Health and Fitness Benefits of Performing Aerobic and Resistance Training

Before delving into concurrent training (CT) and the interference effect, we need to clearly define aerobic training (AT), resistance training (RT), and discuss their health and fitness benefits. For the purposes of this column, we will define AT as a form of exercise during which cyclical, submaximal contractions are repeated for an extended period of time (Hughes Ellefsen & Baar 2018). Several applications of AT exist including cycling, hiking, jogging and rowing. Consistently performing AT leads to central (e.g., increased cardiac output) and peripheral (e.g., increased capillary density) adaptations, which ultimately improve maximal oxygen consumption, lactate threshold and movement economy (Hughes Ellefsen & Baar 2018). The health benefits of AT are well-documented and include reduced risk for metabolic diseases and sudden cardiac episodes (Brellenthin Bennie & Lee 2022).

On the flip side, RT will be defined as a form of exercise during which muscular contractions are performed to overcome external loads (Hughes Ellefsen & Baar 2018). Common applications of RT are bodybuilding, circuit training, high-intensity functional training and powerlifting. Adaptations to RT include skeletal muscle hypertrophy and increased neural drive, which lead to increased muscular endurance, strength and power (Hughes Ellefsen & Baar 2018). RT also reduces risk for cardiometabolic diseases (Brellenthin Bennie & Lee 2022), and the abovementioned RT-adaptations stave off sarcopenia, improve functional task performance and reduce fall risk (Green et al. 2022). Clearly, AT and RT provide a bevy of health and fitness benefits.

Concurrent Training and the Interference Effect

CT is defined as the concomitant performance of aerobic and resistance training during the same session, day, week, month or training block (Fyfe Bishop & Stepto 2014). The health benefits of CT are vast and synergistic, as the combined execution of AT and RT confer a greater reduced risk for all-cause mortality compared to either modality alone (Phillips Ma & Rawson 2023). For example, a recent meta-analysis reported that RT decreased risk for all-cause mortality by 21%, but the addition of AT to an RT program decreased risk for all-cause mortality by 40% (Saeidifard et al. 2019). Hence, to maximize health benefits, exercise enthusiasts should strongly consider implementing a CT program (Brellenthin Bennie & Lee 2022; Phillips Ma & Rawson 2023). Despite the synergistic benefits of CT, those who prefer RT may hesitate to participate in AT because of the so-called interference effect: the theoretical phenomenon suggesting that AT dampens the hallmark adaptations to RT such as hypertrophy, strength and power (Fyfe Bishop & Stepto 2014). In a colloquial sense, the interference effect suggests that cardio will ruin your gains. To address the interference effect, let’s take a walk through history and discuss the original CT study conducted by Dr. Robert Hickson in 1980.

The Original Interference Effect Study

According to legend, Dr. Hickson, who was a prolific powerlifter, joined his PhD mentor, Dr. John Holloszy, for his daily jogs, and noticed that his muscle size and strength decreased over time. Thus, Dr. Hickson designed the first-ever CT experiment to explore the phenomenon that he’d experienced during his personal jogging endeavor. Briefly, recreationally active men and women were randomly assigned to AT, RT or CT, and each group performed their prescribed training program for 10 weeks (Table 1.). Readers should notice that each exercise program was high-volume/high-intensity, and the CT subjects simply ‘doubled the dose’ of aerobic and resistance training. By the end of the trial, RT and CT groups increased maximal strength (measured via weekly 1-RM back squat), but RT was superior to CT, and CT regressed in strength between weeks 8-10. Although a crude measurement of hypertrophy was used (i.e., thigh circumference), RT and CT improved similarly, which implies that hypertrophy was unbothered. Moreover, CT and AT similarly improved maximal oxygen consumption/uptake (VO2max) on a bike and treadmill, which demonstrated that RT did not interfere with typical aerobic adaptations (a common finding in the CT literature) (Hickson 1980).

It is interesting that the original CT study concluded that AT interfered with RT adaptations despite the fact that hypertrophy was not blunted, and strength was unaffected until the 8th week of the program. The latter outcome is likely explained by fatigue and overtraining rather than a fanciful neuromuscular phenomenon, as the CT subjects performed 11 exercise sessions per week (Kreher & Schwartz 2012). Nevertheless, it is nearly a half century later and 50+ publications can be found on CT and the interference effect. We will discuss the collective findings of these studies below.

