How might aerobic exercise reduce gains in muscle size?

Chris Beardsley
Mar 14 · 8 min read

Many recreational lifters do aerobic exercise in addition to strength training, either for health reasons, or to help with fat loss.

However, some research groups have found that performing aerobic exercise regularly while also doing strength training workouts leads to smaller increases in muscle strength and size, compared to just doing strength training workouts. Unfortunately, despite a huge amount of time and effort, there is still no clear answer regarding how this might happen.

I think this is because we have been looking in the wrong place.

Let me explain.


Why do we think aerobic exercise could reduce the hypertrophy achieved from strength training?

In 1980, a groundbreaking study was published showing that gains in strength were smaller when strength training and aerobic exercise programs were performed simultaneously, compared to when only strength training was done. The strength training involved 5 workouts per week, and the aerobic exercise involved 6 workouts per week. When the workouts were done on the same day, there was at least 2 hours of rest in between.

Many later studies duplicated this effect successfully (although it is worth noting that some recent studies have failed to find the same negative effects of aerobic exercise on strength training adaptations).

Some investigations that successfully duplicated the effect have also explored the effects of simultaneous strength training and aerobic exercise programs on other outcomes, such as changes in the ability to produce force at high-velocities (commonly measured as power outputs) and changes in muscle size. Overall, it was found that there was a negative effect of simultaneous strength training and aerobic exercise programs on all outcomes.

Over time, terminology was developed to provide appropriate labels.

Performing strength training and aerobic exercise programs simultaneously was termed “concurrent training,” and the negative effects of aerobic exercise on the adaptations that typically result from strength training was termed the “interference effect.”

But why might this interference effect happen?


Why might the interference effect happen? (part I)

To explain why the interference effect might happen, researchers have looked (pretty much exclusively) at the negative effects of concurrent training on changes in muscle size.

In retrospect, this might seem like an odd decision, given that we now know that the adaptations that underpin increases in maximum strength are quite different from those that underpin gains in high-velocity strength and power. Moreover, it seems like a particularly strange course of action to take, given that increases in muscle size support increases in maximum strength to a greater degree than they support increases in high-velocity strength and power, because of a range of negative effects of increased muscle size on the ability of muscles to shorten quickly.

Nonetheless, some researchers devised the hypothesis that the post-workout molecular signaling that is stimulated by aerobic exercise might suppress the post-workout molecular signaling that is triggered by strength training, which is what leads to muscle growth.

During any type of exercise, sensors in various parts of the body detect the specific stresses and strains that the body has been exposed to, and these sensors then trigger signaling processes that lead to physical adaptations. In the case of aerobic exercise, adaptations include an increase in mitochondrial content inside muscle fibers (which improves the ability to consume oxygen and produce ATP), and an increase in the capillary density around muscle fibers (which improves the delivery of oxygen). In the case of strength training, adaptations include increases in muscle fiber length and diameter, which together produce increases in muscle fiber volume.

Initially, it was suggested that the 5’ adenosine monophosphate (AMP)-activated protein kinase (AMPK) that is activated after aerobic exercise caused a reduction in the activity in the mechanistic target of rapamycin (mTOR) pathway. The mTOR pathway is commonly activated after strength training, and has been implicated in producing signaling effects that lead to increased post-workout rates of muscle protein synthesis, which cause muscle fibers to increase their protein content, and thereby increase in either diameter or length, and therefore in volume.

AMPK is a cellular energy sensor that is activated in response to low energy stores (glycogen and ATP) inside the muscle, which occur commonly during sustained periods of exercise. It is also involved in triggering some of the adaptations that occur after endurance training, including an increase in mitochondrial content, by regulating PGC-1⍺ signaling. Consequently, it is to be expected that AMPK activity increases to a greater extent on days in which both aerobic exercise and strength training were performed, compared to after days when only strength training was done.

Unfortunately for this hypothesis, research found that when strength training and aerobic exercise are carried out in close proximity, this does not have a negative effect on mTOR signaling, even when there are measurable increases in AMPK and PGC-1⍺ signaling.

If AMPK signaling cannot explain the interference effect, what does?


Why might the interference effect happen? (part II)

Once they discovered that the activation of AMPK could not explain the interference effect, researchers looked for other molecular signaling pathways that might be triggered by endurance exercise, which might suppress anabolic signaling.

However, this assumes that we know that endurance-related molecular signaling interferes with anabolic signaling more generally, and that we are now just trying to find the specific details of the interfering pathways, which is not actually the case. Consequently, such investigations may still be looking in the wrong place.

Even so, researchers have suggested at least two alternatives that may provide an interference effect at the molecular signaling level.

While interesting, these two possible lines of research are still new, and we do not yet know whether they will withstand the rigors of future investigations.


