Concurrent Training (CT) is defined as the combination of resistance and endurance training in a periodized program to maximize all aspects of physical performance. Unless an athlete is in a pure-power sport like Olympic Weightlifting, or a pure-endurance sport like long distance cycling; a combination of both power-related and endurance-related attributes are required to excel in mixed-type sports. Mixed type sports are sports that depend on several different energy systems and different strength and speed properties. MMA, boxing, basketball, soccer, hockey and many other team-based sports fall under this category.

In the world of bodybuilding and strength sports, cardio is used as an umbrella term for all types of endurance training protocols. Often as a joke among lifting circles, cardio has been stigmatized to "steal your gains", so far to the point that some lifters see it as a badge of honor to be out of shape and possess almost no cardiovascular conditioning in return for being able to lift a massive amount of weights.

In the world of endurance sports like running and cycling, strength training can be seen as an unnecessary training method that adds unwanted muscle mass to the frame of an endurance athlete, possibly slowing them down and being detrimental to their performance. The term "meathead" might even be applied to people who lift weights.

This article will shed some light on what cardio and strength training has to offer to each training demographic/niche and how a mixed-typed athlete can best organize their training so they reap in the benefits of both training modalities with little to no interference.

The Science and theories behind ct

In one of the first research studies carried out on the effects of concurrent training, Hickson (1980) observed that training both strength and endurance qualities simultaneously had detrimental effects on strength development but did not negatively impact aerobic qualities. Building off of research by Hickson, more recent studies have shown a wide variation of responses in concurrent training, both positive and negative. This suggest that CT methods are still inconslusive and variables such as genetic differences, modality of endurance training, nutritional status and training time may play a role in mediating the effects.

Termed the molecular signaling theory, it has been hypothesized that the distinct molecular signaling pathways of strength and endurance exercise adaptation may be incompatible and inhibit the development of each other.

Other theories speculate that the poor management of both resistance and endurance exercise variables may expose athletes to higher incidents of overreaching and overtraining. For example, the high volume nature of endurance training paired with heavy strength training may make recovery more difficult, increasing the risk of overtraining and injury to an athlete. In conjunction with observing the effects of CT, uncovering the potential mechanisms behind the molecular signaling theory is needed to understand how strength, power and endurance can be developed simultaenously.

Molecular signaling theory

Taken from "Molecular Responses To Strength & Endurance Training - Are They Incompatible?"

Resistance training adaptations such as muscle fibre hypertrophy, strength and power acquisition are known to be mediated by molecular signaling pathways known as the AKT and mTOR pathways. Resistance exercise that create large force outputs, mechanical tension and stretch, as well as muscle damage and swelling are activators of these hypertrophic pathways (Brad Schoenfeld talks more in depth about this in his frequently cited research article - "The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training"). Many of the compound powerlifts that recruit larger amounts of muscle mass have the capability to create large force outputs and mechanical tension, while exercises such as isolation exercises taken to failure, drop sets, supersets, giant sets, contribute more to metabolic stress and cell swelling.

In contrast, prolonged and repetitive low intensity muscle contractions activate signaling pathways involving the enzymes AMPK and CaMK, which are responsible for adaptations related to endurance training such as mitochondrial biogensis; allowing you to walk, run, swim and bike further and more efficiently.

Capturing the attention of researchers, the observed suppression of mTOR signalling pathways by increased AMPK activation has been the focus and basis behind the molecular signaling interference theory. AMPK activation downregulates and can blunt the hypertrophic response to a resistance training workout or program by inhibiting mTOR and can increase protein degradation through other pathways we won't get too deep into. mTOR can even be downregulated indepedent of AMPK activation, through a family of proteins called SIRT (more information here for the nerds). What you need to know is that when performing cardio training during a resistance program, muscle hypertrophy, strength and power can be compromised.

So were the bros right all along? Does cardio in-fact, kill your gains?

