by Chad Kerksick PhD July 28, 2010 5 min read
I guess you could call this Q/A a lecture titled “Muscle Physiology and Strength Research 101”. I must warn you that you are flirting with dangerous territory asking a muscle geek to talk about muscle. I could go on and on and will seemingly get more excited the longer we talk about it. If my sister was here right now she’d either be leaving the room or mumbling under her breath, “Oh God, not again…” while she looked for a way or reason to escape.
First, let’s start with just the general anatomy of the body. All of our bodily tissues (think heart, lungs, muscle, bone, etc.) when added together are called systems; the muscular system, skeletal muscle, respiratory system, etc. The human muscular system is comprised of three types of muscle: cardiac, smooth and skeletal muscle. These three types of muscle can be further characterized as well, but cardiac muscle refers to our heart muscle and is only found in one place. Smooth muscle is located throughout many of our blood vessels and organs while skeletal muscle is that muscle tissue which is connected to bones (our skeletal system, hence skeletal muscle). A unique difference between cardiac and smooth muscle from skeletal muscle is that skeletal muscle is under our voluntary control. This means that a system of communication goes from our eyes (or other primary senses) to our brain and to our skeletal muscles which results in these muscles contracting. Our heart and organs are under involuntary control meaning they function (contract) automatically. Shoot, you think resistance training or running is hard work, imagine having to tell your heart to contract more or less…even less people would exercise!
An extremely unique thing about our muscle tissue is that it is extremely adaptable. The terms scientists use for this is plasticity, but adaptable will work. A close inspection of our muscle reveals three predominant fiber types. Each fiber type possesses a somewhat distinct and unique set of characteristics. This fact, however, is extremely important as these attributes should dictate to some extent how you train, especially if you have specific attributes you would like to develop such as endurance, strength, etc. The three fiber types are characterized in many ways and I’m going to outline and explain the most popular. The three fiber types are commonly called type I, type IIa and type IIx. NOTE: you may seem some people refer to the type IIx fiber as a type IIb fiber, but this a dated designation that is present in rodent (rat, mouse) species, but are not found in humans. Type I fibers have a greater blood flow, more energy producing parts of the cells, more oxygen-carrying proteins, and have great endurance but have a low ability to generate force (strength) and produce force rapidly (also known as power) and possess a low tendency to grow or hypertrophy. The type IIx fibers are on the other end of this spectrum and as a result are much more likely to experience hypertrophy (growth) and are able to produce high amounts of force and power. On the down side, these fibers do not have very good endurance, fatigue easily, have a low blood supply and don’t contain as many muscle components which relate to energy production and oxygen carrying.
So if your head is spinning an easy way to remember is that type I fibers are often considered to be ‘endurance’ fibers while type II fibers are ‘strength/power’ fibers. Why? Well type I fibers have a much greater capacity to produce energy inside them and are much better suited to transport the extremely high amounts of oxygen needed to complete prolonged bouts of exercise like running a 10K or biking for several miles. These fibers are found in higher amounts in athletes who complete a substantial amount of endurance or cardio type of training. If you are a resistance training athlete, you don’t want to train in a manner that will provide a stimulus to your body to convert more of your power and strength-producing fibers (type IIa and IIx) into type I fibers as that would be counterproductive. This is one key reason why resistance training to failure or near failure is so critically important. This level of training is difficult and requires the muscle to produce higher amounts of force say as opposed to completing one or two sets of high (>20) repetitions. As a result of training at near-failure levels, the muscles sense the need to keep the bigger, force-producing fibers around. That’s a good thing!
It works the other way as well. Heavy resistance training to failure requires much greater amounts of force and power to be produced and the type IIa and IIx fibers are much better suited to meet these demands. If you find yourself, however, needing more endurance then performing cardio type exercise will be needed. This concept introduces two key training principles: the specificity and overload principles. Unfortunately, both topics are so critical to effective exercise training that they deserve their own discussion (look for these articles on future Q/A’s). What happens, however, if you want a good mixture of both attributes? For example, what if you enjoy going for a short jog or bike ride or enjoy playing a game of pick-up basketball or football, but you still want to maintain or build your strength and muscle mass. This is ultimately where the type IIa fiber comes in. It’s often considered the hybrid fiber. While the other fibers are somewhat polar opposites, the type IIa fiber bridges the gap between them. Thus and as you might expect, type IIa fibers are able to produce more force than a type I fiber, but not as much as a type IIx fiber. In the same respect, type IIa fibers have a greater resistance to fatigue than type IIx fibers, but don’t have as much endurance as a type I fiber. How should you train to build these fibers? Good question. Interestingly, the more metabolically challenging your resistance training is the more type IIa fibers are present and this is a great thing because they are good at so many things. How do you make your workouts more metabolically challenging? Several ways, but again a full discussion will follow in subsequent Q/As. To start, perform exercises that use the most muscle and rest as little as you can.
In conclusion, having a general understanding of your fiber types and muscle physiology is a key factor when choosing your own workouts and especially if you operate as a trainer for someone else. Entire books and studies have been written on these topics, especially as they relate to specificity of training and so forth. Remember your muscles are capable of making very specific adaptations to the physical and metabolic challenges you place upon them. Make sure you are training how you want to end up and not just because your boyfriend trains that way, the ripped girl or huge dude at the gym does or it was the featured workout in the latest Muscle & Fitness. Until next time…