An alternative (more likely) explanation for the interference effect

In contrast to the complex signaling hypotheses that have been proposed as explanations for the interference effect, there is a simpler alternative that also fits the available data.

Endurance exercise causes central nervous system fatigue.

Fatigue is a reduction in the ability to produce force. Our ability to exert force can be reduced either by reducing the ability of the muscle itself to exert force (which is called “peripheral fatigue” and which is measured as involuntary, electrically-stimulated force), or by reducing the ability of the central nervous system to activate the muscle so that it produces force (which is called “central nervous system fatigue” and which is measured as the difference between involuntary and voluntary force).

When the muscle experiences peripheral fatigue, this reduces the amount of force that each muscle fiber can produce, and so the central nervous system increases the level of motor unit recruitment to compensate. This increase in motor unit recruitment increases the number of active muscle fibers. When peripheral fatigue is very high (as when strength training to failure with light loads), the central nervous system recruits all available motor units, which activates the majority of the muscle fibers inside the muscle. By activating all of the muscle fibers (while they are shortening at slow speeds, due to fatigue), peripheral fatigue effectively contributes to the hypertrophic stimulus.

When a muscle experiences central nervous system fatigue, this reduces the level of motor unit recruitment that can be achieved for the muscle. This means that not all of the muscle fibers inside the muscle can be activated.

If we exercise to the point where we experience central nervous system fatigue for a muscle group, then this will mean that we are unable to recruit all of the motor units for that muscle group until we have recovered. During the period of time in which we are recovering, if we perform a strength training workout, we will not be able to reach full motor unit recruitment at the point when we reach muscular failure. This means that we will not be able to recruit the high-threshold motor units, which are the ones that control the large numbers of highly responsive muscle fibers that are the ones that grow after strength training.

Most research has shown that central nervous system fatigue after strength training is fairly short-lived, but can last up to 3 days if the strength training workout is high volume or if muscle damage occurs during the workout. This explains why high frequency strength training is not as effective as it we might expect, if we just look at the time course of typical post-workout muscle protein synthesis rate increases. Endurance exercise produces larger and more long-lasting central nervous system fatigue than strength training. Therefore, it seems reasonable to assume that the central nervous system fatigue caused by aerobic exercise workouts done immediately before (and perhaps even the day before) a strength training workout will still be present in that strength training workout.

This means that aerobic exercise done soon before strength training workout reduces our ability to recruit motor units in the trained muscle groups. This decreases the number of muscle fibers that are stimulated by the strength training workout, and it will not stimulate the muscle fibers that are most responsive to strength training.


Testing the alternative (more likely) explanation for the interference effect

Assuming that this alternative explanation is correct, then based on our current understanding of central nervous system fatigue, we might predict that the proximity of aerobic exercise to strength training (and the order of aerobic exercise and strength training workouts) should have a major impact on the interference effect.

We should find that aerobic exercise done after a strength training workout should not reduce muscular adaptations (unless it causes central nervous system fatigue that persists until the next strength training workout). And we should find that the interference effect is greater when the aerobic exercise is done immediately before a strength training workout, and smaller when done either immediately afterwards, or on a separate day.

Indeed, we know that when strength training is performed before aerobic exercise, the interference effect is small, even in highly-trained individuals. And the literature shows that the interference effect is greater when the aerobic exercise is done immediately before a strength training workout, compared to when it is done either immediately afterwards.

Also, we should find that the interference effect on performance measures, such as maximum strength and power, is greater than the interference effect on muscle size (and indeed, this is what the literature shows). This happens because the ability to recruit motor units is also an adaptation that happens with long-term strength training, and this adaptation benefits both maximum strength and power. An inability to recruit all motor units during training will mean that we do not develop the ability to recruit more motor units after training, which will impede strength gains.


What does this mean in practice?

In practice, we still know quite little about the time course of central nervous system fatigue after various types of exercise, although the amount of central nervous system fatigue seems to be greater when exercise duration is longer.

We also know little about what types of exercise produce greater central nervous system fatigue than others, although muscle damage is known to be a trigger for central nervous system fatigue, even when exercise duration is fairly short.

In practice, this means that where we want to include aerobic exercise in a strength training program, we probably want to avoid performing that aerobic exercise too soon before a strength training workout, we want to avoid doing long durations of exercise, and we want to avoid doing types of exercise that involve a high degree of muscle damage, such as running.


What is the takeaway?

Performing aerobic exercise programs at the same time as strength training programs may reduce our ability to gain muscle size. Although the popular explanation for this interference effect is the potential suppression of post-workout anabolic signaling by post-workout endurance-related signaling, it is more likely caused by central nervous system fatigue resulting from aerobic exercise, which reduces our capacity for motor unit recruitment during subsequent strength training workouts.

Chris Beardsley

Written by

This is how strength training works