflicting findings exist in youth and athletic populations, suggesting that exercise order may influence specific outcomes differently depending on the population and goal. For instance, performing aerobic endurance first (intermittent shuttle runs) in youth soccer club players (aged 13+ years) showed an increase of 17.5 kg in 1RM squat after 12 weeks compared to 13 kg when strength training was completed first (15). Interestingly, 10-m sprint and 15-m agility times saw greater improvements with aerobic training first; whereas 30-m sprint times, squat jump heights, and countermovement jump heights elicited greater improvements with strength training first. These conflicting findings demonstrate that altering the sequence of aerobic exercise and resistance training can have either detrimental or beneficial effects on certain outcome variables and may differ between given populations.

WHAT IF CARDIO IS PERFORMED ON A SEPARATE DAY?

While performing resistance and aerobic exercise within the same session is time efficient; performing cardio on a separate day has shown inconsistent results regarding performance and health outcomes (1,11,18). For instance, when aerobic exercise was performed immediately post resistance training, versus a separate day, there was a 12% difference in 1RM leg press strength (13% versus 25%, respectively). When researchers examined muscular endurance, same day training increased 80% 1RM repetitions to fatigue by 39% versus 64% on separate days in untrained college-aged males (18). Despite these changes, trivial differences in skeletal muscle hypertrophy and VO₂max have been observed between same day versus separate day training. For example, physically-active young men experienced similar changes in VO₂max when combining strength and endurance in the same session (+7% increase) versus those who trained cardio on a separate day (+6% increase) (18). Although, the group that performed separate sessions increased 1RM leg press to a greater extent than the CT same session group (25% versus 11%, respectively) (18). Oddly, despite the changes in strength with separate day CT, CSA of the knee extensors increased similarly between the same day (+12%) and separate day (+14%) groups. These changes were similar to the equivalent of a one kilogram of fat-free mass gain and 2% reduction in body fat experienced by untrained, college-aged males at the end of 12 weeks when performing either same day CT or separate day CT (1). These findings suggest that separating sessions may be beneficial for maximizing strength outcomes, yet may not impact other key health outcome variables.

WHAT ABOUT TRAINING FREQUENCY?

Many resistance training programs typically include 2 – 4 weekly sessions, consisting of various periodization protocols, volumes, and intensities. However, the addition of aerobic exercise sessions has the potential to increase total weekly training frequency to 5 – 7 days per week; thus, some individuals may consider completing resistance and aerobic exercise on the same day, or even within the same session, to reduce total trips to the gym or total training days per week. Like most programs, increasing frequency of exercise has been shown to improve health outcomes, hypertrophy, strength, power, and weight loss; although, with greater frequencies, recovery time is decreased potentially interfering with long-term adaptations. For instance, a 12-week study of CT using a single day versus multiple days of aerobic activity along with two days of strength training demonstrated greater gains in strength with lower frequencies of cardio (17). Individuals improved 1RM half squat and 1RM leg press by 35±4% with one day per week of cardio compared to 25±2% in the multiple session cardio group (17).

Similarly, performing one strength-based and one endurance-based exercise session per week elicited similar increases in CSA of the quadriceps (~11%) after 16 weeks compared to two strength-based workouts per week (12). While no difference in CSA was observed for these sedentary individuals, the two-time per week strength group had a 3% and 14% greater increase in 1RM half squat and 1RM bench press over the CT group, respectively. Although, when assessing aerobic endurance, the CT group outperformed the strength group by about eight percent during an incremental cycling test to exhaustion (12). These findings suggest that total training load and recovery are important considerations when designing CT programs.

DOES THE INTENSITY OF CARDIO MATTER?

While performing aerobic exercise at different intensities has specific cardiovascular benefits (e.g., endurance versus increased VO₂max), the intensity at which aerobic training is performed during CT may alter neuromuscular adaptations (6,7,9,22,23). For instance, recreationally-active adult males exhibited greater increases in 1RM squat strength performing aerobic exercise at low intensities (+12.8%) compared to moderate (+7.4%) and high intensities (+8.1%) over a period of eight weeks (22). However, 10-m and 20-m sprint times, as well as countermovement jump, had the greatest improvements in the moderate intensity CT group (-5.1%, –3.7%, and 10.3%, respectively) compared to the low-intensity (-3.7%, -2.8%, and 8.9%) and high-intensity (-1.6%, –2.1%, and 7.4%) groups (22).

