How Do Muscles Work?

What Are Muscles Made Of?

Internal stucture of muscleMuscles are the contractile tissue of the body, and produce all movement, and circulation in the body. One muscle fibre is a cell and is composed of a number of thick and thin filaments.  The thick filaments are made of a protein known as myosin and the thin filaments of a protein called actin.  These are arranged to form a three dimensional cylindrical structure, the fibre.  This arrangement has units along it length known as sarcomeres and it is the sarcomere that is the contractile unit of the muscle cell or fibre. Within them, are supporting structures to generate energy, the mitochondria and some fuel stores. There is also a network of channels within the muscle to transmit signals from the surface throughout the muscle (the sarcoplasmic reticulum). Individual cells are linked together by fibrous tissue, the fascia and tendons to join muscle to bones.  Muscles are rich in arteries and veins to carry fuel and oxygen to the muscles and veins to carry waste products and carbon dioxide away. Muscles are the meat which we eat, and the red colour is due to another iron rich protein (myoglobin) which can store oxygen. Muscles also contain a temporary store of energy in the form of creatine, and some fat and a starch (glycogen).  Strength training and high intensity exercise can increase the cross sectional area (csa) of a muscle.  The outward appearance of this is generally larger looking muscles and this increase in muscle csa is directly related to increases in strength and power.

How do Muscles Contract?

Contracile proteins in muscle, actin and tropomyosinWhen you want to move, electrical impulses come from the brain, down through the spinal cord and are transmitted through the motor nerves to the muscles. At the junction between the nerve end and the muscle (the motor end plate), chemical signals are released from the nerve endings (acetylcholine). This binds to a key on the surface of the muscle (the receptor). The binding of this chemical to the receptor causes calcium to enter the muscle cell, and this enables the troponin proteins to move the myosin up the actin molecule. This causes the functional unit, the sarcomere, to shorten and when several of these shorten along the length of the fibre, the muscle as a whole  contracts and shortens. To release the bond between actin and myosin needs energy, to shorten the muscle further or to cause it to relax. When the signal for contraction ends, the calcium is pumped back into the sarcoplasmic reticulum (sr), and the muscle relaxes. One aspect of fatigue is when there is less affinity of ATP for myosin (in response to changes in the environment within the fibre) or there is a slowing of the rate of calcium uptake by sr after contraction and hence a slowing of relaxation of the muscle. Another aspect of fatigue is the availability of fuels.  Glucose and its availability is extremely important for exercise in general to continue. To release the bond between actin and myosin needs energy, to shorten the muscle further or to relax the muscle.  When the signal for contraction ends, the calcium is pumped back into the sarcoplasmic reticulum, and the muscle relaxes.

How Does the Muscle Get and Use Power?

The mitochondrion, the power house of muscleMuscle needs energy to contract and as stated previously the ‘universal energy currency’ of living systems is ATP (adenosine triphosphate).  This is largely produced within mitochondria, organelles which are often referred to as the ‘powerhouse’ of the cell. The ATP that results is used to provide the power for the muscle fibres to contract. Contraction itself (i.e. actual shortening movement) occurs when a bond is broken between ATP and one of its three phosphate bonds.  It is the energy that is liberated by the breaking of this bond that causes the movement. Hence ATP is broken down to ADP (adenosine diphosphate). ADP is reconverted to ATP by donation of a phosphate from another high energy phosphate store in the muscle, creatine phosphate (CP). Mitochondria can burn glucose, fats and ketones to make carbon dioxide and water. Doing so ensures that a greater percentage of aerobic metabolism can be sustained, i.e. a subsequent slightly greater availability of oxygen and production of ATP. A diet rich in creatine has the potential to increase the availability of creatine phosphate, which can increase high energy phosphate supply during intense exercise. Mitochondria can burn glucose, fats and ketones to make carbon dioxide and water. They will do so give an adequate supply of oxygen.

Exercise Types

Aerobic Exercise

During the first few seconds of exercise, muscles use internal stores of ATP and with maximal exercise this is supplemented by high energy phosphate compounds (CP) as exercise continues this is also supplemented by burning of glucose which is converted from muscle glycogen and burned in the mitochondria.  If the rate at which energy is demanded is low then energy production is ‘aerobic’ utilising oxygen.

Anaerobic Exercise

If the rate at which energy is demanded is high then increasingly this is supplemented by contributions from anaerobic metabolism.  The immediate consequence is the production of lactic acid in the muscle which immediately breaks down to lactate and hydrogen ions. Lactate provides a huge fuel source and hydrogen ions can contribute to fatigue by interfering with contractile processes and by changing the pH of the muscle cell. The tolerance for lactate, or more specifically for its attendant hydrogen ions and decreasing pH is limited. This is a protection mechanism, as if pH inside the muscle drops too low damage can occur. However, training can improve tolerance of pH and maintenance of it within an appropriate range which can allow exercise to continue.  Because tolerance of lactate and its attendant challenges very high intensity (‘anaerobic’) exercise is of limited duration..

Training can increase both your capacity for aerobic exercise. Repeated bursts of anaerobic exercise can increase the ability to tolerate and metabolize lactate, and can therefore increase the intensity and duration of maximum work.