The immediate source of energy for muscular contraction is the high-energy phosphate compound called adenosine triphosphate (ATP). Although ATP is not the only energy-carrying molecule in the cell, it is the most important one, and without sufficient amounts of ATP most cells die quickly. The three main parts of an ATP molecule are: an adenine portion, a ribose portion, and three phosphates linked together. The formation of ATP occurs by combining adenosine diphosphate (ADP) and inorganic phosphate (Pi). This formation requires a large amount of energy to and it is called a high-energy bond. In order for a muscle to contract, the enzyme ATPase breaks the ATP bond and releases energy which is used to do work. ATP is the energy produced from the breakdown of food into a useable form of energy required by cells.
Muscle cells store limited amounts of ATP. Therefore, because muscular exercise requires a constant supply of ATP to provide the energy needed for contraction, metabolic pathways must exist in the cell to be able to produce ATP rapidly. Muscle cells can produce ATP by three metabolic pathways: creatine phosphate (CP), formation of ATP, formation of ATP through the degragation of glucose or glycogen (glycolysis), and oxidative formation of ATP. The formation of ATP through the CP pathway or glycolysis is called anaerobic metabolism because they do not use oxygen. Oxidative formation of ATP by the use of oxygen is called aerobic metabolism.
As rapidly as ATP is broken down to ADP and Pi during exercise, ATP is reformed through the CP reaction. However, muscle cells only contain small amounts of CP, so the total amount of ATP formed through this action is limited. The combination of stored ATP and CP is called the ATP-CP system and provides energy for muscle contraction during short-term high-intensity exercise. CP is reformed only while you are recovering from exercise. For this process to occur, there has to be ATP present.
A second metabolic pathway capable of producing ATP rapidly without the involvement of oxygen is called glycolysis. Glycolysis involves the breakdown of glucose or glycogen to form two molecules of pyruvic acid or lactic acid. Glycolysis is an anaerobic pathway used to transfer energy from glucose to rejoin Pi to ADP. Glycolysis produces a net gain of two molecules of ATP and two molecules of pyruvic or lactic acid. Although the end result of glycolysis is energy producing, you must add ATP at two points at the beginning of the pathway. In conclusion, glycolysis is the breakdown of glucose or glycogen into pyruvic or lactic acid with the net production of two or three ATP. This depends on whether the pathway began with glucose or glycogen. Since oxygen is not directly involved in glycolysis, the pathway is considered anaerobic. However, in the presence of oxygen in the mitochondria, pyruvate can participate in the aerobic production of ATP. In addition to being an anaerobic pathway capable of producing ATP without oxygen, glycolysis is the first step in the aerobic degragation of carbohydrates.
Although several factors serve to control glycolysis, the most important rate-limiting enzyme in glycolysis is phosphofructokinase (PFK). PFK is located near the beginning of glycolysis. When exercise begins, ADP/Pi levels rise and enhance PFK activity, which serves to increase the rate of glycolysis. In contrast, at rest when cellular ATP levels are high, PFK activity is inhibited and glycolytic activity is slowed. Further, high cellular levels of free fatty acids also inhibit PFK activity. Similar to the control of the ATP-CP system, regulation of PFK activity operates through negative feedback. Another important regulating enzyme in glycolysis is phosphorylase, which is responsible for degrading glycogen to glucose. This reaction provides the glycolytic pathway with the necessary glucose at the origin of the pathway. At the beginning of exercise, calcium is released from the sarcoplasmic reticulum in muscle. This rise in sarcoplasmic calcium concentration indirectly activates phosphorylase which immediately begins to break down glycogen to glucose for entry into glycolysis.
In addition, phosphorylase activity is stimulated by high levels of the hormone epinephrine. Epinephrine, released at a faster rate during heavy exercise, results in the formation of cyclic AMP. It is cyclic AMP, not epinephrine, that directly activates phosphorylase. Therefore, the influence of epinephrine on phosphorylase is indirect.
It is important to emphasize the interaction of anaerobic and aerobic metabolic pathways in the production of ATP during exercise. Although it is common to hear someone speak of aerobic versus anaerobic exercise, in reality the energy to perform most types of exercise comes from a combination of anaerobic/aerobic sources. The contribution of anaerobic ATP production is greater in short-term high-intensity activities, while aerobic metabolism is mainly found in longer activities. In conclusion, the shorter the activity, the greater the contribution of anaerobic energy production. The longer the activity, the greater the contribution of aerobic energy production.
