Concurrent Endurance And Resistance Training: Are You Compromising Your Strength?

Experiments have been done to investigate the possible effects anaerobic training may have on aerobic training and vice versa. In this article, I will present some of the current thoughts as to what happens when you train on a concurrent schedule.
Many people feel that performing aerobic endurance training along with your anaerobic resistance training you are making a big mistake as it will compromise your strength gains. Selected others may feel that performing both modes of training may serve to complement each other and produce a better athlete/healthier individual.

What belief is the most accurate? It depends on many different factors, some which are controllable and others that are not. Every person has a different physiological make up and will respond to training in different ways.

Experiments have been done to investigate the possible effects anaerobic training may have on aerobic training and vice versa. In this article, I will present some of the current thoughts as to what happens when you train on a concurrent schedule.

Factors That Come Into Play With Concurrent Training

First off, there are a number of variables that will determine how concurrent training will affect your progress. These include

  • How well your body recovers from exercise.
  • Your age and current fitness level.
  • The type of endurance training you are performing.
  • How long you have been strength training for.
  • Whether you are male or female (will affect the amount of type of hormones you have circulating in your body).

All of these factors together can play a role in how much strength and power improvement you display from your training.

The mode of endurance training you are performing will have a considerable role in your strength improvement. If your primary form of endurance training is running, you may see more noticeable performance hindrance in your strength training than if you were performing other forms of endurance training.

Because running is very physically demanding on the lower body muscles, strength training of the lower body will be affected and your strength and power output levels will most likely not be as great as they would be if you were just training strength alone. Studies done comparing the mode of cycling have shown inconsistent results on performance affects (Leveritt et. Al, 1999).

Finally studies done on rowing or arm cranks have shown to have no effect on strength training performance levels.

Training History
The next factor to consider is training history. Athletes who are or have trained for endurance type sports suffer fewer conflicts when they combine both aerobic and anaerobic training than athletes who have not.

Since these athletes bodies' are more accustomed to the longer duration type activities they have a better recovery ability and can therefore workout at a great intensity level during their strength training workouts.

Endurance athletes in general however, usually have body types that are more suited to endurance activities than strength athletes, which can also affect the progress they make in terms of strength performance. They tend to be naturally leaner and have different muscle fibre compositions (with regards to fast and slow twitch) and therefore may have a tendency to show a lower level of muscular hypertrophy and strength increases.

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When compared with sedentary controls though these athletes will be able to better tolerate concurrent training (Leveritt et Al., 1999).

So what causes concurrent training to possibly have an affect on your strength and power results? A few different theories exist.

The first one is overtraining. If you take an athlete who is training for only strength and power, and put them in a gym 5 days a week for an hour each time, then take an athlete who is training for endurance and have them do similar amounts of endurance type training, both athletes will show improvements in their chosen activities.

However, if you choose training concurrently and spend both 5 hours strength training and 5 more hours training your aerobic system there is a much higher chance that you will become overtrained as the volume of work you are performing is surpassing your recovery capabilities.

The second possibility on the negative effects of concurrent training is that muscle fibre hypertrophy, metabolic enzyme activity, contractile protein structure, and endogenous substrates that result from strength training are considerably different from the adaptations that take place during endurance training.

When the athlete trains both forms together, the muscle is in a conflict state as it is trying to adapt to both forms of stress. However, since they are basically at opposite ends of the spectrum, it is unable to progress in the most desirable manner. One particular adaptation to look at is that of aerobic enzymes.

When one performs endurance activity these enzymes tend to be activated whereas after a bout of strength training these enzymes are often inhibited for quite some time. This prevents the muscles from optimally adapting and progressing.

Muscle Fibre Characteristics

Muscle fibre characteristics are also important to look at. Generally, when performing endurance activities the body relies primarily on slow twitch muscle fibres (type I). These types of fibres are best suited to prolonged activity, use a great deal of oxygen, and are fatigue resistant.

The next type of fibres are type IIA. These fibres are not as fatigue resistant as type I and are more targeted towards producing movements that involve more strength and quicker contractions.

The final type of fibre is the type IIB fibres. These are the maximal force fibres and tire quickly. They produce the greatest amount of force in the shortest period of time.

When we look at different types of training, endurance training relies primarily on type I and type IIA fibres whereas strength training relies more on type IIB fibres. When we train in a certain form of exercise, our muscle profile begins to adapt and become more and more like the particular form of fibre that is best suited to that activity.