These findings agree with previous research examining a 12-week CT program for older adults with metabolic syndrome (6). After completing resistance training with either moderate (walking at 60 – 70% HRmax) or high intensity (3 x 3 80 – 90% HRmax) aerobic exercise three days per week, results demonstrated that certain health biomarkers responded more favorably to the different types of aerobic exercise intensity. For example, moderate-intensity aerobic CT benefited fat-free mass, systolic blood pressure, high-density lipoproteins (HDL), and blood triglycerides to a greater extent compared to the high intensity CT group. Whereas variables such as waist circumference, diastolic blood pressure, blood glucose, total cholesterol, and low-density lipoproteins (LDL) reacted more beneficially to high-intensity aerobic exercise CT (6). Alternatively, recreationally-trained males completing moderate-intensity aerobic versus high-intensity aerobic CT showed trivial differences between groups in 1RM leg press, 1RM bench press, VO₂max, and countermovement jump over an eight-week training period (9).

PRACTICAL APPLICATIONS

While muscular strength and hypertrophy are often primary goals, the belief that CT interferes with these adaptations lacks consistent support. Research shows that CT elicits a plethora of benefits for overall health, and in many cases can lead to increases in performance. However, the potential performance detriments brought on by CT are nuanced and situational, requiring the personal trainer to consider the following variables when attempting to minimize interference:

• Program goals
• Volume
• Frequency
• Intensity
• Recovery
• Training status
• Population

When programmed appropriately, CT can improve both neuromuscular and cardiovascular outcomes. Higher volumes and intensities of aerobic training may increase the likelihood of interference, particularly with insufficient recovery or caloric restriction. Thus, personal trainers should understand that while numerous variables influence CT outcomes and potential interference, when programmed and performed properly, clients can achieve cardiovascular benefits along with increases in muscular endurance, hypertrophy, and strength.

This article originally appeared in NSCA Coach, a quarterly publication for NSCA Members that provides valuable takeaways for every level of strength and conditioning coach. You can find scientifically based articles specific to a wide variety of your athletes’ needs with Nutrition, Programming, and Youth columns. Read more articles from NSCA Coach »

Related Reading

Balabinis, CP, Psarakis, CH, Moukas, M, Vassiliou, MP, and Behrakis, PK. Early phase changes by concurrent endurance and strength training. Journal of Strength and Conditioning Research 17(2): 393-401, 2003.

De Souza, EO, Tricoli, V, Franchini, E, Paulo, AC, Regazzini, M, and Ugrinowitsch, C. Acute effect of two aerobic exercise modes on maximum strength and strength endurance. Journal of Strength and Conditioning Research 21(4): 1286-1290, 2007.

Jones, TW, Howatson, G, Russell, M, and French, DN. Performance and neuromuscular adaptations following differing ratios of concurrent strength and endurance training. Journal of Strength and Conditioning Research 27(12): 3342-3351, 2013.

Makhlouf, I, Castagna, C, Manzi, V, Laurencelle, L, Behm, DG, and Chaouachi, A. Effect of sequencing strength and endurance training in young male soccer players. Journal of Strength and Conditioning Research 30(3): 841-850, 2016.

Shaw, BS, Shaw, I, and Brown, GA. Comparison of resistance and concurrent resistance and endurance training regimes in the development of strength. Journal of Strength and Conditioning Research 23(9): 2507-2514, 2009.

Sousa, AC, Neiva, HP, Gil, MH, Izquierdo, M, Rodríguez-Rosell, D, Marques, M, et al. Concurrent training and detraining: the influence of different aerobic intensities. Journal of Strength and Conditioning Research 34(9): 2565-2574, 2020.

Wong, PL, Chaouachi, A, Chamari, K, Dellal, A, and Wisloff, U. Effect of preseason concurrent muscular strength and high-intensity interval training in professional soccer players. Journal of Strength and Conditioning Research 24(3): 653-660, 2010.