There are over 300 pairs of skeletal muscle in the body and its principal role is to pull on bone to move the body in some manner. In addition to movement, skeletal muscle is a major metabolic tissue and a principal depot for carbohydrate and fat. Apart from reflex mechanisms, such as the knee tap by a physician, movement of our skeletal muscle is under the command of our brain. Skeletal muscle is made up of very specialized cells that can shorten when they are stimulated.

Because muscle cells are very long, they are often referred to as muscle fibers. Muscle fibers are bundled up like a box of dry spaghetti or straight wires in a cable. The muscle fiber bundles are themselves bundled up and are part of larger collection of similar bundles which make up a muscle. Skeletal muscle is so named because it is generally anchored at both ends to different bones of our skeleton. When muscle contracts, it pulls on a specific bone, which moves the bone, thus moving a body part.

Like neurons, skeletal muscle fibers are also excitable. In fact, the excitability process of muscle cells is very similar to that of neurons, while the result is different. Excitability in muscle fibers leads to the contraction of the muscle cell while neurons merely carry the electrical nerve impulse to another neuron or to skeletal muscle or other tissue and organs.

The inside of skeletal muscle fibers appears unique from other cells because of the structural apparatus of contraction (shortening). Each muscle fiber contains a tremendous amount of small fibrous units called myofibrils. The prefix myo refers to muscle and fibril means little fiber. Each myofibril is a stalk-like collection of proteins. The predominant proteins are actin and myosin, which are referred to as the thin and thick filaments, respectively. They are organized into a series of tiny contraction regions called a sarcomere. Myofibrils are composed of thousands of sarcomeres situated side by side and are tethered together. 

Calcium isn’t just important for bone, it is also the key factor that initiates muscle contraction. When skeletal muscle fibers become excited, calcium channels open and calcium floods myofibrils and bathes the sarcomeres. Calcium then interacts with specific proteins associated with actin and promotes contraction. The contraction of one muscle fiber is really the net result of the shortening of all the tiny sarcomeres in each myofibril within that cell. Further, the contraction of the muscle itself is the net result of contraction and shortening of muscle fibers that make up that muscle.

Skeletal muscle cells have another unique characteristic. They contain an organelle called the sarcoplasmic reticulum which is a modified version of the endoplasmic reticulum found in other cells. This organelle stores large quantities of calcium. In fact, when a skeletal muscle cell is stimulated, most of the calcium that bathes the sarcomeres comes from the sarco-plasmic reticulum.

For muscle fibers to contract, a lot of ATP must be used. Roughly 40% of the energy released from ATP is used to power muscle contraction, while the rest is converted to heat. Interestingly, ATP is also necessary for a contracted muscle cell to “relax” as well. When the muscle is no longer being stimulated, ATP helps the thick and thin filaments to dissociate from each other so that each sarcomere can return to a relaxed (or unstimulated) position. Also, ATP is necessary to pump calcium out of intracellular fluid of the muscle fiber which allows the muscle cell to reset.

As a side note, if ATP is deficient, muscle fibers become locked in a contracted state called rigor. Rigor mortis occurs when the human body dies as the integrity of muscle cell membranes decrease. This allows the calcium pumped out to leak back in and as a result, calcium bathes myofibrils and contraction is invoked as there can be enough ATP in the dying cells to power the contraction but not enough for it to then relax.

Scientists refer to skeletal muscle cells fibers because they are thin and long. In fact, some muscle fibers can extend the entire length of a muscle group, such as in the biceps. That is several inches! In addition to their unique design, skeletal muscle cells are not all the same. In fact, humans are blessed with more than one type of skeletal muscle cell, which vary in performance and metabolic properties. This allows us to efficiently perform a broad range of activities or sports that vary in nature. This includes sports that are longer duration/lower intensity and short duration/higher intensity.

Muscle cells into grouped into two general categories or “types” (Type I and II). Type II muscle fibers are often sub¬classified as IIa, IIx. Furthermore, there are hybrids of these types whereby muscle cells can have characteristics of more than one type. For instance, there are distinct Type I, IIa and IIx muscle cells, which are often referred to as pure, as well as those that can classified as hybrids (e.g. Type I/IIa and IIa/IIb, etc). For the most part, pure Type IIx are, less than 1% of the pure muscle fibers.

The difference really lies in the weight of key muscle contraction protein called myosin. Different weight versions of myosin are found in different pure muscle fiber. Myosin of different weights can be measured by scientists to assess muscle fiber type, either pure or hybrids, and because they are associated with different performance, they are used to classify muscle fiber types. For instance, the pure Type I, IIa and IIx muscle fibers will each have a singular weight myosin within their type, but different compared to the other pure types.

Most sedentary people have 60-80% pure and 20-40% hybrid muscle fibers. Furthermore, active individuals, these numbers tend to shift to 80-90% pure and 10-20% hybrid muscle fibers. And, as you might guess from the activity progression, highly trained athletes tend to have pure Type I, IIa and IIx muscle fibers and no significant amounts of hybrids.

