In Part 1, I explained the relationship between intensity and duration. Now I will explain what is actually happening in your body when performing higher and lower intensity work, respectively. 

Part 2: Muscle Fiber Types, Henneman's Size Principle, and Applying High/Low Reps to Your Training

The muscles in your body are comprised of tons of individual muscle fibers. Comprising these muscle fibers are two primary types of muscle: Type I and Type II. Every person possesses both types, and also have different proportions of each type. 

Maybe you have heard of "slow twitch" muscles and "fast twitch" muscles? Slow twitch= Type I, and fast twitch= Type II.

The table above shows that Type I fibers have great fatigue resistance, but their force production ability is low. Type II fibers are just the opposite. They can produce high amounts of force, but they fatigue quickly.

From Part 1, just as intensity and duration are always inversely related, so are the force production capabilities and fatigue resistance qualities of both fiber types. 

If you follow along so far, you can make the connection of slow twitch fibers being responsible for lower intensity exercise - for longer durations (lower weight, more reps), and fast twitch fibers being responsible for higher intensity exercise - for shorter durations (higher weight, less reps).

Having muscle fibers is great and all, but without your central nervous system, the muscles in your body would be rendered useless. Your nervous system is the control center that signals muscle fibers to contract. Muscle contractions are what moves your body through space, allowing you to live and perform all daily tasks (including exercise). Thankfully, you are a highly adapted specimen, and this signaling process takes place involuntarily, for the most part.

A motor unit is essentially what links the nervous system to muscle fibers. Henneman's Size Principle states that under load, motor units are recruited from smallest to largest. What does this mean? It means that your body will try to move a load with as minimal muscle activation as possible. Once your nervous system senses that it needs more help, it will activate more motor units. This process will gradually continue until enough motor units, and therefore muscle fibers, are working to overcome the load.

Consider that this motor unit recruitment sequence happens in a matter of microseconds. Your body is a highly advanced computer. It is very impressive to know that your body will instantaneously tap into its fast twitch muscle when exerting a maximal effort!

What is the purpose of training with high and low rep ranges?

I'll start with lower rep training. Please remember to respect the principle of intensity vs. duration. If you are not using enough load, you will have a difficult time recruiting fast twitch muscle! By the way, you don't always need to use a super heavy load to do this. Fast twitch fibers are also trained by moving sub-maximal loads, as fast as possible (speed work). But even speed work will adhere to the principle of intensity vs. duration. It is always done at a very high intensity, for a short duration.

Reasons to train your Type II muscle fibers...

• increased speed, strength, explosiveness.
• stronger connective tissues.
• better nervous system response.
• improved hormone function.
• more agility and coordination.
• better bone and cardiovascular health.
• more energy expenditure. 
• speeds up metabolism.

If your primary goal is to lose weight or "tone up", it is supremely important for you to lift heavy and move fast in your training. If you never stimulate these powerful muscle fibers, they will start to go dormant. If you have tissues in your body that are inactive, you will have a hard time reaching your fitness potential, and perhaps your fitness goals. The more tissue you stimulate within your body, the stronger you will be, the leaner you will be, the healthier you will be. Heavy and fast training works the system that is the human body far better than light and slow training does (that is, to train only Type I fibers). 

But high rep, low intensity exercise still have some benefits...

• anyone can do it.
• good blood flow to working muscles.
• builds contractile endurance.
• maintains joint and connective tissue health.
• can help bring up lagging muscles.

To wrap up, I say you are better off spending more time training squats, deadlifts, sprints and presses, and less time jogging, doing crunches, performing lateral raises, and watching Real Housewives on the elliptical. There is simply much more bang for your buck with exercises that require more effort. 

Of course, a blend of all forms of exercise is preferred. I see plenty of Instagram accounts, and publications such as Shape magazine, that will have you believe that doing a circuit of bodyweight lunges and squats, 10lb. kettlebell swings, and some bicep curls while standing on a BOSU ball for 25 reps, will get you to where you want to go. I think that's fine, unless that's all you do. I think there needs to be some heavy training in there as well...

What do you think?