Interference or Not? A Tale of Two Reviews

Rather than dwelling on data from one randomized-controlled trial, we will conjure the results from two critical meta-analyses in this area. As a reminder to the reader, a meta-analysis is essentially a ‘study of studies’ in which data from several individual trials are combined and analyzed at once. This greatly increases sample size (e.g., from 20 to 200), which boosts statistical power and helps scientists identify truly meaningful effects. First, Wilson et al. (2012) combined 21 studies, with a total of 422 individual data points, and reported that compared to CT, RT alone elicited better outcomes for muscular hypertrophy, strength and power. Although this lends credibility to an interference effect occurring, it should be pointed out that CT still produced medium-large effects for the abovementioned variables. In other words, individuals who performed CT experienced positive RT-related adaptations, but performing RT alone was simply better. Additionally, the authors identified specific AT variables that had the biggest influence on blunting hypertrophy, strength and power, which are covered in the practical application section below.

Years later, Schumann et al. (2022) updated the previous meta-analysis, and analyzed the combined effects of 43 CT studies (n = 1,090). Results indicated that hypertrophy and strength were not significantly different between CT and RT, but explosive strength (commonly thought of as power) was attenuated if AT and RT were performed within the same session. However, this negative effect was eliminated if aerobic and resistance training sessions were separated by ≥ 3 hr. In contrast to the findings of Wilson and colleagues (2012), Schumann et al. (2022) reported that the magnitude of interference, or lack thereof, was not influenced by any particular AT characteristic, such as duration, frequency or modality. Data were also not influenced by the subject’s age or training status. Therefore, based on current research, CT does not seem to negatively affect hypertrophy or strength; power may be slightly impacted, but this can be avoided by performing AT and RT sessions separately (≥ 3 hr).

Conclusions

People with general health and fitness goals can reap many benefits from CT and should not be overly concerned with a potential interference effect. Regarding the mixed evidence surrounding CT, it is plausible that interference stems from excessive fatigue accrued when collective training volume outpaces one’s capacity to recover/adapt. Capacity to recover may be reduced in those with lower training age, poor diet/sleep habits, or who are dealing with significant life stress. Special considerations should be made for people who befall these circumstances. Clients with a specific RT-related goal, like winning a powerlifting meet, may fail to maximize strength and power if an excessive amount of AT is added to their program. Contrarily, clients with a specific AT-related goal, like running a fast marathon, may not reach their maximal performance potential if too much RT is added to their program. For these situations, aerobic and resistance training can be introduced with minimum-effective approaches, and higher volumes of each can be systematically periodized into the plan based on the current phase of the training program (e.g., off-season versus pre-season).

Practical Application Tips

The interference effect may be less daunting than previously believed, but fitness professionals may be interested in taking steps to ensure that minimal interference occurs in their client’s CT program. Based on the results from Wilson et al. (2012) and Schumann et al. (2022), the following recommendations should be considered.

Note: It is important to highlight that these are evidence-based guidelines, but they may not perfectly fit every client’s goals or preferences; adjust your training plan accordingly.

Due to greater eccentric loading and impact, running may cause more interference than other AT modalities such as cycling or elliptical training.

Higher training frequencies ( > 3 days/week) and weekly training volume ( > 150 min/week) may cause more interference than lower training frequencies ( < 3 days/week) and weekly training volume ( < 150 min/week).

Low intensity, long duration AT sessions ( > 30 min) generally lead to more interference. Consider planning short, intense AT sessions that last 10-20 min. Several sprint interval training programs fit this paradigm.

Interference is specific to the musculature that is trained. Therefore, running and cycling may be added to an upper-body-dominant-RT day, while rowing and battle ropes may be added to a lower-body-dominant-RT day.

Interference generally does not occur when AT and RT sessions are separated by ≥ 3hr. Program AT and RT on opposite days or provide morning/evening sessions for a two-a-day client.

If aerobic and resistance training sessions must be completed in the same session, RT should be performed before AT as fatigue induced by AT may reduce the quality of the RT session.

Table 1. A description of the training programs applied by Hickson (1980).

Aerobic Training

• 6 days/week
• M/W/F: 30-40 mins of running to exhaustion on a treadmill.
• T/Tr/Sat: 6 reps of 5 min maximal effort bike sprints; 2 min rest between reps.

Resistance Training

• 5 days/week
• M/W/F: 3-5 sets of 5 reps for knee extension, leg curl, and squat.
• T/Tr: 3 sets of 5 reps for leg press and 3 sets of 20 reps for calf raises.

Concurrent Training

• 5-6 days/week
• Subjects performed the AT and RT programs on the same day.
• 30-120 min of passive rest was allotted between sessions.

Want to learn more? Resistance Training and the Cardiorespiratory Response