Aerobic Respiration is the metabolic process that generates ATP in association with a chemiosmotic process driven by a respiratory chain that depends on the use of oxygen as the ultimate ele ctron acceptor. Water is the ultimate reduced end product and this process occurs in the mitochondria where ATP is made by oxidative phosphorylation. In mitochondria tricarborylic acid cycle activity and fatty acid oxidation provide most of the reducing equivalents that fuel this process but reducing equivalents released by metabolite oxidation reactions in the cytosol can be shuttled into mitochondria to supply a small proportion of ATP needs.
The abdominal muscles help to maintain the trunk, maintain posture and compress the contents of the abdomen. There are four different sets of abdominal muscles involved. The first is the rectus abdominus. This is the straight muscle of the abdomen. It is medial, and it is divided into segments laterally by connective tissue. The rectus abdominus flexes and rotates the trunk and compresses the abdomen.
The external obliques are the most superficial of the lateral muscles. Its fibres run obliquely from the ribs to the linea alba. The linea alba is the midline seam of connective tissue which binds all of the abdominal muscles. The external obliques flex and laterally flexes the trunk, and compresses the abdomen. The internal obliques are deep to the external obliques. The fibres run at right angles to the externals, which increases the strength of the muscular abdominal wall. The internal obliques flex and laterally flexes the trunk, and as well assists in compressing the abdomen. The transversus abdominus is the deepest of the lateral muscles. Its fibers run transversely from the ribs and top of the ox coxa to the linea alba. The only function it has is to compress the abdomen. When performing a regular crunch exercise, you can hit all four of the abdominal muscles discussed. There is no abdominal exercise that is better than the rest, but it is important that you switch exercises every so often. The reason for this is due to the fact that each exercise hits the abdomen in a different way and in order to prevent your muscles from adapting, you must not only increase intensity, but the exercise as well.
Muscular contraction is a complex process involving a number of cellular proteins and energy production systems. The final result is a sliding of attin over myosin, which causes the muscle to shorten and therefore develop tension. The process of muscular contraction is best explained by the sliding filament theory of contraction; muscle fibres contract by a shortening of their myofibrils, which results in a reduction of distance from Z line to Z line. As the sarcomeres shorten in length, the A bands do not shorten but move closer together. However, the I bands decrease in length. Filament sliding occurs due to the action of the numerous cross-bridges extending out like arms from myosin and attaching on the actin filament. The head of the myosin cross-bridge is oriented in opposite directions on either end of the sarcomeres. This orientation of cross-bridges is such that when they attach to actin on each side of the sarcomeres they can pull the actin from each side towards the center.
The energy from contraction comes from the breakdown of ATP by the enzyme ATPase. The breakdown of ATP to ADP and Pi and the release of energy serves to energize the myosin cross bridges. The ATP released energy is used to cock the myosin cross-bridges, which in turn pull the actin molecules over myosin and shortens the muscle. A single contraction cycle, or power stroke of all the cross-bridges in a muscle would shorten the muscle by one percent of its resting length. Since the muscles can shorten up to sixty percent of their resting length, it is clear that the contraction cycle must be repeated over and over again. In order for this to occur, the cross-bridges must detach from actin after each power stroke, resume their original position and then re-attach to actin for another power stroke.
Relaxed muscles are easily stretched which demonstrates that at rest, actin and myosin are not attached. The regulation of a muscle contraction is a function of two proteins called troponin and tropomyosin, which are located on the actin molecule. The actin filament is formed from many smaller protein pub units arranged in a double row and twisted. Tropomyosin is a thin molecule that lives in a grove between the double row of actin. Troponin is attached directly to the tropomyosin. They work together to regulate the attachment of the actin and myosin cross-bridges. In a relaxed muscle, tropomyosin blocks the active sides on the actin molecule where the myosin cross-bridges must attach in order for contraction to occur. The trigger of contraction to occur is linked to the release of stored calcium from the sarcoplasmic reticulum. Most of this calcium is stored within expanded portions of the sarcoplasmic reticulum. In a relaxed muscle the concentration in the saroplasm is very low. However, when a nerve impulse arrives at the mernomuscular junction it travels down the transverse tubules to the sarcoplasmic reticulum and causes a release of calcium. Some of this calcium binds to troponin, which causes a position change in tropomyosin such that the active sites on the actin are uncovered. The energy released from the breakdown of ATP cocks the myosin cross-bridges. This energized cross-bridge then attaches to the active sites on actin and contraction occurs.