So, if you all of a sudden took up an endurance training program, your type IIB muscle fibres would begin to change their properties so that they more closely resemble type IIA fibres. When you train both endurance and strength, your muscles will once again be in conflict since each activity relies on a certain type of fibre. The body will be forced to keep characteristics of both and will therefore not adapt as optimally as it could if you trained for just one training mode.

Below is a picture showing the 3 different types of muscle fibres. Notice the slow twitch fibres produce force for the longest duration where as type IIB produce force for the least duration. The Type I fibres also have the most myoglobin as this is the property that allows them to utilize the most amount of oxygen.

Click To Enlarge
The 3 Different Types Of Muscle Fibres.

Muscular Hypertrophy Adaptations

The next factor to consider when taking into account the effects of combining resistance training with endurance training is how muscle hypertrophy may be affected. When an overloading stimulus is placed on a muscle, the muscle responds by adapting and growing stronger so it can deal more efficiently in the future if a similar load is placed upon it.

Often one of the results of this adaptation is an increase in the size of the diameter of the muscle, known as muscle hypertrophy. One of the prime determinants of how great a load the muscle is able to lift is determined by the size of the cross-sectional area of the muscle fibres so this is an important consideration to think about when talking about maximal muscle strength.

Some studies have demonstrated that concurrent training increases the size of the slow twitch, fast twitch a and fast twitch b muscle fibres, while strength training alone only increases the size of the slow twitch b fibres. It is interesting to note however, that in this study strength was not compromised in anyway during either the concurrent group or the strength only group.

One may conclude that the hypertrophy seen may be altered by concurrent training, however strength gains are not. More studies need to conducted to get a firm understanding of how concurrent training effects muscular hypertrophy but this gives us a general idea of what may be happening.

Hormone Production And Release

One of the biggest influences on an individual's training results is determined by the action of hormones. Hormones are released by many glands in the body and essentially instruct the body on what to do as a result of a current situation. Using different training methods we can elicit different hormonal responses and thus change the results we see from our training.

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In general, endurance training has a tendency to release catabolic hormones which serve to break down muscle tissue whereas strength training releases anabolic hormones which promote muscle growth. The catabolic hormone most often released is cortisol and the anabolic hormones released include testosterone, insulin and growth hormone.

Some of the factors that influence the release of the different hormones are the total workload performed, the duration of the workout, the intensity of the workout, the length of the rest periods and the current status of glycogen stores in the body. Exercise of any type that is quite lengthy in duration tends to be catabolic.

When we look at concurrent exercise training, if the individual is performing both strength and endurance workouts in the same workout there will no doubt be a much greater chance that cortisol will be released. One main reason for this will be because the body will run out of stored glycogen and blood glucose and will turn to it's muscle tissues for energy. This will put a dramatic halt to any muscle building that is taking place.

Another important consideration with anabolic hormones is rest periods. Studies have shown that performing resistance training with shorter rest periods (1 minute) produces greater amounts of mmol of testosterone/L of blood than resistance training that utilizes longer rest periods (3 minutes and greater).

High intensity exercise performed for shorter duration also increases the chances of anabolic hormone release which is why many people are now changing their cardio from longer moderate intensity duration to short high intensity sprint sessions. This type of cardio is very similar to the hormonal environment an intense weight lifting situation produces and thus helps promote anabolism rather than catabolism as the former does.

A good example to illustrate this is to picture a long distance marathon runner vs. a sprinter. Both athletes are incredibly lean, however the sprinter carries a great deal of muscle mass with them whereas marathon runners often have a long and thin look to them.

Click To Enlarge

When one performs long distance endurance activities the body tries its hardest to reduce any unnecessary mass that will slow it down and thereby cause more work, and since muscle tissue is quite dense it often one of the first tissues to go. So, when considering concurrent training versus strength or endurance training alone, concurrent training appears to produce a more catabolic environment than strength training alone, however not as catabolic as endurance training alone.

One way an athlete can offset some of the catabolic effects of concurrent training is to ensure that any endurance work is done separately from strength work and to ensure they have a full supply of muscle glycogen going into their endurance training so the body will have enough fuel for the activity and muscle repair.

Making sure to take in plenty of post exercise carbohydrates is also vital to ensure the muscle tissues get restocked with glycogen after an endurance training session and to promote better performance at the next workout session whether it be strength or endurance.

Motor Unit Recruitment

Mechanical Model For Analyzing Adaptation To Training.
In the following sections, our objective is so see if we can come to any conclusions about our training by analyzing a simple mechanical model that conforms to the SAID principle.

