In this chapter we will cover basic information about muscles. As you progress to the Become a purestretch Instructor online course you will have a better understanding of movement and muscles.


The Neuro Muscular System

The neuromuscular system includes all the 600 (plus) muscles in the body and the nerves serving them. Every movement the body makes requires communication between the brain and the muscles, even things we just take for granted like our heartbeat, the muscles in our digestive system… this is all down to the neuromuscular system!

It’s the Cardiac muscle that is responsible for the heart. This muscle is involuntary (it just works, there is no conscious control) which has a striped appearance and is controlled by the sino-atrial node (SAN), which is the pacemaker of the cardiac muscle, we will be covering this in more detail in chapter 4.

You also have Smooth muscle which is responsible for the digestive system. This also is involuntary which has a smooth spindle shape and is controlled by the autonomic nervous system.

And then you have the Skeletal muscle, and these are the muscles that create movement! They have a streaked (also known as striated) appearance and are controlled by the somatic nervous system, so these muscles are voluntary … (a muscle that you choose to move)

What are the functions of the Skeletal muscles?


Skeletal muscles have several key functions, including:

  • Maintaining posture and body position.
  • Producing movement.
  • Helping return venous blood from lower limbs to the right side of the heart
  • Storing nutrients
  • Maintaining body temperature.

So just think about the muscle having to shorten, lengthen and then returning to its original size and getting very excited about its ability to respond to stimuli from the nervous system.

Muscles work in opposition to each other to create movement. When a skeletal muscle receives a signal from the somatic nerve, it shortens, pulling one bone towards the other.  As one muscle in the pair contracts, the other muscle relaxes; the process is then reversed to straighten the bone joint. …we’ll go over this later in the purestretch course


Bursas allow structures to glide smoothly over each other, reducing friction during movement. On occasion, they may become inflamed and painful due to infection, arthritis or repetitive motion and ‘overuse’ of the joint, a condition known as bursitis.

A medical illustration of a knee with an inflamed prepatellar bursa

The structure of skeletal muscle


In very simple terms, skeletal muscles consist of tightly packed muscular bundles surrounded by connective tissue.

Skeletal muscle structure with anatomical inner layers outline d


The Epimysium protects the muscle from friction against other muscles and bones. It also continues at the end of the muscle forming the tendon (along with other connective tissues) which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.


Inside each skeletal muscle, muscle fibres are organized into bundles, called fascicles, surrounded by a middle layer of connective tissue called the perimysium.


The Endomysium surrounds each muscle fibre. It is another connective tissue that insulates each muscle fibre.


Beneath the Endomysium and surrounding the muscle fibre is the Sarcolemma. This is the muscle fibre’s cell membrane. Beneath this is the Sarcoplasm, a gelatinous fluid containing glycogen, fats, and mitochondria. Mitochondria is the cell’s powerhouse that produces energy.


Each muscle fibre contains hundreds to thousands of Myofibrils. The myofibrils are bundles of interconnected protein filaments of striated muscles. The filaments (myofilaments) included in the myofibrils are mainly thick filaments (myosin), and thin filaments (actin). The contractile functional unit of the myofibril is called the sarcomere.


Sarcomeres are the repeating units of myofibrils. Sarcomeres have two important protein filaments (actin and myosin) that slide past each other during relaxation and contraction. This is why the theory of muscle contraction is called the Sliding Filament theory.

So very simply the SLIDING FILAMENT THEORY is the process of muscle contraction involving the sliding of actin (a thin protein strand) & myosin (thick protein band) myofilaments past each other to shorten the length of each sarcomere.

Fast or Slow?

There are 2 muscle fibre types.

Fast twitch = Type 1
Slow twitch = Type 11

It’s pretty much what you’d expect by their titles.

Fast twitch muscles … these are called upon with rapid movement like strength training, and sprinting. They fatigue faster than slow twitch muscles and are used for high intensity exercise.

Whereas slow twitch muscles help with maintaining your posture, holding that position.  .. they like endurance-based exercises and don’t fatigue as easily as the fast twitch, this is also due to the fact they get increased oxygen delivery unlike the fast twitch muscles.  These muscles are used for lower intensity aerobic exercise.


The more intense exercise you do will increase the size of your fast twitch muscles and endurance exercises will develop the capacity of slow twitch fibres.

Heavy Squats, Deadlifts and Bench Presses are good ways to increase Type II fibres, as well as running sprints, agility drills and med ball training. Strength training and speed training can be very effective in developing maximum force.

Long-distance endurance training (running and cycling) and lifting lighter weights for high reps will develop slow-twitch muscle fibres.

So how do we move?

It’s straight forward really, firstly the muscles receive a message from the brain to shorten or use a force and as one muscle shortens its other half (they work in pairs) must lengthen.

For a muscle to shorten it needs to be attached to the bone. Origin is relatively the less movable end of the muscle/tendon that is attached to a bone. Insertion is the more flexible end of the muscle that is usually attached to a bone via tendons.

Types of muscle action

Remember that muscles work in pairs … think of something with a front and back … for the front to shorten the back needs to lengthen.

Concentric contraction occurs when the total length of the muscle shortens as tension is produced. For example, the upward phase of a biceps curl is a concentric contraction of the biceps.

Eccentric contraction occurs when the total length of the muscle increases as tension is produced. For example, the lowering phase of a biceps curl is an eccentric contraction of the biceps.

 Isometric contraction occurs when muscle length remains relatively constant as tension is produced. For example, during a biceps curl, holding the dumbbell in a constant/static position rather than actively raising or lowering it is an example of isometric contraction. Although the forces generated during isometric contractions are potentially greater than during concentric contractions, muscles are seldom injured during this type of contraction. Exercises are often used during the early phases of rehabilitating a musculotendinous injury because the intensity of contraction and the muscle length at which it contracts can be controlled.

The role of the muscle.

If a muscle contracts the muscle is then an Agonist … think Agony!

When that muscle contracts its other half needs to relax … Antagonist… think of it just sitting back and antagonising the working muscle, so in kicks the Synergist to help the prime movers (agonist) by adding a little extra force to the same movement or by reducing undesirable or unnecessary movements. Now whilst all this is going there will be a muscle who is holding it all together, the Fixator … the name is a bit of a giveaway… this muscle is stabilising whatever action/pose you need for the Agonist to perform.

Muscle teamwork!



A summary of muscle, attachment, and the action it creates when contracted is covered in the Concise Book of Muscles book. 


Helpful Tip:

Many muscles are named according to their various characteristics, such as: brevis (short), longus (long), maximus (large) and minimus (small). Their names may also indicate the direction of the muscle – for example, the transverse abdominus runs transversely and the rectus abdominis muscle runs vertically (‘rectus’ means straight) – and can also indicate the function; as an example, flexor pollicis longus signifies a muscle that flexes.

Knowing this can really help you remember muscle names and movement.

Describe each of the following muscle contractions.

Name the Antagonist of the following Prime Movers/Agonist