Attachment of fresh ATP to the myosin cross-bridges allows the cross-bridge to detach and re-attach to another active site on an actin molecule. This contraction cycle is repeated as long as free calcium is available to bind the troponin and ATP is available to provide the energy. The signal to stop contraction is the absence of the nerve impulse at the neuromuscular junction. When this occurs, an energy requiring calcium pump located within the sarcoplasmic reticulum begins to move the calcium back into the sarcoplasmic reticulum. This removal of calcium from troponin causes tropomyosin to move back to cover the binding sites on the actin molecule and cross-bridge interaction ceases.
It is possible for skeletal muscle to exert force without the joint angle changing. This might occur when an individual pushes against the wall of a building. Muscle tension increases bu t the wall does not move, so neither does the body part that applies to the force. This is called an isometric contraction. Isometric contractions maintains a static body position during periods of standing or sitting. In contrast most types of exercise involve contractions that result in movement of body parts. This is called an isor isotonic contraction. Tension within the muscle increases but the joint angle changes as the body parts move.
Skeletal muscle can be divided into three types of fibers. These are: fast-twitch fibers (fast-glycolytic), low-twitch fibers (slow oxidative), and intermediate fibers (fast oxidative glycolytic). Fast-twitch fibers have a small number of mitochondria, a limited capacity for aerobic metabolism, and are less resistent to fatigue than slow-twitch fibers. However, fast-twitch fibers are rich in glycogen stores and glycolytic enzymes, which provide them with a large anaerobic capacity. In addition, fast-twitch fibers contain more myofibrils and ATPase than slow-twitch fibers, and are therefore able to contract more rapidly and develop more force than the slow-twitch fibers.
Slow-twitch fibers contain larger numbers of mitochondria and are surrounded by more capillaries than fast-twitch fibers. In addition, slow-twitch fibers contain higher concentrations of the red pigment myoglobin. The high concentration of myoglobin, and the high content of mitochondrial enzymes provide slow-twitch fibers with a high capacity for aerobic metabolism and a high resistance to fatigue.
Intermediate fibers contain biochemical and fatigue characteristics that are somewhere betweeen fast-twitch and slow-twitch fibers.
The amount of force exerted during muscular contraction is dependent on a number of factors. These include the types and the number of motor units recruited, the initial length of the muscle, and nature of the neural stimulation of the motor units. Variations in the strength of contraction within an entire muscle depends on the number of muscle fibers that are stimulated to contract. If only a few motor units are recruited, the force is small. If more motor units are stimulated the force is increased. As the stimulus is increased, the force of contraction is increased due to the recruitment of additional motor units. The peak force generated by muscle decreases as the speed of movement increases. However, the amount of power generated by a muscle group increases as a function of movement velocity. The muscle spindle functions as a length detector in muscle. Golgi tendon organs continuously monitor the tension developed during muscular contraction. In essence, Golgi tendon organs serve as safety devices that help prevent excessive force during muscle contractions.
The 206 bones of your body protect and support your organs and allow movement. Bones are living, changing structures that require adequate calcium and weight-bearing exercise to build and maintain their density and strength. Bones are joined together by different types of joints: fixed joints (as in the skull), hinged joints (as in the fingers), and ball-and-socket joints (as in the shoulders and hips). The bones function as a lever. The bones of the upper and lower limbs push and pull, with the help of muscles.
Bones are also a calcium store. 97% of the body’s calcium is stored in bone. Here it is easily available and turns over fast. In pregnancy the demands of the fetus for calcium require a suitable diet and after menopause hormonal control of calcium levels are impaired which can cause brittleness and a chance for osteoporosis to occur. In addition, bones are a marrow holder. This is secondary to produce maximum strength for minimum weight. The cavities produced in unstressed areas are used for marrow, or in some places just for storage. Around the outside is a layer of strong, hard, heavy compact bone. In the middle is a branching network of trabecular bone which usually follow lines of force. Marrow sits in the interconnecting cavities between those plates or rods of bone.
