Length-Tension Relationship

Tension

When fibers in one fascicle contract, tension is created. All the fasciclesBundles of nerve fibers within a nerve.
or
Bundles of nerve fibers within a muscle. in an entire muscle, like the biceps brachiiFlexor / Supinator Front of upper arm; bends elbow and turns palm upward., contract. Together, they create tension.  Tension is the force that will create movementA fundamental property of life involving motion of the body or its parts., or prevent movement, in a muscle.  We are using skeletal muscle as our example. This allows us to talk about a muscle moving one bone closer to another.  Use your arm to pretend you are picking up an ice cream cone to your mouthThe opening of the digestive tract where food enters and mastication begins..  Your muscle creates tension. It produces enough tension to overcome the downward force of gravity acting on your ice cream cone.  By producing that amount of tension, the muscle can create movement, allow you to eat your lovely ice cream.  Mmmmmm.

In the smallest microscopic sense, tension results from all of those little cross bridges. These bridges are created between myosin and active. Remember that as a muscle contracts cross bridges are created and broken in cross bridgeThe connection formed between myosin heads and actin filaments. cycles.  But how many cross bridges does a muscle know to make and for how long? A motor neuron coming from the brain stimulates a skeletal muscle. It will continue to excite that skeletal muscle until the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission., the stimulus, is removed.  If there are many, subsequent action potentials, the muscle will continue to contract. 

Tetany is a condition where a muscle refuses to stop contracting.  The masseterMasseter Elevator Jaw closer Runs along the jaw; raises the mandible to close your mouth. muscle, which is a chewing muscle in your face is subjected to tetany and results in lockjaw.  The jaw is immovable due to the sustained contraction.  This usually results form certain types of bacteria that are associated with rusty metal and soiled food.  The tetanusIn this context, sustained muscle contractions due to calcium or electrolyte imbalances. shot you get every 5 years provides antibodies. You are supposed to get one every 5 years. These antibodies fight those types of bacteria.

Resting Length

The frequency of the action potential or stimulus determines how long tension is applied. The resting length of the sarcomere also makes a difference.  When we first moved into our house, we purchased a stackable washer and dryer.  My back was in good shape back then. I could help my husband lift it. A dryer is not as heavy as you think; it’s just big. I had to bend down all the way to get my fingers under it.  My knees were flexed as much as they could be and even my hips were slightly flexed.  I could get the initial movement to lift it.  With my muscle stretch out like that, I couldn’t create tension!  Yes!  In my case, the actin and myosin were too far apart to make cross bridges.  The zones of overlap or A bandsThe part of the sarcomere containing thick filaments. in my sarcomeres were too small. 

The opposite situation could prevent you from making contractions as well.  Flex your arm at the elbow as much as you can.  Even if you want to flex it more, you can’t.  There’s no room.  In this situation, your cross bridge are fully contracted.  The zones of overlap or A bands are as large as they possibly can be.  All the cross bridges that can be formed are formed.  The ability to create tension to move a load against gravity is not only the result of an action potential. It is also determined by the resting length of the muscle.  You should aim for your resting length to be between these two situations. This helps you pick up an ice cream cone or a dryer.

Of course, there are some things that affect that resting length.  If you stop stimulating your ice cream cone holding arm, gravity will act. It will make your arm fall back to a resting state.  Once it falls back, the elastic forces in your muscles come into play. The elastin in the epimysium and perimysium prevents your arm from falling off. Additionally, the elastin attached to the myosin in the sarcomeres helps in this process.  In my case with the dryer, I was also working again opposing muscle contractions.  I had to contract some muscles to bend down and needed the muscles opposing them to lift the dryer.  I just created a tug of war that went nowhere.

Length-Tension Relationship

What the heck is this graph?!?!?!  This is a graphical representation of all the things we just said.  This graph shows the relationship between sarcomere length and its ability to generate tension.  Along the bottom is the sarcomere length.  More is not better.  There is an optimum length that your muscles want to be at to produce the most tension.  That length is obviously different for muscles.  I mean the muscles in your calf have a larger optimum length than do the muscles in your eye. 

The side of this graph shows the muscle tension. This tension is produced when starting at any of those lengths along the bottom.  Let’s read this graph. Let’s start at that optimum there. This is the point on the graph where the most tension can be created.  That is this peak right here.  If we move away from this peak to the left, the sarcomere resting length decreases. The tension the muscle can produce also decreases.  If we move to the right on this graph away from the optimum length, the sarcomere length increases. Meanwhile, the tension decreases.  I don’t want to be on either side of this hill, I want to be on top of it.  The extremes of this graph represent the two situations with the dryer and the ice cream cone.  But are unable to produce tension, but for different reasons.