While we saw in part 3 that the impact of HIIT on body composition is overplayed based on misleading references to the differences in the physiques of track sprinters versus endurance runners, there is also the fact that the few studies showing that intervals are associated with slight muscle gains were conducted on untrained subjects with no weight training background. Once we add the fact that most of the people reading this will be performing weight training for muscle building in addition to any cardiovascular work being performed then the relevance of HIIT for building muscle drops to zero. Aside from its effect on body composition we have to consider the effects of HIIT and steady state cardio on performance. We will look first at steady state cardio.
Steady State Cardio & Performance
Steady state cardio has been shown in research (1,2) to counteract the effects of strength training due to the fact that the two activities produce differing molecular adaptations in the body, with the activation of AMPK by endurance exercise inhibiting the activity of the key anabolic signalling protein mTOR which is critical for the muscle building effects associated with weight training. In addition, the Australian research study shows that performing an acute bout of endurance training before weight training blunts the activity of anabolic hormones such as IGF-1 and MGF while performing a bout of cardio after weight training was shown in the same study to increase the production of genes associated with muscle breakdown. It was notable that in this particular study the actual endurance workout performed was not particularly excessive being only 30 minutes in length and 70% Vo2 peak. Although other research has in the past showed that performing cardio after weights enhances fat loss (3), the Australian research did not track performance or body composition, only measuring the levels of various signalling hormones associated with muscle anabolism and catabolism respectively. It should also be noted that the researchers in this study failed to provide participants in the study with any nutritional support for 3 hours after their exercise bout which would definitely have contributed to a negative environment for muscle growth.
Steady State Cardio – Empirical Evidence
Setting aside the research for a moment, we can see from the real world that excessive endurance training is associated with much lower muscle mass in athletes in sports requiring high endurance levels although even then we see that in some sports such as cycling, the riders will often display pronounced muscle hypertrophy despite the high degree of endurance activity performed.
Away from endurance sports it is worthwhile looking at athletes in mixed sports requiring both anaerobic and aerobic fitness as these will often utilise both forms of training in a more balanced ratio than seen with either competitive strength athletes or endurance athletes. Athletes in such sports will often structure their use of endurance and weight training through the year in cycles and for the most part, will employ endurance training including steady state cardio (but also intervals) in increasing amounts closer to the start of their season whereas weight training is emphasised during the off season. What tends to happen is that they increase their strength and muscle mass during the off season and then during the in-season only manage to maintain their strength levels (or try to lower the decline in strength that can occur especially during the in-season of contact sports).
Based on research and real world outcomes there is both theoretical and empirical support to suggest combining endurance training with weight training can have negative consequences for both performance and body composition. A sensible strategy to mitigate this would be to ensure provision of adequate nutrition especially BCAA's during the training session as well as to try to keep exercise sessions for cardio and weights separate to avoid sending mixed signals to the body. Other than this, for anyone who emphasises weight training as a priority, the intensity, duration and frequency of steady state cardio should be carefully monitored to prevent unnecessary muscle breakdown. Moving on from the effects of steady state cardio it will be time to discuss how intervals can impact performance in the gym in part 5, before we wrap up this series with a conclusion and make some recommendations for different kinds of subjects.
- Nader (2006): Concurrent strength and endurance training: from molecules to man.
- Coffey VG (2009): Consecutive bouts of diverse contractile activity alter acute responses in human skeletal muscle.
- Goto K et.al (2007): Effects of resistance exercise on lipolysis during subsequent submaximal exercise.