A joint is formed by the meeting of two or more bones. A joint can allow full movement (synovial), little movement (cartilagenous), or no movement (fibrous). With immovable joints, bones are joined by cartilage (ex: rib meets sternum) or a series of dove tailed edges (ex: skull). Slightly movable joints are where bones are joined by ligaments only (ex: where tibia and fibula meet) or by ligaments and fibrous cartilage (ex: between vertebrae). Freely movable joints are where both ends of the bone are covered with cartilage and surounded by a fibrous capsule. This capsule is lined with smooth tissue called synovial membrane which secretes a fluid to lubricate the joint. This type of joint is strengthened by ligaments and is the most common type of joint. There are six different types of freely movable joints:
1)Pivot – bone rotates on a fibrous ring;
2)Saddle – thumb joints, the articular surfaces fit together concave
3)Condyloid – convex surfaces fit into concave, free movement,
but no rotation (ex: wrist);
4)Gliding – vertebrae of spine, two nearly flat surfaces glide over
5)Hinge – joint movement is in one plane (ex: elbow);
6)Ball-and-socket – the shoulder and hip joints are the only ball
and socket joints in the body. Bone head fits into cup-like
cavity, movement is allowed in any direction. They are the
most freely movable synovial joints.
Types of movement of synovial joints:
- flexion – decreasing the angle between two bones
- extension – increasing the angle between two bones
- adduction – moving the bone towards the midline
- rotation – moving the bones around a central axis
- circumduction – complete circular movement
- elevation – raising a part of the body
- depression – lowering a part of the body
The nervous system is the body’s means of perceiving and responding to events in the internal and external environments. Receptors capable of sensing touch, pain, temperature, and chemical stimuli send information to the central nervous system (CNS) concerning changes in our environment. The CNS responds by either voluntary movement or a change in the rate of release of some hormone from the endocrine system, depending on which response is appropriate. The nervous system is divided into two major divisions, the central nervous system and the peripheral nervous system. The central nervous system includes the brain and the spinal cord, and the peripheral nervous system includes the nerves outside the central nervous system.
Nerve cells are called neurons and are divided anatomically into the cell body, dendrites, and axon. Axons are covered by schwann cells, with gaps between these cells called nodes of ranvier. Neurons are specialized cells that respond to physical or chemical changes in their environment. At rest, nerve cells are negatively charged in the anterior when compared to the electrical charge outside the cell. This difference in ele ctrical charge is called the resting membrane potential. A neuron fires due to a stimulus changing the permeability of the membrane, allowing sodium to enter at a high rate, depolarizing the cell. When the depolarization reaches threshold, an action potential or nerve impulse is initiated.
Repolarization occurs immediately following depolarization due to an increase in membrane permeability to potassium, and a decreased permeability to sodium. Neurona communicate with other neurons at junctions called synapsis. Synaptic transmission occurs when sufficient amounts of a specific neurotransmitter are released from the presynaptic neuron. Upon release, the neurotransmitter binds to a receptor on the post synaptic on the postsynaptic membrane. An excitatory transmitter increases neuronal permeability to sodium and results in excitatory postsynaptic potentials. However, some transmitters are inhibitory and cause the neuron to become more negative or hyperpolarized. This hyperpolarization of the membrane is called an inhibitory postsynaptic potential.
Propriosceptors are position receptors located in joint capsules, ligaments, and muscles. The three most abundant joint and ligament receptors are free nerve endings, golgi-type receptors, and pacinian corpuscles. These receptors provide the body with a conscious means of recognition of the orientation of body parts as well as feedback relative to the rates of limb movement.
Reflexes provide the body with a rapid unconscious means of reacting to some stimuli. The vestibular apparatus is responsible for maintaining general equilibrium and is located in the inner ear. Specifically, these receptors provide information about linear and angular acceleration.
The spinal cord plays an important role in voluntary movement due to groups of neurons capable of controlling certain aspects of motor activity. The spinal mechanism by which a voluntary movement is translated into appropriate muscle action is termed spinal tuning.
The brain can be divided into three parts: the brain stem, the cerebrum, and the cerebellum. The motor cortex controls motor activity with the aid of input from subcortical areas. The cerebellum receives feedback from proprioceptors after movement has begun and sends information to the cortex concerning possible corrections of that particular movement pattern.
The basal ganglia are neurons involved in organizing complex movements and the initiation of slow movements. The premotor cortex operates in conjunction with the motor cortex to refine complex motor actions and may be important in acquisition of motor skills.
The autonomic nervous sytem is responsible for maintaining the constancy of the body’s internal environment and can be separated into two divisions: the sympathetic division and the parasympathetic division. In general, the sympathetic portion tends to excite an organ, while the parasympathetic portion tends to inhibit the same organ.