Type I fibers are often called slow-twitch or slow-oxidative muscle because they are better designed for prolonged exercise performed at a lower intensity. The term twitch refers to the how quickly a muscle cell can contract and generate force. In comparison, Type I muscle fibers will have more mitochondria and rely more heavily on the aerobic generation of ATP than Type II fibers. The primary energy molecules used to generate ATP in these muscle cells will be fatty acids and glucose. 

Since ATP production in mitochondria requires oxygen, proper function of these muscle fibers is very dependent upon oxygen supply via the blood. Luckily, Type I muscle cells always seem to have many capillaries around them to deliver oxygen-endowed blood. In addition, Type I fibers contain a substance called myoglobin, which is an iron-containing protein that binds oxygen and serves as a limited oxygen reserve for these cells during exercise. So, in general, Type I muscle fibers:

• Always activated (recruited) during exercise
• Develop force more slowly than Type II muscle fibers
• Have more mitochondria and capillaries and thus are more aerobic
• Generate little lactic acid (lactate)
• Do not fatigue quickly

Type II muscle fibers are often called fast twitch or fast-glycolytic fibers, which can contract and generate force more rapidly than Type I muscle fibers. This is to say that Type II muscle fibers are designed to generate force more rapidly, thereby allowing them to be more powerful as they will allow a job to be performed in a shorter amount of time. Meanwhile, Type II muscle fibers are relatively limited in their ability to generate ATP by aerobic means. When Type II muscle fibers break down carbohydrate to generate ATP, lactic acid will be formed. This is because these muscle fibers, especially Type IIx, generate ATP at the fastest rate and have less mitochondria and receive less oxygen as they are served by fewer blood vessels. Type IIa muscle fibers tend to be more between I and IIa as far as aerobic capacity. So, in general, Type II muscle fibers:

• Activated (recruited) in a progressive manner as more force is needed to perform an exercise
• Develop force more quickly (more powerful) than Type I muscle fibers
• Have fewer mitochondria and capillaries and thus are more anaerobic
• Generate more lactate 
• Fatigue quickly

All muscle movement, apart from reflex actions, begins in the brain in an area called aptly enough the motor cortex. So how does the brain know which muscle fibers to activate to perform a movement? This is a no-brainer for the brain as there is a specific order of activation. Regardless of the intensity or force required for muscle to perform a movement, the brain will always activate Type I muscle fibers. That is because there is always going to be some oxygen available in muscle tissue and circulation for aerobic ATP generation. In fact, for lower intensity exercise like walking, ATP needs can be met by Type I muscle fibers alone.

When more force is required for an exercise movement, more Type II muscle fibers will be activated simultaneously. The major factor will be the requirement of force to perform the exercise with Type II muscle fibers fulfilling the more extreme ATP needs of high-intensity exercise that cannot be covered by Type I. For instance, when an exercise requires less force (e.g., jogging, fast walking, casual cycling) the brain recruit or activate primarily Type I muscle fibers. Here, there will be ample oxygen available through increases in breathing and heart rate to meet ATP demands in the muscle doing the work. As the necessary force to perform an exercise increases (e.g., running, cycling faster, moderate load weightlifting), the brain will also call upon Type IIa and IIx.

Successful athletes seem to have an imbalance in muscle fiber types that favors excelling in a sport. For instance, successful sprinters often have a higher percentage of Type II fibers, allowing them to generate more force in a very brief period of time. This then allows the athlete to be more powerful, generate greater speed, and complete a sprint distance more quickly. Conversely, successful endurance athletes tend to have a greater percentage of Type I muscle fibers. This allows them to generate more force through aerobic energy systems in muscle cells. They can perform at a higher intensity before they generate critical amounts of lactic acid. However, this is more of a common trait, not necessarily an absolute.

Often the question is asked whether top athletes are born or conditioned. The answer is both, but the former may be more important in setting the platform for performance excellence. However, there are many other factors that have genetic ties that can separate the successful athlete from the rest of the field. So, as an athlete’s genetics may direct the formation of more pure Type I or Type II muscle cells with training, body design as well as the potential for skill development. Beyond that, an athlete must train and practice to optimize that performance. Here again genetics can play a role in an athlete’s mental attitude, training diligence, performance anxiety, ability to excel in the sport climate, altitude and more.

Muscle fibers do seem to be able to change in response to exercise training. The biggest effect is the reduction to minimization of hybrid muscle cells. This means that training may force cell to “pick a Type” so to speak. For instance, sedentary people have 60-80% pure and 20-40% hybrid muscle fibers, while highly trained athletes tend to have primarily pure Type I, IIa and IIx muscle fibers and minimal amounts of hybrids. Here the fibers, particularly the hybrids, are changing in their composition of myosin type as well as other proteins that allow them to adapt to support subsequent exercise bouts.