To get the best results from fitness training, you need clearly defined fitness goals and the right workouts to help you achieve those goals. In some cases, people unknowingly exercise in a way that makes it harder for them to achieve what they want. Here’s an example. You want to build lean body mass and lose body fat at the same time. You begin resistance training several days a week and when you aren’t pumping iron, you step on the treadmill or head outside for a very long-distance run. After all, running is one of the best ways to burn calories and the longer you go the more calories you’ll burn, right?
Unfortunately, there are drawbacks to this approach. Doing long endurance workouts, especially if you’re restricting the number of calories and carbs in your diet makes it tougher to build the lean body mass you want even if you’re lifting hard on resistance training days. When you follow this type of fitness plan, you’re really sending your muscle “mixed messages.” These incompatible messages make it more difficult to achieve your fitness goals. Here’s why.
Do Endurance Workouts Interfere with Strength Gains?
One of the benefits of cardiovascular exercise, other than the good things it does for your heart, is it burns fat. This increase in fat burning is partially due to an increase in a molecule called AMPk. AMPk rises when glycogen levels dwindle, common during extra long periods of endurance training. One of the effects low levels of AMPk have is to increase fat oxidation. That’s the fat-burning effect of endurance exercise – and one that people who are trying to lose body fat want more of. It’s desirable for another reason as well. When AMPk rises, it boosts insulin sensitivity – a good thing for fat loss.
Unfortunately, there’s a downside to having more AMPk floating around. When AMPk is activated, it turns off muscle protein synthesis or at least turns it down. That’s something you don’t want if you want to get stronger or build lean body mass. So there’s a price to pay for the rise in AMPk that happens during very long periods of cardio. You may be burning more fat but, in turn, you’re making it hard to build new muscle by thwarting the ability of muscle cells to synthesize proteins. There is some good news. You can reduce the negative impact of AMPk by fueling your workouts with carbs to keep glycogen levels from dwindling but there’s a better way – change the way you do cardio.
Another drawback to higher AMPk levels is it impacts appetite hormones like leptin and ghrelin and the stress hormone cortisol. These hormones impact appetite so you may find yourself hungrier after a long endurance workout. Eating more to satisfy that hunger can more than compensate for the calories you burned during your endurance workout. Plus, your body gradually adapts to long periods of endurance training. You may initially experience fat loss from this type of workout but as your body adapts the weight loss typically slows or stops. At the same time, you may be LOSING lean body mass because your cortisol levels are driven up by the stress of extra long cardio sessions.
The Importance of Adjusting Your Fitness Goals
When you do endurance training, you stimulate slow-twitch muscle fibers rather than the fast-twitch ones you target with resistance training and power workouts like plyometrics. When you do long, endurance workouts, your muscles don’t adapt by growing larger, as with resistance training. Instead, your muscles experience a greater capacity to produce energy to fuel those long endurance sessions by developing greater capillary density and more mitochondria, the energy-generating organelles inside cells. Muscle cells also become more adept at using fat as an energy source and are more resistant to fatigue.
In contrast, resistance training at a high enough intensity breaks down muscle fibers and gives them the stimulation they need to grow and become larger. Compare the lack of muscle development in most long-distance runners to the well-developed muscles of sprinters who do short, intense bursts of high-intensity exercise. The adaptations and the end results are completely different.
What Does This Mean?
If your goal is to build lean muscle, extra long periods of steady-state cardio may interfere with your ability to reach your fitness goals. A better approach is to do high-intensity, interval cardio and shorten the duration of your workouts. Shorter, more intense cardio creates a metabolic environment that boosts fat burning even after you finish. Plus, it’s less likely to interfere with your ability to build muscle through resistance training.
Nutrition counts too. It’s important to refuel with a protein/carbohydrate snack to help replenish glycogen stores so you don’t reduce muscle protein synthesis due to the effect of increased AMPk. Make sure your fitness goals aren’t conflicting with one another, that you’re not thwarting your attempts to build lean body mass by doing too much steady-state cardio.
References:
J Biol Chem (2002) 277: 23977-23980.
Sports Med. (2004). 91-103.
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