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Muscles are made up of cells and each cell is between a few micrometers to a few centimeters in length. An actual muscle is comprised of thousands muscle cells that are organized at multiple different levels. The following picture is a good reference about this organization:
Fig. 1: Skeletal muscle organization. Many hundreds (if not thousands) of muscle cells make up each individual muscle fiber and a sheath of connective tissue called endomyosium surrounds each muscle fiber. A muscle fiber, or myofibril, is a series of repeated sarcomeres, which are the functional units of a muscle.
Fig. 2: A myofibril and sarcomere
Sarcomeres are overlapping thick and thin filaments that form cross bridges during muscle contraction. Without getting into too much detail here, we can simply state that each myofibril is a series of sarcomeres that contract simultaneously, thus shortening the muscle fiber as unit. When many myofibrils contract in unison, large-scale muscular movements can occur.
Muscle fibers (myofibrils) are grouped together as fascicles, which in Latin means, “little bundle of sticks.” Each muscle contains multiple fascicles that are each individually covered by another connective tissue sheath called perimysium. The entire muscle is additionally covered by yet another connective tissue sheath called epimysium, which blends into the muscle’s tendons at its origin and insertion. Muscular contractions are transmitted between the specific muscle’s origin and insertion, where the insertion is pulled, rotated, or otherwise moved towards the origin. In this manner, the origin remains relatively stable, whereas the insertion is the actively moving end of the contraction.
The skeletal muscle’s origin is the more proximal (closer to the axial skeleton-ribs, vertebrae, skull, etc.) connection to the skeleton and the insertion is more distal, or further away. Each skeletal muscle has its own innervation by a motor nerve, which receives signals from the central nervous system (CNS), i.e. the brain and spinal cord. A motor neuron, or nerve that provides innervation to a skeletal muscle, ends at what is known as a motor end plate (MEP). The motor end plate represents the junction of the CNS and the skeletal muscle, whereby through a series of events- excitatory or inhibitory signals are transmitted to the muscle fibers. The motor unit is the basic unit of the innervated skeletal muscle. It is defined as the motor nerve and all of the muscle fibers (myofibrils) that it innervates. Moreover, when the nerve sends an excitatory signal to the muscle (i.e. contract) then all of the muscle fibers of that motor unit contract. Similarly, all muscle fibers of a single motor unit are of the same type, i.e. either fast twitch or slow twitch.
Skeletal muscles are organized by what myosin heavy chain they possess and their oxidative phosphorylation ability of fuel (e.g. carbohydrates, fats, and protein metabolites). The two main categories are Type I (slow-twitch) and Type II (fast-twitch). There are many differences between these two types of fibers and in principal, we care pare the discrepancies down to the following three things: time to exhaustion, contraction strength, and size. Type I fibers, in general, take longer to fatigue, provide less strength when they contract (but can contract for long periods of time), and are small. These types of fibers are present in motor units, whose functions include maintaining posture, locomotion, and similar long-term tasks. Because they are resistant to fatigue, they must have high concentrations of mitochondria, which make energy for the muscle. They also are rich in capillaries and other vasculature, which allows them to remove metabolic byproducts that cause fatigue like hydrogen ions, for instance. Finally, these fibers tend to be smaller than their type-II counterparts, thus they are the first motor units to contract. Type II fibers come in a variety of flavors depending on the text used to describe them, but in general they are less resistant to fatigue, have the potential to generate high levels of force, and are larger than slow-twitch muscle fibers. Motor units are summoned to be active based on the needs of the muscular contractile force. That is, the higher the force, the more motor units are required to be active. Additionally, they are recruited from smallest to largest to produce the contraction and furthermore, as less and less force is needed the largest (read high threshold) motor units become less and less activated. So for a simple task like picking up a pencil off a desk, it is likely that only slow-twitch motor units are functioning, since there is a low force requirement for successful completion of this task. In a task like a limit squat or deadlift however, more motor units are required to complete this task, if it is possible to do so, and so the higher threshold motor units that are larger and more difficult to activate, must be summoned to contract. It would be appropriate to call slow-twitch muscle fibers “low threshold” and classify fast-twitch muscle fibers as “high threshold”. Imagine a muscle group like the quadriceps muscle. Whilst standing, walking, or kneeling there is certainly some low level activity by the low-threshold motor units, i.e. the slow-twitch (type I) fibers at all times to maintain posture, provide contraction for locomotive movements, and balance. Then during a squat, these motor units’ contractile strength are not sufficient to complete the task, thus other motor units of this muscle group must be called upon to help in providing contraction of the muscles during the movement. The heavier the weight or the quicker the movement requires incrementally more extensive motor unit recruitment. Thus, to effectively train more muscle, more motor units, and subsequently stress the muscle in a more complete fashion, there exists a certain intensity (weight and/or speed) that the load must represent of that individual’s particular ability.
This generalized distinction between the two main types of muscle fibers provides us with a framework for how muscles adapt to specific stressors we impart upon them during training. We will soon compare and contrast two different modalities of training, endurance exercise via running long distances and strength training via barbell exercise in order to showcase how the different fibers respond differently based on their own individual properties listed in Figure 3.
In summary, skeletal muscle fibers are organized at various different levels with the sarcomere representing the most basic unit of a fiber. Muscles receive nervous innervation from the CNS and are grouped together as motor units. Motor units can contain anywhere from 10-10,000+ muscle fibers, which is dependent on the amount of fine motor control necessary in the area. For instance, the motor units of the muscles of the hand are much smaller than those of the back or legs, as the back and legs do not require very fine movements, whereas the hands and fingers do. Motor units consist of only a single type of muscle fiber and each type of muscle fiber has specific properties that characterize its function in movement of the skeleton. Furthermore, muscles can either contract or relax from excitation or inhibition stemming from the nervous system. When they contract, they exert force between their origins and insertions to produce movement about the insertion. These movements are known as the actions of a muscle and they are only produced about the joint or series of joints that the muscle crosses. In short, if a muscle does not cross a joint it does not act upon that joint.
To complete this aerial overview of skeletal muscle anatomy and function we must briefly describe some anatomical terms so that all further explanations are clear. When we talk about anatomy, we do so with normal anatomical position in mind. Normal anatomical position looks like Figure 4.
Fig. 4-Standard anatomical position
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