Motor unit recruitment can also be influenced by concurrent versus strength or endurance training. Endurance training of moderate intensity tends to recruit slow twitch muscle fibres. These fibres, as described above are more fatigue resistant, supply more oxygen to the muscle cells and are therefore more suited to endurance activities.

Resistance training on the other hand is aimed at maximal force production and recruits all types of muscle fibres in an effort to generate the most force possible. Concurrent training may interfere with maximal recruitment during strength training and thus result in less force production.

Studies indicate that vertical jump results are decreased during concurrent training versus isolated training thus demonstrating that maximal motor unit recruitment may be jeopardized when endurance exercise is performed (Leveritt, 1999).

One hypothesis for the reduction in strength performance resulting from concurrent training is the residual fatigue hypothesis. This hypothesis states that the primary reason for a reduction in strength shown is because of the timing of training rather than the actual training itself.

It explains that when endurance training and strength training are performed together is when there is a reduction in strength gains. The study looked at subjects who performed endurance training first and strength training second, and at subjects who did the reverse.

The subjects who performed the endurance training first showed a great reduction in generated force with lower body strength workouts however not with upper body workouts. After endurance exercise the lower body becomes fatigued and the muscle fibres are not able to recruit as efficiently or quickly and maximum force production declines.

While performing endurance exercise after strength training will reduce these effects, some effects may still be seen due to a psychological component where the athlete tries to "conserve" some energy in an effort to ensure they will be able to perform the endurance training after the strength training is completed.

Therefore, if concurrent training is necessary in the individuals training program the ideal goal would be to perform them during separate sessions, preferably on separate days. If they must be performed together, a lesser risk of compounding effects would most likely be seen by performing the strength training first and endurance training second.

Build Up Of Metabolic Byproducts And Reduced Glycogen Stores

A final factor that plays a role during concurrent training is the accumulation of metabolic byproducts such as lactic acid and the depletion of muscular stores of energy such as ATP and muscle glycogen.

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After endurance training, strength is decreased for 4 hours immediately afterwards and will still not be completely recovered after 25 hours. When lactic acid is produced in the body, muscle pH levels become lower which is the primary cause of fatigue during the first hour after the exercise is complete.

Even once the levels have returned to normal again however strength is still less therefore indicating that muscular pH levels are not the only determinant of fatigue with endurance training. The other factor that often causes muscle fatigue is the depletion of stored muscle glycogen.

Muscle glycogen is the primary fuel source that supplies endurance activity so once these run out performance begins to decline at an extremely fast rate. With regards to strength training, when muscle glycogen stores are low, it is harder for the body to produce maximal tension and recruit all muscle fibres so strength levels will also decline.

Post workout nutrition is a critical factor in both isolated and concurrent training in ensuring glycogen stores are refilled. Getting carbohydrates into the muscles cells immediately after performing either strength or endurance training is necessary if one hopes to improve performance at the next training session.

If you are performing concurrent training it is even more important as these types of sessions tend to require an even greater load of muscle glycogen (assuming the total workout time would be longer than an isolated training protocol). To reduce the effects of a decrease in strength with concurrent training, an athlete may wish to take in some carbohydrates halfway through training to provide the body with some quick burning fuel.

In both isolated and concurrent training, nutrition plays an important role in reducing fatigue, but concurrent trained athletes need to be even more aware of this factor as it will have a greater impact on how they feel during their training sessions.


As more studies are done investigating the effects of concurrent training on muscular and strength development we will no doubt find new and valuable information regarding how to reduce any negative side effects that may come along with performing both endurance and resistance training.

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Not all athletes can focus just on strength or just on endurance training as many sports require a combination of both. We can reduce the amount of concurrent training that is needed with a good periodization program utilizing on and off seasons during the year, but most athletes will still not completely drop one form of training.

Factors that come into play when you are performing concurrent training are:

  • Mode of endurance training used.
  • Prior training history.
  • Muscular hypertrophy adaptations.
  • Muscle fibre type and motor recruitment.
  • Hormone production and release.
  • Build up of metabolic byproducts and reduced glycogen stores.

Current evidence suggests, on the whole, that performing endurance training while also performing strength training will inhibit the amount of muscle and power gains more than isolated training would, however by taking certain measures to protect against it we can significantly reduce the negative side effects.


  1. Abernethy, P.J., Barry, B.K., Leveritt, M., & Logan, P.A.. Concurrent Strength and Endurance Training: A Review. Sports Medicine (1999). 28(6).
  2. Kravitz, L. The effect of concurrent training: does cardiovascular exercise performed prior to strength training have a negative impact on strength performance? IDEA personal training (2004). 15(3).