Structure of muscle

Click images to enlarge.

The muscle belly is completely surrounded by a thick connective tissue called epimysium. This is continuous and eventually forms tendons which join the muscle onto the periosteum of bones. Each part of the muscle is covered by connective tissue to help provide shape and add strength. The muscle belly is composed of many bundles of fibres known as fascia, and these are covered with perimysium. Each fibre within a single fascia is surrounded by a membrane called the sarcolemma and contains many smaller fibres called myofibrils which provide the contractile unit of the muscle. These myofibrils are covered by endomysium and have light and dark bands which represent a sarcomere. Sarcomeres are the smallest contractile units of the muscle.
muscle structure

Sliding filament theory

Each muscle fibre consists of large numbers of microscopic threads called myofibrils, which in turn consist of two rows of microscopic protein filaments called actin and myosin.
sliding filament theory 1
It is the ability of the actin and myosin filaments to slide over each other that brings about the contraction and relaxation of the muscles. This is known as the sliding filament theory. When a muscle contracts the actin and myosin slide over each other (like patio doors).
sliding filament theory 2
Every muscle fibre is linked to the Central Nervous System (CNS) by nerves. A motor unit is made up of ONE motor nerve and all muscle fibres that are stimulated by that nerve. When a muscle contracts, not all the fibres are used. Only the number of fibres necessary for the task involved are used. When muscle fibres contract, they obey the all or none law which states that a muscle fibre contracts fully or not at all. The force of muscle contraction depends on the cross-sectional area of the muscle, how many motor units are recruited and how often they are stimulated.

Types of muscular contraction

(1) Isotonic contractions occur when a muscle moves during contraction. This can be divided into two types:

  1. Concentric contractions involve the muscle shortening while contracting.
  2. Eccentric contractions involve the muscle lengthening whilst contracting.

isotonic contractions
Gains in muscular strength and endurance are developed through a full range of movement with isotonic exercises. Isotonic exercise helps to promote the neuromuscular pathways and progress can be easily monitored.

(2) Isometric contractions occur when the muscle remains the same length whilst contracting. It has been reported that this form of training can lead to rapid gains in muscle strength and may be suitable for good postural development. However, any gains made are only specific to the joint angle used in the exercise. Isometric exercises require little or no equipment and can be of excellent use in muscle rehabilitation programmes. However, isometric training encourages breath holding which can lead to a rapid increase in blood pressure and it can also increase the risk of muscle strain.
isometric contractions

Muscle fibres

There are differences in the physiological make-up and type of fibre. There are two types: slow twitch and fast twitch. Each type of fibre is suited to different activities:

  1. Slow twitch fibres are reddish in colour because they have a good blood supply and are therefore oxygenated. They contract slowly, and exert little force. They can contract repeatedly and do not tire easily. They are more suited to endurance activities such as jogging, swimming and cycling, i.e. aerobic activities. They contain many mitochondria (where aerobic energy is produced).
  2. Fast twitch fibres are white in colour and do not have a good blood supply. These are generally larger than slow twitch fibres and contract quickly to produce fast and powerful contractions (up to five times faster than slow twitch). They tire very easily. Activities such as sprinting, jumping and throwing rely on fast twitch fibres, i.e. anaerobic activities, strength, speed and power events. They contain few mitochondria.

All muscles contain a mixture of both types of fibre, and most sporting activities often use both types of fibre at different stages. Slow and fast twitch fibres are distributed fairly equally throughout the body. The ratio of slow to fast twitch fibres is determined at birth, and cannot be changed. However, by training for specific sports it is possible to develop either type, developing their speed and force of contraction.

Paired muscle action

Muscles never work alone. Muscles can only contract, so in order to produce a range of movements at the joints, they must work as a group or team, with several muscles working at any one time. Muscles that cause joints to bend are called flexors, whilst the muscles that straighten a joint are called extensors. The muscle that produces the desired joint movement is called the prime mover or agonist. The muscle that relaxes in opposition to the agonist is known as the antagonist. The agonist and the antagonist must work together to produce the required movement. Muscles that are agonists for one movement, act as antagonists for the opposite movement.
agonist antagonist
When performing a movement, other muscles may help out and assist the prime mover. These are called synergists. For example, with the biceps curl above, two smaller muscles called the brachialis and the brachioradialis help out the biceps. Synergists also help to prevent any unwanted movements taking place at the joint being moved.

Muscles can also contract statically (isometrically) to fix parts of the body so that a stable position can be maintained throughout the movement. These muscles are known as fixators. They usually fix the point of origin (see below). With the biceps curl, deltoids (amongst others) act as fixators.

The point of attachment for each muscle is known as the origin and insertion. The origin is the end of the muscle which is attached to a stable bone through a tendon, against which the muscle can pull. This is usually the nearest flat bone. The point of origin does not move. The insertion of a muscle is where the tendon joins a bone which the muscle moves. The point of insertion moves when the muscle pulls on it. The muscle belly is the thick portion of the muscle found between the origin and insertion.
upper arm
joint action table 1
joint action table 2

Muscle tone

Muscle fibres are never completely relaxed. A little tension is always present, maintaining the shape of the body even though no movement takes place. This partial contraction of muscles is called muscle tone. Muscle tone is important for maintaining good body posture, placing the minimum amount of stress on the musculo-skeletal system.


Tendons are strong flexible cords or straps that connect muscles to bones. They transmit the pull of the muscle to the bone to create movement. They are strong, but inelastic – they do not stretch.

Short-term effects of exercise on muscles

  • Capillaries dilate
  • Increased blood flow to working muscles
  • More oxygen taken from blood by muscle
  • Muscles able to contract more often and more quickly
  • More muscle fibres contract in muscle
  • Temperature of muscle increases – increased pliability
  • Metabolism increases

Long-term adaptations of muscles

  • Individual muscle fibres get shorter and fatter, increasing thickness and strength of muscle
  • Blood supply to muscle is improved
  • Muscle reacts more quickly to stimuli
  • With correct training, fast twitch muscle fibres contract more powerfully, improving performance in maximum effort activities
  • Fatigue is delayed for a few seconds
  • With correct training and diet, muscle’s energy supplies become more efficient
  • Metabolism increases

Adaptations to strength training

  • Strength training usually leads to growth in muscle tissue (hypertrophy) – especially true for fast twitch muscle fibres.
  • Growth in response to strength training is made faster by male hormone testosterone – which may partly explain differences between men and women in terms of strength.
  • The efficiency of recruiting muscle fibres (i.e. the neuromuscular pathways) is improved, leading to a stronger muscle contraction – especially true in first few weeks of resistance training when rapid gains in strength can be observed.
  • A larger amount of fibres within the muscle can be through used at any one time while strength training.

Adaptations to muscular endurance training

  • Whereas strength training is mainly fuelled by the CP system (see energy systems), muscular endurance is fuelled to a larger extent by the lactic acid system and aerobic energy system.
  • An increase of capillaries in and around the muscle.
  • An increase in both size and number of mitochondria.
  • Improved supply of oxygen to the muscle and its ability to use this oxygen. This offsets fatigue resulting from build-up of lactic acid, improving muscular endurance.
  • Efficiency of muscle fibre recruitment is improved, benefits occurring in slow twitch muscle fibres.
  • Because hypertrophy of slow twitch muscle fibres is not as marked as that of fast twitch fibres, the muscle looks more toned rather than bulky.