Now that I have a little more time on the weekend I thought I'd expand a little on why I feel low intensity endurance building exercises are the way to go at least over P90X:
First off P90X is a brilliant program and certainly delivers on its promises - done correctly it will give that trim low fat% body - but in the manner it does this fails in the long term and in my opinion its these popular notions that high intensity regiments are necessary to lose weights that are partly to blame for the increase in obesity trends in this country.
To understand why it comes down to understanding how our muscles work. So first - why does P90X work? P90X is a program that primarily focuses on activities that utilize fast twitch muscle fibers (push-ups, curls, squats, ect - here is a list found online:
P90X Workout list). Energy utilization proceeds as follows - as you do each exercise cellular ATP energy is used up and the muscle begins to utilize glucose (glycolysis) and then glycogen as a fuel source. P90X is great because it burns up glycogen stores in a multitude of different muscle groups. Following the exercise the body will convert consumed foods to recover blood glucose and glycogen reserves - as there is a caloric deficit the liver will begin to break down fats (gluconeogenesis), stored in adipose tissue, into glucose to begin to restore these reserves - which over time leads to a leaner body.
Additionally, on the muscle side, as you first begin the program the intensity of the workout is beyond what your muscles are prepared to do resulting in microtears. These make the person very sore in the first stages but promote myofibrillar development of the muscles - building strength. However, as the program progresses the muscles are adequate to handle the intensity so at this point sarcoplasmic development of the muscles occurs. This is an increase in the size of each muscle cell allowing the storage of more glucose and glycogen - resulting in greater muscle endurance and ultimately more fat burning power. Ultimately this swelling and the loss of adipose tissue gives the "sculpted" look that makes P90X so popular.
So why does it fail? In the end the intensity of the workouts will typically result in an injury or quitting because of reaching desired results, tired of the program, lifestyle complications, ect. This leaves a person with an abundance of fast twitch muscle fibers. The problem here is these fibers only burn glucose (and thus calories) when they are active - the gains on resting metabolism are minimal. As inactivity sets in the caloric imbalance begins again. Glucose and glycogen levels are high in the muscles and other tissues - thus glucose from food is converted into triglycerides for fat storage - the decline into obesity begins again.
So why low intensity workouts? Low intensity workouts primarily focus on the slow and intermediate twitch fibers of muscles and here energy usage is different. In fast twitch fibers, as glucose is broken down to energy the byproducts of this reaction are converted into lactic acid. However, in slow and intermediate twitch fibers the presence of mitochondria (and adequate supply of oxygen) allow these byproducts to be further utilized for energy. This allows for the usage of these muscle fibers to proceed for longer periods of time and as you do so – endurance exercise promotes activation of signaling pathways to produce more mitochondria. Here is the key point – mitochondria are always active so whether you are exercising or not they are constantly breaking down glucose into energy – this is where resting metabolism comes from. Inactivity has its effects too – obesity works to slow down metabolism by preventing further mitochondrial biogenesis. It also works to limit the efficiency of mitochondria. Still the decline of the body into obesity will be much slower for a person who is predominately slow and intermediate twitch fiber compared to one who is predominately fast twitch.
In summary, the battlefield in the war between fat gain and fat loss happens in the diversity of muscle fiber type. Biopsies from muscle tissue of obese participants show that they are predominately fast twitch. Studies also show shifts from predominately fast twitch to slow twitch through exercise results in better glucose sensitivity in animals. Although glucose utilization while exercising (and subsequent recovery) is higher in predominately fast twitch muscles this is only effective if these muscles are under constant use and the price for inactivity is high.
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