Not so fast. It is not uncommon to see increased endurance performance or improved resistance training outcomes with CT, thus several studies have made arguments for the limitations of the interference theory. In some cases, resistance training can upregulate AMPK, and aerobic exercise can also induce increases in mTOR activity; therefore a positive transfer effect might be present when intensity, volume and frequency of each training modality are strategically manipulated.

It should also be noted that the results of research studying the acute effects of exercise and molecular signaling cannot always be predictive of future chronic adaptations. If athletes have no CT experience, the interference effect might be just occur until the athletes acclimatizes the the CT methods. It is also possible that the interference effect may not be present until later into a training cycle where there is an accumulation of resistance and endurance training fatigue due to increased training loads, or intensity. For example, the original CT study by Hickson (1980) found there were no interference effects until the 8th week of training.

Since molecular signalling has been found to be highly variable depending on the training status of the individual (strength trained vs. endurance trained vs. completely untrained), CT variables must be prescribed based on athletes' current training status and previous training experience.

Concurrent Training Effects on Resistance Training Adaptations

Varying modality, intensity, frequency and volume of training has been shown to affect the magnitude of molecular signaling and protein synthesis. Therefore we know that the degree of interference between signaling pathways can also vary depending on programming variables. Since AMPK downreulgates mTOR signaling and NOT vice versa, its hypothesized CT can be more detrimental to resistance training (RT) related adaptations compared to endurance training (ET) related adaptations.

Muscle Hypertrophy

Muscle size and hypertrophy is a highly sought after adaptation in the fitness world, both as a means to improve metabolic health, and a way to achieve an aesthetic physique.

With several studies concluding that CT blunts muscle hypertrophy, excessive training volume has been predicted to be the cause of overtraining and fatigue when comparing CT groups to resistance training only groups. Based on our knowledge of exercise adaptations, we should know that RT volumes performed by an elite bodybuilder cannot be concurrently trained successfully with ET volumes performed by an elite triathlete due to nutritional and time constraints. However, to date, there is no conclusive evidence that hypertrophy is blunted when low-volume aerobic exercises is added into a training program; some studies have even shown it might mitigate muscle loss (Study #1, Study #2). High volume ET on the contrary, can be detrimental due to the factors discussed above (interference theory OR overtraining/fatigue theory OR... both). For practical recommendations on how to avoid interfering with your hypertrophic workouts, read below in the practical application section.

Strength & power

In a large scale meta-analyses of CT effect sizes, Wilson et al (2012) observed that in many studies, power was significantly lowered during CT while muscle cross sectional area and strength was maintained. This suggests that force at high velocities (think of a vertical jump, NOT a heavy squat) may be affected to a more significant degree with concurrent ET than force at lower velocities. The mechanism behind this could be attributed to motor unit and specific muscle fiber type innervation. During classical long duration endurance exercises, the majority of muscle action and force production comes from low threshold, fatigue-resistant type I muscle fibers. There may be a shift from type IIx fibers to type IIa fibers or type IIa to type I fibers to accommodate the oxygen-demanding adaptations of endurance exercise during concurrent training therefore aerobic exercise can be detrimental to athletes that require a high rate of force development rate and power when poorly programmed into a periodized plan.

To better compliment the demands of strength and power sports, prescribing lower volume - higher intensity and velocity interval type ET may be more beneficial in terms of maintenance and improvement of power. This may be because of the similarities in the motor unit/muscle fibre recruitment patterns in RT and high-intensity based ET. When viewing this from an interference theory stand point, the high energy costs and high activation of AMPK from high-intensity ET could potentially magnify the interference effect but the research does not support this claim as there has been no cases of muscle mass loss when high-intensity ET is prescribed in a low-volume fashion. Research isn't conclusive but it is clear the intensity and volume of endurance training affect muscular strength and power outcomes.

When it comes to training modality, it is hypothesized that modalities that require a lot of eccentric muscle action can cause excessive muscle damage, further impeding the muscle recovery process from a challenging RT session. Because of this, predominantly concentric movements, are favored over running (a movement that includes a lot of eccentric contractions - think of every time your foot strikes the ground), like cycling and prowler pushes. Cycling, specifically hill climbing, can also resemble resistance-like loading patterns and can induce lower body hypertrophy that may compliment RT adaptations. Following this stream of thought, other ET modalities that possess lower eccentric muscle action and lower impact stress like swimming or modalities that have similar loading patterns to resistance training such as prowler-pushes and sled drags can also be used to improve cardiovascular conditioning when training concurrently.

