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.