The Muscular System

Unit 1 - Anatomy for sport and exercise

Muscle Classification

There are 3 different classifications of muscle; smooth muscle, skeletal muscle and cardiac muscle. The purpose of these muscles are for movement and support as well as providing heat. Muscles are generally composed of 75% water, 20% protein, 5% mineral salts and glycogen & fat

Smooth muscle - Smooth muscles are located in hollow organs (such as the stomach or bladder) and blood vessels. They are involuntary muscles which produce long, slow contractions. They are generally found in the body where conscious thought is not required in order for movement to occur for example the stomach and intestine's role in the digestive system. Smooth muscle makes up the majority of the muscles in the walls of almost all of the body's hollow organs, the only exception being the heart. The main roles of smooth muscle are to help regulate digestion, by contracting to act help transport things in the digestive system, and also to help aid blood pressure. Smooth muscles measure 5-10 mm in diameter and 30-200 mm in length, they obviously cannot be too large as they generally line an organ or blood vessel. Also there is a large size range as there will be a massive difference in the size of the muscles in a major hollow organ such as the stomach compared to a small blood vessel. They primarily use an aerobic metabolism to get fuel.

Cardiac muscle - Cardiac muscles are located only in the walls of the heart, this makes it unique compared to the other 2 types of muscle as it is specially made for one organ/part of the body and is found nowhere else. The contractions of cardiac muscle helps force blood through the heart and blood vessels and around the whole body. Each contraction of the cardiac muscles as an entirety represents one heartbeat. Cardiac muscles work only aerobically and are in constant contractions as the heart must constantly be pumping blood around the body. One of the main factors of a cardiac muscle compared to the other types is that they are incredibly fatigue resistant - a heart beats about 100,000 times per day and it goes constantly until you die (or go into cardiac arrest) the fact that it can constantly contract for this length of time without tiring shows how fatigue resistant it is. Cardiac muscle is made up of a specially formed type of strained tissue which has its own blood supply. Cardiac muscle is also involuntary, like smooth muscle, as you do not consciously control it. Cardiac muscle measures 10-20 mm in diameter and 50-100 cm in length. Cardiac muscle also gets fuel from an aerobic metabolism but also from some lipid or carbohydrate substrates.

Skeletal muscle - Skeletal muscles will be attached to the skeletal system. It accounts for a large amount of muscle tissue throughout the whole human body. Skeletal muscles are unique as they are the only muscle type which is under voluntary meaning that it is consciously controlled. Skeletal muscle is attached to and around the bones/limbs which it allows us to move; when stimulated the skeletal muscle will move along with the part of the body which it is moving. Skeletal muscles measure around 100 mm in diameter and up to 30 cm in length making them far larger than either of the other types of muscle. For fuel skeletal muscles use an aerobic metabolism during moderate-low level activity and use glycosis during more intense (anaerobic) activity. Skeletal muscles are often called "striped muscles" due to their stripy appearance which is caused by long, thin parts of each individual muscle cell called fibrils which run longitudinally along the muscle. Each muscle also contains multiple nuclei. The human body contains over 400 skeletal muscles and they make up 40-50% of total body weight. There are multiple connective tissues in and around a skeletal muscle; the epimysium surrounds the entire tissue, the perimysium surrounds bundles of muscle fibers and the endomysium surrounds the individual muscle fiber. Each muscle fiber contains many myofibril.

Muscle fibre types

There are 3 different types of muscle fibre types; type 1, 2a and 2b. Each type of muscle fibre has different characteristics and functions.

Type 1 - Type 1 muscle fibres are built for aerobic activity such as marathon running or long distance swimming. They have a slow contractions speed (slow twitch) and can contract repeatedly meaning that they are excellent for endurance activities. In addition to this, type 1 muscle fibres are red in colour due to the high myglobin stores which allow them to carry lots of oxygen meaning that they will be able to use it as energy. Essentially these type of muscle fibres also exert minimal force for a long period of time. In addition to this they also have high mitochondira stores, low phosphocreatine stores, low glycogen stores and high triglyceride stores. Type 1 muscle fibres use oxygen to generate energy through ATP, they are very efficient at this which allows the continuous muscle contractions for long periods of time.