Concurrent Training Effects on Endurance Adaptations

Well-trained athletes often show different responses to exercise compared to untrained individuals, endurance athletes are no different as they may see dissimilar improvements from RT compared to untrained or already resistance-trained individuals. High volume and high frequency endurance training make it hard for endurance athletes to improve muscle size, strength and power without cutting into the recovery process of ET. Paired with the high energy expenditure and AMPK levels from ET, CT can be problematic for endurance based athletes.

Wang et al (2012) showed that resistance training following endurance training elicited greater PGC-1a actavation (a regulator of energy metabolism and endurance adaptations) and therefore more oxidation capacity improvements than endurance training alone. The increase in activation of PCG-1a was thought to be related to the high amount of reactive oxygen species (ROS) and lactate concentrations produced during CT vs. endurance training alone. AMPK activation however, was not a plausible explanation for the improvements in oxidative capacity as AMPK was similar in both the CT and ET-only group. The researchers suggest that the RT portion of the CT group upregulated mTOR, which had a positive effect on PGC-1a. The interactions between mTOR and PGC-1a also have implications for muscle and endurance performance.

Another study concluded that resistance training has positive implications for endurance performance, mainly due to increases in type IIa muscle fibers and a greater potential for force production (Study). This means endurance athletes that concurrently resistance train can improve their average and peak power outputs, which play a big factor during races and time trials. In addition, a study looking at the effects of CT found that pairing strength circuit training along with an ET program (same workout session) improved aerobic performance more than an ET program alone where VO2max and 4km time trial performance both increased slightly more in the CT group compared to the ET only group. The beneficial effects of RT for endurance athletes cannot be overlooked!

Resistance training can also benefit endurance events of different durations. For shorter, more anaerobic dominant endurance events like the 400m/800m run, most swimming events and many team sports, performance can be improved by increasing muscular strength and neuromuscular function that can't be achieved with ET alone. For long aerobic endurance events and competitions like marathon runners, and triathletes, improvements in performance can be attributed to the higher economy of movement induced by RT (study), which simply means endurance athletes that resistance trained were able to more efficiently use their energy to travel at any given velocity, saving them energy in a long race.

Concurrent Training Timing

(AE = Aerobic Exercise) (RE = Resistance Exercise) Disclaimer: This is not my chart. — (AE = Aerobic Exercise)
(RE = Resistance Exercise)
Disclaimer: This is not my chart.

If we base our training order off the molecular signaling theory of the interference effect, ET would be best performed prior to RT (within the same day). Since AMPK downregulates mTOR and not vice versa, an ET session that raises AMPK levels will not have a chance to interrupt mTOR signalling if RT is performed after. But we also need to take into account other facts to help us maximize training adaptations and minimize any interference.

The interference theory can manifest in the form of negative interactions between protein activity and molecular signalling, but coaches cannot overlook the more simple explanations as to why there might be an interference effect when training several physical attributes or modalities. Reduced training quality can also be an explanation as to why CT causes interference problems. For example, after performing a ET session, performance in the subsequent RT session may be diminished due to pre-exhaustion. This is problematic if a particular athlete needs to prioritize his/her RT session because of their personal goals, weaknesses or position on a sports team. Coaches and athletes must plan and prioritize which training sessions are more important and the introduction of rest and nutrition must be taken into account to mitigate any interference effects.

