Oct 2014 11

All about muscle contraction

By: Dave Wheeler

Muscles make us move, but how?

Muscles connect to bones (often via tendons). Muscle contraction is when the muscle fibres shorten, causing a pull on the joint, meaning movement.

Think of someone flexing their bicep: the muscles of the biceps contract, pulling on the lower arm causing it to pivot up at the elbow.



pic by stockimages @ freedigitalphotos.net


But actually, there's so much more to muscle contraction than a bicep curl.


Concentric isotonic contraction

When a muscle shortens and pulls on a limb it's known as concentric isotonic contraction.  

The word isotonic comes from a combination of two words:

  • iso meaning "same"
  • tonic meaning "tension or force"

The idea is that tension progresses in the muscle and then remains constant while the muscle changes in length. So, isotonic contraction causes movement.

The word concentric means moving towards the centre - both ends of the muscle move towards the centre, in other words the muscle shortens.

In the example of the bicep curl, the muscle develops enough force to overcome the weight of the dumbell; once enough tension has been acheived, the muscle shortens towards the middle, causing the arm to move and the bicep to curl.


Eccentric isotonic contraction

Still on the subject of the bicep curl, let's think about lowering the weight back down.

As the dumbell is being lowered, the muscle of bicep will be lengthening as it supports the weight going down.

This is called eccentric contraction.

Eccentric means "away from the centre".

Eccentric contraction still isotonic - the amount of force remains the same as the weight is lowered during the bicep curl, but the muscle lengthens out.


Isometric contraction

Now think of a skiier who maintains the same pose going downhill - a partial squat. 

The legs of the skiier remain bent at the same angle, the muscles neither shortening nor lengthening, but the muscles at the front of the thigh (the quadriceps) are still contracting to maintain the pose.

As any skiier will tell you, this type of contraction can still cause the thigh muscles to ache afterwards.

This tyird type of contraction is called isometric.

Isometric comes from the combination of:

  • iso meaning "the same"
  • metric meaning "length"

So isometric contraction is where the muscle contracts, but remains at the same length (which of course means that the angle of the joint remains the same, too).


Strength training and muscle contraction

For undamaged muscles, strength training usually starts with an emphasis on concentric contraction.

Once a degree of muscle tone has been achieved, controlled eccentric contraction can be used rapidly to build tone. Eccentric contraction is the most powerful type of muscle contraction.

The fastest way to build muscle is to focus on eccentric contraction

But it's also the type of contraction that can causes the most damage to muscles: because the muscle is contracting whilst the fibres lengthen, there's a considerable shearing force applied to them.

This can easily lead to muscle tears. 

So eccentric contraction is great for building muscle, but it needs to be done with care, and certainly not overdone.


Rehabilitation and muscle contraction

After an injury the best type of exercises are those that use short duration isometric contractions

Isometric exercises avoid both the direct weight bearing force of concentric contraction and the additional excessive force of shearing of eccentric contractions.

But because the length of the muscles don't change, the pressure inside the muscle bundles (called the intramuscular pressure) increases. If the isometric contraction continues for a period of time, this build up of pressure can compress the blood vessels, meaning less oxygen flowing into the muscle fibres.

So whilst isometric contraction is best for rehabilitation after injury, to start with it should be for short bursts of 10-15 seconds at a time. As recovery proceeds, this length of time can be increased.

Isotonic concentric and then isotonic eccentric exercises can then be added to the rehab regime to gain strength once the muscle has started to recover properly.


If you find yourself with sore muscles after exercise, or need help recovering from an injury, give me a call.