Muscle Physiology.

1. Energy systems.

There are three main energy systems that provide power for muscles to contract. The most basic system in the ATP-PCr system. In this system, ATP is converted to ADP, and creates Pi and 3 units of energy. This system has enough capacity for about 6 seconds of maximal effort, or 15 seconds of sub-maximal effort. To replenish the system, PCr (Phosphate Creatine) splits into Cr (Creatine) and Pi (Inorganic Phosphate). 1 unit of energy is required to combine the Pi to ADP to make ATP.

The second system is the Glycolytic System. This has the capacity to provide maximal energy for about 30 seconds, or 120 seconds of sub-maximal effort. In this system, glycogen or glucose is first converted to Glucose-6-Phosphate. 1 unit of energy is required to convert glucose, none to convert glycogen. The Glucose-6-Phosphate is then converted to Pyruvic Acid, and 3 ATP are created. In the absence of oxygen, the Pyruvic Acid is then converted to Lactic Acid and then to Lactate.

The third energy system is the Oxidative System. Unlike the first two systems, which do not require oxygen, this system is heavily dependant on the availability of oxygen to function, and can be limited by a lack of oxygen. This system is able to provide energy almost indefinitely. In the Oxidative System, the Pyruvic Acid from the Glycolytic System is converted to acetyl co-enzyme A (Acetyl CoA). The Acetyl CoA enters the Kreb's (Citric Acid) Cycle where it is broken down into carbon and hydrogen, and 2 ATP are created. The carbon combines with oxygen to form Carbon Dioxide (CO₂), which we eventually breathe out. The hydrogen then enters the Electron Transport Chain where the hydrogen is split into protons and electrons. The H⁺ ions combine with oxygen to form H₂O (water). The electrons are used to produce ATP from ADP. All up, the Oxidative System produces 39 ATP.

2. Muscle Fibre Types

You have probably all heard about the different muscle fibre types: but what relevance does this have to weight-training?

Well, the fibre types are grouped by some common properties including speed of contraction, fuel sources and the ability to resist fatigue. These factor dictate what types of fibres are recruited (used) in different types of exercise.

The Fibre Types

Seven different muscle types have been identified: they are named as types I, IC, IIA, IIB, IIAB, IIC, and IIX.

Of these seven types, the most commonly found are types I and II A and B. The following table summarises some of the characteristics of these fibre types.

Type I or Slow-Twitch (ST) muscle fibres are smaller in size, have a lower recruitment threshold and slower conduction velocities which match their functional task of postural maintenance and other movements which are not critical of speed.

Type II or Fast-Twitch (FT) muscle fibres are larger in size and have faster conduction velocities which enable them to contain more contractile proteins for faster and more forceful muscle contractions.

Other structural aspects of the FT and ST fibres are seen to support their different metabolic functions. ST fibres have higher mitochondrial and capillary density in support of aerobic metabolism. FT fibres are structurally suited to anaerobic metabolism and display high myosin-ATPase and glycolytic enzymatic activity.

Functionally, FT fibres reach peak tension twice as fast as ST fibres. ST fibres are more energy efficient however, which means that they produce more isometric force per unit of energy used, and are, therefore, relatively resistant to fatigue.

The elastic component of ST fibres is reduced due to their higher collagen content, although this doesn't interfere with ST fibre function due to their slower contraction time. However, FT fibres have high elasticity to enable initiation of rapid, forceful contractions.

ST fibres are tonically active and are found in higher proportion in the muscles requiring this function, such as postural muscles. In contrast, higher proportions of FT fibres, which are of a phasic nature, are found in the muscles used for quick, forceful movements. These properties allow another way to classify muscles according to function, as either Tonic Muscles or Phasic Muscles.

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