In order to minimize the fatigue and the interference effect, a 24-hour recovery period between training sessions is suggested; the longer the better. However, this suggestion is often not practical for subelite or elite athletes that want to or are required to train 2 and up to 3 times a day. Based on the time course of AMPK elevation and it's downregulation of mTOR signaling, a minimum recovery time of 3 hours is suggested between ET and RT sessions. Although 3 hours is enough to reduce the molecular signaling part of the interference theory, its also suggested that 6 hours or more is needed to reduce the muscular fatigue from the previous ET bout and retain muscular performance during a subsequent RT session. When high-intensity ET was performed prior to RT, force production was reduced for at least 6 hours, while the capacity to perform higher volume RT was also diminished for up to 8 hours. As noted by Robinuea et al (2014), technical and tactical training sessions in team sports also have an cardiovascular and endurance component to it, not scheduling adequate rest before a subsequent RT session could be detrimental to performance.

Nutritional Protocols For Concurrent Training

Scheduling adequate recovery time in between RT and ET sessions can also allow for nutritional interventions to decrease the interference effect. Since AMPK is increased when the energy status of an athlete is low, RT in theory, is better performed in a fed-state to allow for optimal mTOR signaling post-workout. Ingesting carbohydrates and high-quality, leucine-containing protein supplements or whole foods to fuel post-exercise-induced increases in muscle glycogen and protein synthesis is crucial when training concurrently. In contrast, performing ET while in a fasted state (low-intensities) can promote PGC-1a and AMPK signaling, exemplifying ET adaptations.

Here is my article on nutritional periodization.

Here is a research review on nutritional methods used to maximize concurrent training.

Practical Application & Recommendations

In light of this research, endurance athletes should focus on programming moderate volume, moderate to high intensity RT around their prioritized ET, sport-specific, sessions.

Strength and power-based athletes should include low to moderate volume ET in order to attenuate the interference effect while still reaping in the benefits of ET such as improving blood flow for recovery or building some general work capacity in the off-season or restoration period.

For mixed-type and team sport athletes, the proportion of RT and ET should be strategically programmed based on the energetic and muscular demands of the sport, an athlete's strength & weaknesses, as well as their position on the team.

The Dilemma

So when it comes to within-day training order, there is a dilemma on whether RT should come before or after ET.

If RT comes before ET, performance during the RT session will hypothetically be higher quality due an absence of residual ET fatigue but hypertrophic signaling will be downregulated after the ET session.

If RT comes after ET, the glucose/substrate depletion and residual fatigue from ET will reduce the training quality of the RT session and may also blunt the hypertrophic response. Specifically, AMPK is upregulated by increased energy expenditure and substrate depletion, negatively affecting the mTOR pathway.

So what do you do? It really depends on your goals.

For me to offer any rigid periodization schemes or training methods would be counterproductive. Coaches and trainers need to plan their athletes' demands, circumstances, strength, weaknesses and limitations. Training should be evidence-based but there is also a lot of room to be creative in their program design. I'll give you some tips:

If You Are training for hypertrophy...

Endurance training after resistance training if your ET session is low to moderate volume. The ET session ideally will use different muscles than the ones emphasized in your RT session. It is still unknown whether the molecular signaling theory of the interference effect is peripherally driven - meaning muscle specific, or if it is systemic (whole body interference regardless of muscle trained) sessions. If the interference effect is peripherally driven, a viable strategy would be to perform an upper body lifting session, followed by a lower body only cardio session like cycling.
Endurance training before resistance training if you have 3-6 hours of recovery time in between the two sessions and are able to refuel with carbohydrates and some protein.
Use low-impact ET modalities like prowler, cycling, sled drags, elliptical, swimming, etc... to avoid any more stress on your joints and muscles
Use moderate-volume, low-intensity ET to improve heart health, blood flow to working muscles as a form of recovery, control energy expenditure to mediate diet/macros and calories intake.
Use Zone 1 to warm up prior to lifting sessions.
Use Zone 2 on off days or as a separate training session to improve the benefits discussed above (heart health, energy expenditure, etc.)
Additionally, you can use Zones 4 and 5 as a "finisher" to hypertrophy-based workouts as a form of metabolic stress. These zones recruits similar muscle fibers and uses the same energy systems as strength training sessions.

If you are training for strength...

