Although concurrent training can help to accelerate the loss of body fat, there is evidence that it may compromise muscle growth in the process. This phenomenon has been termed the “chronic interference hypothesis,” which contends that muscle cannot simultaneously adapt to both strength and endurance exercise. 15 The chronic interference hypothesis is predicated on the concept of the so-called “AMPK-PKB switch” whereby competing anabolic and catabolic intracellular signaling pathways are believed to cancel each other out and thereby limit exercise-induced growth. Specifically, PKB is an anabolic pathway that regulates activation of mTOR, which has been shown to be critical for promoting hypertrophy while AMPK is a catabolic signaling pathway that antagonizes mTOR and thus blunts muscular development.
While the chronic interference hypothesis has been accepted as gospel by many in the fitness field, research is actually somewhat equivocal as to whether combining aerobic exercise in a lifting regimen actually impairs muscular gains. Some studies have found a negative effect of concurrent training on anabolic signaling, 5,6 but others have not. 1 There is even evidence that adding aerobic exercise to resistance training actually increases the activation of mTOR to a greater degree than resistance training alone. 12 Moreover, concurrent training has not been shown to decrease muscle protein synthesis. 4,8 It is important to note, however, that although acute measures of anabolic signaling and muscle protein synthesis provide a snapshot of the adaptive response, they do not necessarily correlate to long-term increases in muscle mass following a regimented training program.
To get a true sense of the effects of concurrent training on muscular gains, we need to look at training studies that have investigated the topic over the course of at least a couple of months. Fortunately, there have been a number of these studies conducted. Despite somewhat conflicting results, the preponderance of the evidence does indicate that the performance of aerobic exercise blunts the hypertrophic response to resistance training. A recent meta-analysis that systematically pooled results of relevant concurrent training studies showed that the addition of aerobic exercise to resistance training programs reduced the hypertrophic effect size (a measure of the magnitude of gains) by more than 30 percent. 15 Before ditching any cardio, however, understand that the manipulation of aerobic variables appears to have a significant effect on hypertrophy outcomes.
For one, the modality of aerobic training has been shown to have important implications in the concurrent training response. Research suggests that running is particularly detrimental to hypertrophy. It’s been speculated that the high amount of eccentric forces involved in running causes excessive muscle damage, which in turn could inhibit the ability to recuperate from an intense lifting session. 15 Conversely, cycling does not seem to significantly impair muscle gains perhaps due to the lack of ground reaction forces and/ or biomechanical similarities to resistance exercises. Given the choice, cycling seems to be the better alternative.
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