MUSCLES AND EXERCISE

Exercise and physical activity:

I have a few more thoughts about the subdivisions of physical activity, stimulated by an article in the International Journal of Cardiovascular Health – and with a title like that you have to pay attention. Their suggestion is that
*  “Physical Activity” is any bodily movement produced by skeletal muscles that results in energy expenditure.
*  “Exercise” is a subset of physical activity that is planned, structured and repetitive and
*  “Sport” is an activity that most often, but not always, involves exercise and predetermined rules and participation in games or races or competitions. So there you are………..

The benefits of physical activity, exercise and sport

I have come upon an excellent book on the benefits, and lack of benefits, of all forms of lifestyle change, including of course exercise. It is called “This book could save your life” by a New Scientist journalist – Graham Lawton. I particularly like this:

“If you want to be healthy, slim and youthful, there’s a miracle treatment which you should try. It is available to almost everyone, whenever they want it, in unlimited quantities. It doesn’t cost anything except a bit of time and effort. It can make you feel as good as any drug. In return you get protection against heart attack, strokes, diabetes, cancer, Alzheimer’s disease and depression. It can even cure some diseases. Overall it has the potential to prevent more premature deaths than any other single medical intervention, with none of the side effects.

What is the name of this wonder drug? Exercise…….”

Aerobic and anaerobic exercise

A convenient way of subdividing muscular effort is by the need for oxygen to fuel their exertions. If oxygen is needed the effort is “aerobic”. If not, it is “anaerobic”. My main interest is in aerobic exercise – movement which demands a continuous supply of oxygen carried to the muscles by the cardiovascular system. More on this theme next week

The muscles are the body’s engines – all movement depends on muscular contraction and relaxation. All joints and limbs are powered by muscular contraction and the physiology of muscle action is both beautiful and elegant.

 Muscle types

The muscles of movement are the skeletal muscles known also as voluntary or striated muscles. They have a main body, the belly of the muscle, and boney attachments which are very strong fibrous cables called tendons. Most movements depend on muscles contracting thus bending or straightening the joints which connect the bones to which the tendons are attached. A good example is the biceps muscle, attached to the upper arm, the humerus, and inserted into the forearm bones, the radius and ulna. Contracting the biceps bends the elbow joint which connects these bones.

Each muscle is made up of thousands of fibres which in turn are made up of myofibrils. The myofibrils contain the microscopic structures which can bring about shortening of the whole structure – contraction. Each muscle cell contains thousands of myofibrils which are tubular structures extending the whole length of the cell. The myofibrils are made up of thick and thin filaments; the thick filaments made up mainly of myosin and the thin filaments of actin.

Muscle contraction is stimulated by discharge of the nerve supplying the muscle – the motor neuron (I won’t bore you with the physiology of this process. Suffice it to say that its effect is similar to the arrival of an electrical impulse). Each muscle is served by a large number of nerve fibres and the individual motor neuron plus the muscle fibres which it stimulates is called a motor unit.  When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in all the unit’s sarcomeres (the basic component of muscle) between the actin and myosin filaments.  This reaction results in the start of a contraction which is achieved by the two different types of filament sliding alongside each other producing shortening of the whole muscle. Viewed under a microscopic the muscle fibres have a striped appearance – hence striated muscle – which is caused by the highly ordered arrangement of the actin and myosin within the fibrils and the similarly ordered arrangement of the fibrils within the muscle cell. 

Muscle contraction continues until the neuron stops stimulating it or until muscle fatigue sets in. This happens when the supply of energy to fuel contraction is exhausted.

Type I and Type II muscle fibres

One more important fact about muscles. They are made up of two distinct types of fibre – slow twitch (Type I) and fast twitch (Type II) fibres (which are further subdivided into Type IIa and Type IIb fibres but please don’t trouble yourselves with that information). Slow twitch fibres contract slowly and can maintain their shortening for long periods – they are the strength and endurance giving fibres and are particularly used during such activities as distance running and cycling. Fast twitch fibres contract rapidly but tire quickly and are those most used by sprinters and weight lifters. Repeated contractions gives speed to action.

Most muscles consist of a combination of slow and fast twitch fibres but one often predominates in particular muscles – for instance back muscles which move little but contract continuously to maintain posture are predominantly slow twitch. Eye muscles on the other hand are almost all fast twitch. There is also a genetic influence on the predominance and distribution of muscle type. The ratio of slow to fast is about 50% for most of us. However some individuals inherit a much larger proportion of one or another and that may determine their prowess in different sporting endeavours. Those born with a higher proportion of Type I fibres are more likely to become endurance athletes while those born with more Type II fast twitch fibres will do better as sprinters and weight lifters.