Endurance training after resistance training if your ET session is low to moderate volume. The ET session ideally will use different muscles than the ones emphasized in your RT session. Same rule applies from the hypertrophy section.
Endurance training before resistance training if you have 6+ hours of recovery time in between the 2 sessions and are able to refuel with carbohydrates and some protein. Since strength training emphasizes sport technique (powerlifting, weightlifting, Strongman) more so than general hypertrophy training and operates at higher intensities (as a % of 1RM), it is extra important that muscle residual fatigue is as low as possible.
Use low-impact ET modalities like prowler, cycling, sled drags, elliptical, swimming, etc... to avoid any more stress on your joints and muscles.
Use Zone 2 on off days or as a separate training session to improve the benefits discussed above (heart health, energy expenditure, etc.) I personally like doing Zone 2 easier, low-impact aerobic workouts with a mobility and stretching routine to promote recovery on off days.
Zone 4 and 5 can be used as intervals on the prowler, sled, rower or ski-erg to increase top end cardiovascular conditioning to help you improve your work capacity for high volume lifting workouts.

If you are training for power...

Endurance training session after your resistance training session if your ET session is low to moderate volume.
Endurance training before resistance training is definitely not recommended. If it must be done, take as much recovery time as possible between sessions and refuel adequately and avoid working the same muscle groups. Pre-fatigued power workouts can be dangerous! Ideally rest 24+ hours.
Use low-impact ET modalities like prowler, cycling, sled drags, elliptical, swimming, etc... to avoid any more stress on your joints and muscles
Use Zone 2 on off days or as a separate training session to improve the benefits discussed above (heart health, energy expenditure, etc.) I personally like doing Zone 2 easier, low-impact aerobic workouts with a mobility and stretching routine to promote recovery on off days.

If you are an long event endurance athlete...

In almost all situations, resistance training AFTER endurance training is the best choice. You avoid pre-fatiguing yourself before your sport-specific training sessions (endurance training) and avoid downregulating mTOR to any significant degree.
Resistance training should focus on complex and compound exercises that span across several different muscles and movement types. Use weights anywhere from 70-90% of your 1RM. This is important for building a strong and healthy body that can withstand the high volumes of endurance training by building tissue resilience, tendon strength and core strength. Avoid using silly training methods like 30+ rep sets with 50% of your 1RM in hopes of building muscular endurance. Endurance will be built specifically in your sport already.
Time is valuable. Don't fall into the trap of training on unstable training surfaces or overly-specific exercises that try to mimic movements from your endurance sport. Stick with the basics. Push, pull, hip hinge, squats, etc...
Endurance athletes training in the off-season or restoration period can experiment with fasted training. More on this in my nutritional periodization article.

If you are a shorter event endurance athlete...

In almost all situations, resistance training AFTER endurance training is the best choice. You avoid pre-fatiguing yourself before your sport-specific training sessions (endurance training) and avoid downregulating mTOR to any significant degree.
Shorter event endurance athletes like 400-800m runners or track cyclists will greatly benefit from resistance training and is almost a requirement to excel in many short endurance sports. Use weights anywhere from 70-100% of your 1RM to build muscle mass and strength. Power training can be included using anywhere from 30-70% of 1RM to match the demands of the sport and can benefit endurance athletes involved in sports that require bursts of high-intensity efforts.
Again, don't fall into the trap of training on unstable surfaces or overly-specific exercises. Exercise selection though, should be more carefully planned to match some of the movement patterns seen in the endurance sport. There is a bigger carry over for shorter event endurance athletes vs. longer event athletes.
Endurance athletes training in the off-season or restoration period can experiment with fasted training. More on this in my nutritional periodization article.

If you are a mixed type athlete or team sport athlete

Mixed type sport athletes can have vastly different demands depending on the sport itself and the position played. Luckily through a needs-analysis, all sports can be plotted onto a endurance and strength attribute spectrum. Below is a figure outlining the physical demands of several sports.

Start by mapping out the specific strength and endurance demands, movement patterns, and energy demands of your position or sport and follow the principles discussed above to get the most out of your training sessions!

Here are related articles on all things training programming and periodization, goes well with this concurrent training article.