The Muscular System
Movement of the body is possible because of the interaction between the muscular system and the skeletal system. Together these two systems are referred to as the musculoskeletal system, which consists of bones, joints, connective tissue (ligaments and tendons), and the muscles. Muscles alone do not move weights, but rather they function by moving bones that rotate around connective tissue. Our bones provide the structural support and our muscles convert chemical energy into mechanical energy for movement. Our joints transmit forces through the bones of our body to the external environment. In this article I will discuss the muscular system. To read about the skeletal system see my article “The Skeletal System–Bones, Joints, and Connective Tissue”.
The Muscular System
All body movement from running, walking, circulating blood, or digestion, all depend upon the actions of our muscles. The muscular system is comprised of 600 muscles working together with the support of the skeletal system to create motion. Alone, there are 30 or so muscles involved in the digestion of food through our digestive system, to circulate blood, and to operate specific internal organs. As athletes, we are concerned with exercise physiology because muscles are our main operative tissue. Our muscles expend energy, generate wastes, and require substantial nutrition.
Muscles are either involuntary or voluntary. We have very little, if no control over the involuntary muscles in our bodies. An example of an involuntary muscle is the heart. We do not have to think about the involuntary muscle for it to perform its job, but in very rare occasions with proper training they can be controlled. The voluntary muscle are the muscles that we control consciously, such as muscles attached to bones or those to your eye. There are two types of muscles (straight and smooth )which are divided into three types as follows:
1) Cardiac Muscle Tissue: Is an straight-involuntary muscle tissue that composes the wall of the heart. They contract the heart which pumps blood through our body. Cardiac muscle cells are often branched and their nuclei are more centered than that of skeletal muscles. Cardiac muscle tissue does not fatigue easily and the rest period between beats is all it needs. Even during intense exercise the skeletal muscles will fatigue first.
2) Smooth Muscle Tissue: smooth-involuntary muscle tissue is found in the walls of the tubular viscera of the digestive tract, respiratory tract, and genitourinary tract. They are also found in the walls of blood vessels and large lymphatic, in ducts of glands, in intrinsic eye muscles, and in erector muscles of hairs. The smooth muscle tissue have several functions such as: to move substances along their respective tracts, change the diameter of blood vessels, move substances along glandular ducts, change the diameter of your pupils, and erect hairs. Smooth muscle tissue only have one nucleus per cell and contract more slowly than straight muscle and therefore do not fatigue easily.
3) Skeletal Muscle Tissue: straight-voluntary muscle tissue that is found attached to bones, in the extrinsic eyeball, and in the upper portion of the esophagus. Skeletal muscles function to move your bones and eyes, and are involved in the first part of swallowing. Skeletal muscles are unique in the fact that they are made up of many long muscle cells that each contain many nuclei (multi-nucleate). As a general rule, skeletal muscles cannot sustain prolonged all effort contractions and easily fatigue.
Muscles Structure and Function
Locating muscles and knowing their relationship to the joints is critical in understanding the effects muscles have on joints. When a muscle contracts it tends to pull both ends toward the middle of the muscle. The points that the muscles attach to our bones are called the insertion and the origin of the muscle.
The insertion is considered the distal (away from the center) attachment. This is considered the part where the muscle attaches to the bone with the farthest proximity to the midline of the body. It is generally considered the most movable part of the muscle attachment.
The origin is called the proximal (toward the center) attachment. This is considered the part where the muscle attaches to the bone with the closest proximity to the midline of the body. It is generally considered the least movable part of the muscle attachment.
Muscles are largely made up of proteins and are organized in a hierarchical system from large groups to small fibers. The fibers combined together and are called motor units. These motor unit groups are physically separated by a membrane from other groups and are connected to bones through tendons. A motor unit consists of a single neuron and all of the muscle fibers are stimulated to action by it. The ratio of nerves to fibers determines the fine motor control available to that muscle.
Muscle fiber is composed of myofibrils, which are small bundles of myofilaments. The myofilaments are the elements of the muscle that shorten upon contraction. Myofilaments are made up of two types of protein: 1) myosin, the short thick filaments, and 2) actin, the long thin filaments. Two other proteins are involved in muscle contraction and they are: troponin and tropomyosin.
The main function of muscle tissue is contraction. Contraction of muscles is either brought on by voluntary or involuntary stimuli.Voluntary muscle tissue receive nerve fibers from the somatic nervous system (which are your voluntary actions) and therefore can be voluntarily controlled. Skeletal muscles are the major voluntary muscle tissue.Involuntary muscle tissue receive nerve fibers from the autonomic nervous system (which are your involuntary actions) and cannot be controlled (An example of a rare exception is biofeedback training that biathletes use to control there heart rate).
In general a single muscle will not contract alone, but rather sets of muscles contract together or in sequence. The nervous system is responsible for producing the complex movements needed to do even the simplest tasks. The nervous system neutralizes the action of muscles that are not required and causes contractions in the muscles that are required. The exercise of this control comes from the spinal cord and brain through the motor nerve fibers.
