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How much gap should there be between weight training and cardio?

Usually I work out 5 days a week in the morning and sometimes do cardio and some days do weights, but I have been thinking about going for a run during lunch and lifting weights every morning.

So my question really is: Will my running cancel out the benefits of my weightlifting or burn muscle instead of fat?

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How much running? If you're running during lunch then you probably mean running for less than an hour. Maybe 30 minutes? –  Joshua Carmody Sep 12 '11 at 14:03

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up vote 15 down vote accepted

First, let's understand that running is a form of conditioning, which can be done either aerobically or anaerobically. There are several ways to condition, and some are compatible with weightlifting and some do work against it. The first point I want to make is that you do have a mistaken supposition: that running will burn muscle. The two articles referenced in IronStrong's Conditioning forum will help shed some important light on the subject.

Metabolic Conditioning Energy Sources:

  • Muscles pull from Adenosine Triphosphate (ATP) for all their energy needs
  • When demand is low (aerobic), energy is sustained by oxidative pathways such as burning fat
  • When demand is high (anaerobic), energy comes from glycolysis such as burning sugars. This happens without oxygen.
  • The aerobic pathways are still engaged when the glycolitic pathways are engaged (i.e. you are still getting aerobic work)
  • Longer period aerobic work (jogging) processes the lactate buildup of the anaerobic pathways through the aerobic work.

In short your choices are burning sugar and fat. It's not an either/or proposition, when you are doing anaerobic work you are also burning fat just as you would with your aerobic work. The differences then are in the adaptations on where your muscles look for its main source of energy. Purely aerobic work will cause your muscles to adapt towards being more efficient at being aerobic--which is the opposite of what you need for weightlifting. Purely anaerobic work still incorporates the necessary amount of aerobic activity to support the higher demands.

In reality, when it comes to conditioning you want to condition for the types of stress you would normally encounter, or at least would need to be prepared for. A 40 yard sprinter would need a different conditioning plan than a relay racer. A martial artist would have different needs than a football lineman. In short the conditioning should support the work you need.

The "Death by Prowler" article by Matt Reynolds is very informative, and provides a few tips and approaches to customize your conditioning program for your ultimate goals. While he focuses on the prowler as the tool of choice, there is no reason it won't work for sprints, sledgehammer conditioning, etc.

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Awesome very nice explanation and excellent article. –  Java Ka Baby Sep 12 '11 at 20:47
    
I've read when I was doing a research in the topic that anaerobic work will speed up your metabolism so you'll burn more fat even after you finished with your workout. Is this true? –  Adam Arold Jan 24 at 22:58
    
From what I've read there's conflicting results. More studies are necessary, I believe. Anaerobic work does burn fat in addition to glycogen, but resistance training is the most effective form of anaerobic work that keeps fat burning long after training. That has more to do with the energy requirements to build muscle. The more lean mass you have the more your metabolism goes up. –  Berin Loritsch Jan 25 at 2:35

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