Type 2a - Type 2a muscle fibres are fast twitch muscle fibres made primarily for anaerobic activity but can also be used for aerobic activity, they are almost a combination of type 1 and type 2a muscle fibres. They have a medium contraction speed and are fairly fatigue resistant which allows them to be used for both aerobic and anaerobic activity but they will not be able to perform well on either extremity of the scale for example marathon running or 30 yard dash however they are ideal for activities such as 800/1500M running or also for team/invasion sports such as football where you need to do some sprinting and other anaerobic activities but you also need to keep going for the full 90 minutes. Type 2a muscle fibres are red in colour and have medium myglobin and mitochondria stores unlike type 2b fibres due to the fact they do use oxygen. They also, however, have high PC and glycogen stores like 2b fibres and they also have medium triglyceride stores.

Type 2b - Type 2a muscle fibres are also fast twitch fibres which are built entirely for anaerobic activity such as sprinting or explosive movements. They have a rapid contraction speed (fast twitch) however they are easily exhausted, this makes them well suited for short intense bursts of activity. They use mainly ATP-PC as energy as opposed to oxygen. They also have low myglobin stores and are white due to the fact they do not really use oxygen as fuel. They have high phosphocreatine and glycogen stores and also have low triglyceride stores. Type 2b muscle fibres will be used in short bursts of high intensity physical activity such as the 100M sprint or a single explosive action such as jumping up to dunk the ball in basketball as these muscle fibres exert the largest amount of force out of any type of muscle fibre.

Sliding filament theory

This is the theory of how muscles are believed to contract.

Inside each muscle fiber, there are many fibers called myfibrils which will appear as very thin lines when looked at under a microscope. To see how the muscles contract via the sliding filament theory we must look into what goes on in each individual sarcomere (which is a functional section of a myofibril), which are separated by Z-discs. Each sarcomere is composed mainly of 2 types of protein; the actin filament which is thin, and the myosin filament which is thicker. These filaments are spread out around the sarcomere as shown by the diagram; the I band contains only actin filaments, the H band contains only myosin filaments and the A band contains both actin and myosin filaments. During muscle contraction the I band shortens, the A band remains - this is because the myosin pulls the actin across so that the 2 filaments slide closer together however the filaments themselves do not shorten in length.

The whole process of the myosin pulling the actin inwards is called the ratchet mechanism and it happens somewhere between 50-100 times per second for each myosin head when the muscle is contracting. Firstly a bridge is formed between the actin and myosin filaments during muscle contraction. This happens due to the many small heads on the myosin connecting to the binding sites, this is called a crossbridge. When the crossbridge is made the myosin uses its energy from ATP to pull the actin filaments towards each other in an arcing motion, this is called the power stroke. The crossbridge will then detach itself from the actin binding point then reattach itself to another further along the actin filament,. This completes the ratchet mechanism. This process also pulls the Z bands closer together which causes the muscle to shorten and contract.

The actin filament also contains 2 other proteins - tropomyosin and troponin. Tropomyosin is a rod shaped protein which is wrapped around the actin core - this protein prevents the myosin from forming a crossbridge when the muscle is not contracting. The troponin is a complex of proteins attached to the tropomyosin molecule. When muscles contract, calcium ions are released and bind with the troponin molecules. This causes the tropomyosin molecules to move from covering the binding sites, allowing the myosin heads to attach to the actin binding sites.

For this entire process to occur a high energy compound called ATP (adenosine triphosphate) is needed at fuel. One crossbridge requires one ATP molecule to perform the power swing.

Agonistic pairs

Skeletal muscle are generally arranged in pairs; the agonist and the antagonist. There are also fixator muscles and synergist muscles. Generally when one muscle is the agonist for one movement then it will be the agonist for one movement then it will be the antagonist for the opposite for example in the bicep curl the bicep brachii is the agonist muscle and the tricep brachii is the antagonist muscle where as in the opposite muscle, the tricep dip, the bicep brachii is the antagonist and the tricep brachii is the agonist.

Agonist - The agonist muscle (or the prime mover) it the main muscle in a contraction. It will contract to perform an action. In the upwards phase of a bicep curl the bicep brachii will be the agonist. As mentioned before the agonist will be working closely with the antagonist.

