Muscle and Muscle Injury Dr Rod McLoughlin
1. What Is Muscle?
This is the first in a series of articles covering sporting muscle injuries. In this article I hope to outline the basic properties of skeletal muscle.
What is muscle?
Muscle is a fleshy contractile tissue which serves to move parts of the body relative to each other. The ability of a muscle to forcibly change length (contractility) is the property which distinguishes muscle from other tissues.
What types of muscles are there?
There are three main types of muscles;
1. Cardiac (heart) muscle.
2. Smooth muscle - lines the walls of hollow organs such as the stomach, intestines, and blood vessels. This type of muscle is well adapted to producing long, slow contractions which are not under voluntary control.
3. Skeletal ( or striated ) muscles – muscles in the body which are attached to bone. When stimulated it moves a part of the skeleton such as the leg. Contraction of skeletal muscle is under voluntary control.
The human body possesses more than 300 hundred clearly defined skeletal muscles compromising about 40% of the total body weight. Skeletal muscle is the type of muscle that these articles refer to.
Structure of Skeletal muscle;
Each muscle has two attachments to the skeleton (bones). In between is the bulky part of the muscle called the muscle belly. Muscles are often attached to the skeleton by tendons. Thus a muscle consists of a number of distinct areas – muscle belly, musculo-tendinous junction (junction between muscle belly and tendon ), tendon and teno-osseous junction ( site of attachment of the tendon to the bone.)
Skeletal muscle is composed of thousands of long narrow muscle cells or fibres surrounded by a membrane. These fibres are grouped together into bundles (fasciculi) and the bundles are also surrounded by a membrane. Bundles of fibres are bound together to form the muscle belly. Some muscle have more than one belly eg biceps has two heads.
The muscle fibres contain filaments which are interconnected. When a muscle contracts these filaments slide over each other to change the muscle length. The contraction is triggered by a nerve impulse arriving down the nerve which supplies the muscle. Muscles need energy to contract, this energy is supplied by oxygen, carbohydrates/sugars and fats.
Types of fibres;
There are broadly two types of muscle fibres.
1. Slow ( Type I ) fibres - need oxygen to work. Slow fibres are used in endurance sports. These fibres contract slowly and have a high resistance to fatigue.
2. Fast ( Type II ) fibres – work without oxygen. Fast fibres have a fast speed of contraction. These fibres contract fast and have a high power output.
Fibre type varies between individuals and is genetically determined. Training does not alter fibre type. Endurance training improves endurance performance of both fast and slow twitch fibres
Types of muscle contraction;
A muscle contraction is the generation of tension within a muscle which usually produces movement.
1. Concentric contraction – results in shortening of the muscle.
2. Eccentric contraction – results in lengthening of the muscle.
3. Isometric contraction – results in no change in muscle length.
The term ‘contraction’ implies shortening and is thus sometimes confusing. The term muscle action maybe better.
Using the example of lifting a cup in the hand to the mouth and the work done by the Biceps muscle which is on the front of the upper arm.
When the cup is being lifted up from the table to your mouth the biceps muscle is doing work and getting shorter – this is a concentric contraction.
When the cup is being held still at your mouth the biceps muscle is doing work and not changing length – this is an isometric contraction.
When the cup is being put back down on the table from your mouth the biceps muscle is doing work and getting longer – this is an eccentric contraction.
Other topics to be covered in future articles;
1. Assessment of a muscle injury.
2. Management of a muscle injury.
3. Muscle haematomas.
4. Myositis ossificans.
5. Flexibility/ stretching.
6. Strength training.
7. Cramp.
8. Delayed onset muscle soreness (DOMS).
9. Compartment syndrome.
10.Specific muscle injuries.