What is the interference effect?

The interference effect refers to the phenomenon where engaging in endurance training can potentially hinder muscle growth and strength development, particularly when muscle growth is the main fitness goal. Andy Galpin explains that the addition of endurance work can compromise muscle strength and hypertrophy, though the effect of strength training on endurance gains is generally positive and not detrimental. The interference largely depends on the volume and intensity of endurance training in relation to strength training. For instance, high volumes of endurance exercise, especially those involving significant eccentric activity like running, can have more of an interference effect compared to lower impact activities like cycling.

This effect is associated with cellular mechanisms involving signaling pathways like Mtor and Ampk, which impact muscle growth and mitochondrial biogenesis respectively. The activation of Ampk can inhibit Mtor through a molecule called TSC2, leading to reduced muscle growth when both endurance and strength training are pursued simultaneously at high intensities or volumes. However, if energy intake is sufficient and recovery is well managed, the interference effect can be mitigated 1 2 3.