Antagonist - The antagonist is the opposite muscle to the agonist. Whilst the agonist contracts the antagonist relaxes. During the upwards phase of a bicep curl the tricep brachii will be the antagonist.

Fixator - Fixator muscles stop any unwanted movement throughout the body while a contraction occurs. This is done by the fixator muscles stabilising the origin of the agonist and also the joint of which the origin spans. This also helps the agonist muscle contract more effectively. In the bicep curl the fixator muscles would be the rotator cup muscle group in the shoulder.

Synergist - Synergist muscles work with the other muscles to allow the agonist to contract more effectively. They work mainly with the agonist muscle of the contraction to direct movement by modifying the direction of pull on the agonist to a more effective position. They stabilise the joint which movement occurs which also helps increase effectiveness of the contraction. In the bicep curl the synergist muscles are the brachioradialis and the breachialis which helps to stabilise the elbow joint.

Contraction types

There are 4 main types of muscle contractions; concentric and eccentric (which are both isotonic contractions) ,isometric and also isokinetic.

Concentric - Concentric muscle contractions are the main type of contraction which occurs in the body, they are contractions which cause the muscle to shorten in length as the 2 ends of the muscle move closer together in order to manipulate the joint/limb. An example of this is during a bicep curl, when there is flexion at the elbow, the bicep brachiii will concentrically contract in order to bring the arm up.

Eccentric - Eccentric contraction is the opposite of concentric contraction. During eccentric contraction the muscle lengthens, as the 2 ends of the muscle go further apart, while still producing muscle tension like in an isometric contraction however there is not as much muscle tension created in this type of contraction. A good example of this type of contraction is the downward phase of the bicep curl when the bicep brachii will be working and contracting concentrically in order to slowly lower the weight in a controlled way. Eccentric muscle contraction is far less common than concentric muscle contraction, it also requires the control of a certain movement being started by the eccentric muscle's agonist.

Isometric - During isometric contractions the muscle which is contracting does not actually change in length - where as in an isotonic contraction the muscle will noticeably change - the joint angle also stays constant throughout the contraction. In isometric contraction the muscles actively engage in holiding a statuc position and they are in constant tension as they contract to hold something in place. Isometric contractions are easy to perform but lead rapidly to fatigue. They can also make blood pressure increase rapidly due to the fact that blood flow can be reduced when doing this type of contraction. An example of this is if an individual is doing the plank then the abdominal muscles will contract isometrically.

Isokinetic - In an isokinetic contraction the muscle always contracts and shortens at a constant speed. It is very rare and is generally done by some form of equipment so it is not a natural contraction. Isokinetic contraction is often used in physiotherapy as it helps to give an individual good kinaesthesis for a particular movement.

Example - the basketball layup

For this example we are focusing on the arm pushing the basketball up when performing the layup shot.

Sliding filament theory - When the tricep brachii contracts concentrically to push the ball up, calcium ions are released causing the tropomyosin to shift so that crossbridges can be formed between the myosin and actin. There is then continuous power strokes which pull the actin inwards and therefore the Z disks together forcing the muscle to begin to contract and shorten. The myosin heads then detach and reattach somewhere further along the actin filament. This process (the ratchet mechanism is repeated until the elbow is fully extended and the ball is released from the grip of the player.

The muscles involved - The agonist muscle in the layup would be the tricep brachii which contracts to push the ball outwards away from the body. The antagonist muscle is the bicep brachii which relaxes to allow the tricep brachii to contract as the ball is thrown up. The fixator muscles would be the rotator cuff muscles in the shoulder. The synergist muscle would be the brachioradialis and the breachialis which will help stabilise the elbow as it extends.

Contraction types - The only contraction type in the layup shot would be concentric contraction as the tricep brachii - the main contracting muscle - is shortening as the ball is thrown up. However isometric contraction is also very common in basketball for example when boxing an opposition player out (holding them behind you when trying to get a rebound) many muscles in the body are contracting isometrically including the quadricep group and the trapezius. Eccentric contraction is relatively rare in basketball however we can see it in the bicep brachii when pushing an opposing player away to make space to get the ball, this is similar to the way that the bicep eccentrically contracts during the downward phase of a bicep curl.