Learning goals

Learn how muscles contract and relax

Discover the relationship between ligaments, tendons and muscles


* muscles tendons ligaments

* actin, myosin protein filaments, crossbridge


What stimulates muscles to contract?

Muscles are a soft tissue that contract when they receive signals from a nerve. Muscles produce force and motion. But what makes them contract? They receive signals from a nerve.

Nerve cell axon stimulating muscle


Work together with tendons and muscles

A tendon is a fibrous connective tissue which attaches muscle to bone.

This allows the muscle to actually move the bone.


A ligament is a fibrous connective tissue which attaches bone to bone.

They slightly stretch. Their job is to limit how far a joint can move. This helps protect against injury. They hold structures together.

Knee ligament

By Dr. Bertram Zarins of the Mass General Hospital Sports Medicine Service


Muscles work in pairs

They can only contract; they can’t relax

So how we can pull back an arm, once it is extended?

Contract the muscle on the opposite side of the bone!

Biceps Triceps muscles arm

from backyardbrains.com, flexion and extension

Here we see the biceps contract, pulling the forearm up.

Then we see the triceps contract, pulling the forearm back down.

flex muscle extension biceps triceps

from backyardbrains.com, flexion and extension

How do joints work?

When muscles pull bones, they rotate. A group of bones that rotate around each other are basically a simple machine.  The joints in our body are similar to joints we use when building things.

Types of joints muscles machines

See animations of the six types of joints.

Here we see the ankle joint in motion.

foot ankle animation GIF

Dr. Noah Weiss and Cameron Drake http://blog.drake.fm/blog/gifs/


Inside your muscle, how are muscle fibers organized?

Skeletal muscle

muscle fibers bundes

Single, long muscle fiber (one long cell)

These long cells contain proteins, repeated in units called sarcomeres.

As seen below, sarcomere are the basic functional unit.

Muscle fiber

GIF made by SSACC and hosted on imgur.com

Sarcomeres are the machine that make muscle cells contract

Let’s look at muscle under a microscope. We see the sarcomere – the basic unit of muscle tissue

It is made of alternating thin lines that can contract and expand.

We discovered that these thin lines were long, thin strands of protein, working together like a machine.

Sarcomere is the basic unit of muscle tissue

Sarcomere by Sameerb on commons.wikimedia.org

Sarcomere, by Sameerb on commons.wikimedia.org

Here we see the thin and thick lines contracting, and being pulled apart.

Sarcomere actin myosin muscle

Using X-ray microscopy, scientists eventually figured out exactly what these protein fibers look like up close.

We see a long branch from one protein reach out and grab on to a neighboring protein strand. Once it grabs, it “walks” across, pulling them.

This process requires energy. We get energy from ATP molecules for energy. How does our body create ATP? Read about cellular respiration.

* actin, myosin protein filaments, crossbridge

myosin action ATP muscles

Image from Mohammad Attari and Hossein Khadivi Heris at the McGill Univ Bio Active Materials (BAM) Lab

Sample questions

During exercise, a person’s muscles need a constant supply of ATP. To meet this
need, the rate of which of the following processes increases?
A. cellular respiration
B. mitosis
C. protein synthesis
D. transcription

This infographic is by Eleanor Lutz

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