The central nervous system (CNS) does not have a direct line to each cell in a muscle. Rather, impulses travel down the nerve axon from the CNS and branch off to supply a group of muscle cells which then contract together. The CNS to be able to perform this function needs information about the length of the muscle and the tension of the tendon, which attach it to the skeleton. The mechanism that the CNS receives this information is through special sense organs called muscle spindles. The muscle spindles measure the strain in the muscle and can be used to pre-set the tension of muscles. The information is then processed by the brain to determine the position of body parts.
The tiny electrical signal that comes from the CNS runs along a junction between the nerve fiber and the muscle surface. The surface takes the signal and amplifies it stimulating the larger muscle fiber. The arrival of this nerve impulse releases acetylcholine (a neurotransmitter in both the CNS and the PNS) from the motor nerve ending which stimulate the membrane of the muscle fiber. This stimulation is in the form of an electric current which passes along the surface of the muscle causing it to contract. The fiber will release unless another current impulse arrives. The time it takes for this process to happen is 1/1000 of a second.
How Muscles Contract
The external skeletal muscles are made up of small cylinder like fibers and may be several centimeters long. In their length they are divided into bands called striations. Each individual fiber is surrounded by a thin plasma membrane called the sarcolemma. Some 80 percent of the fiber’s volume is filled with fibrils known as myofibrils. The myofibril fibers are the structures that are directly involved in the contraction of the muscle fiber. The remainder of the muscle fiber is filled with sarcoplasm. The Sarcoplasm of a muscle fiber houses unusually large amounts of glycosomes (granules of stored glycogen) and significant amounts of myoglobin, an oxygen binding protein. The calcium concentration in sarcoplasm is also a special element of the muscular fiber by means of which the contractions takes place and regulates.
The fibrils are made of of two types of proteins: actin and myosin. Actin and myosin are found in the long filaments of the muscles. The thick filaments consist of myosin, and the thin filaments consist of actin. These filaments work together by interlocking and sliding over each other. During this process the myosin and actin fibers do not become shorted, but rather stay the same length but just move toward each other. Contraction does not always mean the shortening of the muscle. Technically, it refers to the tension within a muscle. During contraction they slide into on another and a crosslink between the actin and myosin filaments are made. Once made they are almost instantaneously broken and new links are created along the filament. These two filaments moving toward one another cause the muscle to contract. The process is known as the sliding filament theory.
In the muscular system there are two major muscle contractions: 1) isometric and 2) isotonic. An isometric contraction of a muscle generates force without changing length. An example of this is using you hand to grip something while applying constant force. With an isotonic contraction the tension in the muscle remains constant despite a change in muscle length. This can occur only when a muscle’s maximal force of contraction exceeds the total load on the muscle. An example would be doing curls with a heavy barbell. During the upward motion of the curling exercise the muscles shorten and the isotonic contraction is called concentric. During the downward motion of the curling exercise the bicep muscle lengthens while maintaining a constant tension. This type of isotonic contraction is called eccentric.
The energy for muscle contractions comes from the food we eat and the oxygen we breath in which is transported to the blood. Blood brings these nutrients and oxygen to the muscles, and also removes the waste products. In the muscle, the biomechanical energy production process involves breaking down of glucose to carbon dioxide and water. During this breakdown, energy is released and used by the muscle proteins to cause the muscle to contract. This process requires an abundant supply of oxygen, which is not always available. During intense exercise the blood supply is often insufficient to carry the amount of oxygen the muscles require. At this point the muscles start converting glucose into lactic acid without oxygen. Even without oxygen, the conversion can give an ample release of energy.
It is pretty much a given fact that all athletes have felt the effects of lactic acid buildup. Lactic acid accumulation in our muscles limits the intensity at which an exercise can be performed at the same intensity, and fatigue will take over. The excess lactic acid eventually enters the bloodstream and is circulated to the liver. Once in the liver it can be turned into glucose and either be returned to the bloodstream or stored as glycogen. Some lactic acid can also be converted back into pyruvic acid and enter the mitochondria to be catabolized (broken down)for energy.
The energy for muscle contractions comes from glucose in a form of complex carbohydrates called glycogen, which is stored in the muscle. Upon high-intensity/low-duration activities, the body uses this stored energy. For low-intensity/high-duration activates the muscle uses a mixture of glucose from glycogen and fatty acids from fat stores.
Muscle Fiber Arrangements
In the muscular system, there are several different ways that the muscles arrange themselves in our bodies. The specific alignment of our skeletal muscle fibers has a direct effect on their ability to generate force. There are six major grouping of muscle fiber alignment:
2) Unipennate: the fibers are like the plumes of a quill pen and to one side of a tendon, which runs the entire length of the muscle.
3) Bipennate: Bipennate muscle is stronger than unipennate. It shortens less than unipennate muscles but develops greater tension when it does.
