How does a CrossFit WOD effect the Body?

By: Clint Russell, DPT

Our goal is to help each and every one of our clients lead healthier, more fulfilling lives.  Lots of people join a gym with the goal of losing weight.  We have been told for years that in order to lose weight, we need to adhere to the standard American Diet, and do longer duration aerobic exercise for 20-30 min at least.  The fallacies of the SAD diet are for another post, but I will detail how the  before mentioned methodology is largely ineffective, and why shorter higher intensity exercise is significantly more effective.

To lose weight be have been told that we need to expend more calories than we take in.  We can cut back our calories, and increase our exercise, and weight should fall off.  Expect that it doesn’t, as many of you reading this already know.  This fails to take into account the most important factor in weight loss, hormones.  If we cut our calories back, our body down regulates our basal metabolic rate or calories burned at rest.  This causes our body to release cortisol.  This is a stress hormone that will increase fat storage, and lead to elevated blood sugar, which will cause the body  sugar cravings.  When we add long duration exercise, the body releases more cortisol.  This is supposed to happen to a certain level, but it gets out of whack if we have cut our calories and ultimately leads to a dysregulated metabolism.

Now where does CrossFit come in.  The body has different energy systems.  Each has a different fuel source, muscle fiber type, and job.  Our aerobic system is fueled by oxygen, goes indefinitely, and produces low force and speed.  Our glycolytic system produces more power for about 2 min.  A by product of this system is acidosis.  This is the burn in your legs.  Our phospocreating system is max force and speed for about 10 seconds.

Now the number one factor in producing adaption(results), is strength of stimulus(intensity).  During high intensity short duration exercise, we reach a much higher heart rate and recruit a much higher percentage of our overall musculature.  Now the body only uses the muscles it thinks it needs for the task, so if we are in our anaerobics systems, it is because we have exhausted the capacity of our aerobic systems.  In other words, we are never truly anaerobic, we are utilizing both systems.


For weight loss, long duration low intensity training, burns calories while during the activity, and then the heart rate returns to normal.  Anaerobic training burns calories during the activity, but the heart rate remains elevated above resting for sometimes multiple hours after training.  This is called the afterburn.  This type of activity causes you to have an elevated metabolism and burn significantly more calories for the rest of the day while you rest!  Who doesn’t want that.  This allows you to get significantly more bang for your buck, get stronger, and do all this in a lot less time.  Because you are doing so many less repetitions than long duration training, the cortisol release is minimized.

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