Effects of Exercise on Muscles

The volume of blood flow to muscle tissues increases during exercise.

It can increase by up to 25 times (2400%) during especially demanding exercise.

During exercise muscles repeatedly contract and relax, using and requiring ENERGY to do so.

The energy comes from adenosine triphosphate (sometimes called ATP) that is broken down during exercise into another chemical called Adenosine diphosphate (ADP):

ATP Adenosine diphosphate (ADP) + ENERGY + inorganic Phosphate

When there is plenty of oxygen available in the muscle tissues the ENERGY for muscle action is produced aerobically but in cases of prolonged or vigorous activity there may be insufficient oxygen available in the tissues so the ENERGY is produced anaerobically.

In the case of anaerobic energy production, ATP is generated by converting glycogen to lactic acid. Lactic acid is a toxic substance that can only be removed from the body by the supply of further oxygen to the affected tissues - hence anaerobic activity leads to oxygen debt.
Exercise can generate an oxygen debt of 10-12 dm³ (up to 18-20 dm³ in trained athletes).

The need for oxygen to repay the "oxygen debt" built up in exercised muscle tissues explains the increased requirement for air / oxygen indicated by panting after extreme exercise - or even just more demanding exercise than one is used to, e.g. running a few strides for a bus if unfit.

(A very short summary about aerobic and anaerobic activity.)

Muscle fatigue is short-term decline in the ability of a muscle to generate force.
Another way to describe muscle fatigue is as the short-term inability to continue to repeat muscular contractions with the same force (i.e. when the same or greater effort seems to result in output of lower mechanical force).

When exercise continues through muscle fatigue after time it can lead to muscle exhaustion.

Muscles can be damaged by injuries sustained during or as a result of exercise.

A common example is strained muscles or muscle tears due to over-stretching without warming-up the muscles before using them intensely.

Muscle damage is both a possible short-term effect of exercise and a possible long-term effect of exercise because, depending on the damage caused, it may take a long or short time to heal. It is included here as a "short term effect of exercise" because damage can even occur after just a short amount of physical activity, e.g. to someone who has not exercised for a long time then suddenly uses muscles intensely, perhaps without having first warmed-up properly.

Muscle cramp is powerful, on-going, uncontrolled muscle contraction due to over-exercise of muscles receiving insufficient blood circulation. It can be painful.

Glycogen and potassium are both important solutes required by the tissues in the body.

Depletion of these from muscle tissues due to excessive exercise are associated with both fatigue and exhaustion - see above.

Although muscle size (and other physical characteristics such as height) is largely determined by a person's genes, muscle size can be affected to a certain extent by:

• drugs e.g. anabolic steroids
• lifestyle choices e.g. exercise for work or leisure.

Not all forms of sports or exercise have a significant effect on muscle size because some sports rely more on concentration, co-ordination and control than on physical power and strength. However, in general, exercising specific muscles regularly can increase their size by up to approx. 60%. This increase in muscle size is mainly due to increased diameter of individual muscle fibres.

As a result of frequent exercise over a sustained period of time both the quantity of blood vessels (incl. e.g. arterioles and venules) and the extent of the capillary beds increases.
The benefits of these effects on blood supply to muscle tissues include:

• improving delivery of substrates to the tissues by the blood
• improving the blood system's efficiency in removing toxic products from the tissues

Frequent exercise and especially use of specific muscles for the same or similar skilled tasks e.g. dribbling a ball in a game of football leads to improved co-ordination.
For example, antagonistic pairs of muscles work together even more effectively; when the prime mover contracts more rapidly the antagonist (muscle) must also relax as quickly.

Improved muscle co-ordination is not just about muscle cells and tissues but also the nerves that innervate those muscles. The somatic nervous system controls skeletal muscle e.g. the muscles that move the arms and legs together with external sensory organs such as the skin.

Many beneficial biochemical changes take place in muscle tissues as a result of regular long term exercise. These include:

• increase in the size and quantity of mitochondria in the cells
• increase in activity of enzymes in the tricarboxylic acid cycle (which is also known as the TCA cycle, the Krebs cycle and as the citric acid cycle), a series of enzyme-catalyzed chemical reactions that form a key part of aerobic respiration in cells
• increase in fatty acid oxidation (fatty acid oxidation in mitochondria provides energy to cells when glucose levels are low).