4) Parallel: Most of the skeletal muscles in the body are parallel muscles. When a parallel muscle contracts, it gets shorter length and larger in diameter.
5) Triangular: the muscle is spread over a broad area such as your pectoralis muscle.
6) Multipennate: diagonal muscle fibers are in multiple rows with the central tendon branching into two or more tendons.
Fast-Twitch and Slow-Twitch Muscle Fibers
The skeletal muscles tissue is comprised of two general types of muscle fibers: 1) fast-twitch, and 2) slow-twitch.
Fast-Twitch Muscle Fibers
Fast-twitch fibers are used when heavy workloads are demanded, and both power and strength are needed. They are for high-intensity/short-duration activities. The fast-twitch muscles contract quickly giving short bursts of energy. They work together in high numbers for brief, intense exercises. Fast-twitch muscles exhaust quickly and they become vulnerable to lactic acid build-up. Fast-twitch muscles are larger than slow-twitch muscles. They have a higher capacity for glycolytic activity which allows them to perform high force outputs for long periods of time. Fast-twitch fibers have thicker nerves giving them a greater contractile impulse.
Fast-twitch muscles are categorized as either Type IIa or Type IIb.
Slow-Twitch Muscle Fibers
The slow-twitch muscle fibers produce a steadier, low intensity, repetitive contraction that are characteristic of endurance athletes. They are capable of sustaining workloads of low-intensity/high-duration. They are highly resistant to fatigue and injury, but their force output is very low. Exercise activities that are aerobic use slow-twitch muscles. Slow-twitch fibers have smaller nerves, but have a high degree of oxygen using capacity because of the greater amount of mitochondria cells (where ATP is turned to energy). They also have a greater concentration of myoglobin and other oxygen metabolizing enzymes.
Slow-twitch muscles are categorized as Type I Fiber (also called red fiber).
Exercise & Training – The Effects on your Body
Exercise effects the muscular system and the body in many different ways. Exercise triggers many metabolic responses that effect and change the body’s anatomy, physiology, and biomechanics. The amount that exercise changes the body is directly related to the type of training that you do; either anaerobic or aerobic. As we previously discusses, the type, intensity, and the duration of your exercise will stimulate your muscles to develop more fast-twitch or slow-twitch muscle fibers. The type of training and exercising that you perform will also dictate the main source of fuel that your muscles will use for energy.
Aerobic Exercises Effect on the Body
When you think of aerobic workouts think of endurance training, treadmill workouts, biking, stepper machines, and the such. It is a cardiovascular workout. The benefit form these aerobic, cardiovascular workouts include: cardiovascular health, fat burning, and faster heart-rate recovery abilities. Some people fear that aerobic activities will result in muscle loss. If there is a loss, it usually results from the person not eating the adequate amount of calories that the aerobic activity requires.
During aerobic exercises oxygen is forced through your body. This results in an increase in the size and number of blood vessels in your body. With more blood vessels, you can now transport more oxygen and nutrients to your muscles, while at the same time remove more waste products from your muscles. With aerobic exercise, there are metabolic changes that take place within the body at the cellular level. One of these metabolic changes is the increase in the size and number of mitochondria, and an increase in myoglobin. Myoglobin is a protein that transports oxygen from the bloodstream to the muscles. Mitochondria are the cells in the muscles where ATP is produced by the oxidation of glycogen. So, with an increase of the metabolism of glucose, an increase in blood flow, an increase in oxygen and nutrients, more mitochondria, and a higher level of myoglobin, the overall effect is enhanced muscle tissue and a boost in the muscle fiber capability to burn fat.
Anaerobic Exercises Effect on the Body
When you think of anaerobic activities think of power training, bench pressing, squats, a golf swing, and the like. Anaerobic training increases the body’s explosive strength for maximum output for a short time. Anaerobic exercises increase the size and number of fast-twitch muscles. It also increases the muscles ability to tolerate higher levels of blood lactate, and increases the levels of ATP (adenosine triphosphate), CP (creatine phosphate), and Creatine. Anaerobic exercise also increases the amount of both growth hormones and testosterone which are two of the four growth hormones relating to muscle hypertrophy (muscle growth). The other two are insulin and growth factor-1.
Muscle fibers will increase in response to adaptive overload stress. During exercise, the muscle fibers tear apart and then repair themselves resulting in individual muscle cells increasing the numbers of their myofibrils. This is the basic idea of muscle hypertrophy; a breakdown and a rebuilding. At a deeper level there is also an increase in number and size of mitochondria, an increase in myoglobin, an increase in capillarization (the formation and development of a network of capillaries to a part of the body; it is increased by aerobic exercise), and a fusion between muscle fibers and surrounding satellite cells. The satellite cells have only one nucleus and can replicate by dividing. As the satellite cells multiply, some remain as organelles (a specialized subunit within a cell) on the muscle fiber where as the majority differentiate (the process cells undergo as they mature into normal cells) and fuse to muscle fibers to form new muscle protein stands.