Edited by: Danielle Abel

Understanding Muscle Structure

If you're studying for the National Strength and Conditioning (NSCA) Certified Strength and Conditioning Specialist (CSCS) exam, you will need to understand muscle structure and how muscles work, or function. In this article, we're going to cover what you need to know to pass the CSCS exam.

If you'd like to hear this information broken down in video format, you can watch it below too. 

From a muscle layers, or connective tissue perspective, there are 3 key areas to know when it comes to overall muscle structure, the epimysium, the perimysium, and the endomysium. 

• Epimysium is the biggest structural layer of connective tissue surrounding the whole muscle.
• Perimysium is the intermediate structural layer of connective tissue surrounding a fascicle of muscle fibers.
• Endomysium is the smallest structural layer of connective tissue surrounding one individual muscle fiber.

Muscle Fiber Groupings

Muscle fibers are organized into groups called fascicles. Each fascicle acts together as one motor unit, meaning the whole group of fibers is controlled by a single nerve.  

Photo credit: Haff, G., & Triplett, N. T. (2016). Essentials of strength training and conditioning. Fourth edition. Champaign, IL, Human Kinetics.

All or Nothing Principle

The all or nothing principle says that a nerve either creates a full action potential or no action potential. There is no such thing as a "strong" or "weak" action potential. If the threshold voltage is reached, an impulse will be sent down the nerve to the group of fibers it innervates. A stronger stimulus doesn't provide a larger impulse, but rather can increase contraction strength by increasing the rate at which neurons fire (rate coding) or by activating more neurons and therefore more muscle fibers.

Now, importantly the all or nothing principle is different than the size principle. The size principle states that as more force is needed larger motor units will be recruited according to their force output. Therefore type 1 motor units will fire before type 2 motor units.

Muscle Contraction in 7 Steps

This is where we'll dive into actual muscle function now that you have a better understanding of muscle structure. 

 #1 Create an action potential.

 The brain sends a signal down to the muscle via the spinal cord and nerves. The signal is received at the neuromuscular junction, a space where the end of the nerve comes in contact with the muscle fascicle.

#2 Propagating the neuromuscular signal 

The neurotransmitter acetylcholine (ACH) is the messenger helping the nerve signal cross the neuromuscular junction and transmit the signal to the muscle. 

#3 Crossing the neuromuscular junction

The acetylcholine (ACH) crosses the neuromuscular junction (a small gap between the end of the nerve and the fascicle) and activates the sarcolemma. 

The sarcolemma is a membrane around the group of muscle fibers that help send the signal to the whole fascicle, as pictured below. 

Photo credit: Haff, G., & Triplett, N. T. (2016). Essentials of strength training and conditioning. Fourth edition. Champaign, IL, Human Kinetics.

#4 Calcium release

Calcium is released from the sarcoplasmic reticulum (as pictured above) and helps distribute the signal to activate the other muscle fibers inside the fascicle. 

Calcium is what regulates muscle contraction, but after the muscle has been activated, calcium sequestering proteins re-uptake the calcium and bring it back to the sarcoplasmic reticulum. 

#5 Calcium binding to troponin on actin filaments

In order for the fascicle to contract, calcium needs to bind to troponin on the actin filaments. 

#6 Troponin moves tropomyosin to expose actin binding sites

After calcium is bound to troponin, the troponin then attaches to the tropomyosin, a rope-like structure, to move it away so that myosin heads can bind to the actin. 

#7 Forming a crossbridge

The last step is when actin and myosin overlap, forming a crossbridge, which then allows the muscle contraction to occur. This process is referred to as the sliding filament theory. As the overlap occurs, the muscle contracts or shortens and brings that muscle closer together. As it relaxes, it lengthens and pulls the muscle farther apart. 

It might be helpful for you to go back and reread the above steps to understand this process well. You could even jot these steps down on some note cards and practice putting them together in the correct order. Or write them all down and have a friend quiz you on the steps.  You will most definitely need to know this for the CSCS exam as well as the sliding filament theory, so it would be a good idea to spend some extra time on these concepts. 

CSCS Practice Questions

#1 Which fascial layer surrounds 1 individual muscle fiber?

A) Epimysium

B) Perimysium

C) Endomysium

#2 Which contraction type would increase the length or the H Zone?

A) Concentric contraction

B) Eccentric contraction

C) Neither

#3 When released from the sarcoplasmic reticulum, calcium will bind to which of the following?

A) Tropomyosin

B) Actin

C) Troponin

Practice Questions Answers

#1 Which fascial layer surrounds 1 individual muscle fiber?

A) Epimysium

B) Perimysium

C) Endomysium

The answer here is endomysium because it is the layer of tissue that covers only 1 individual muscle fiber.

#2 Which contraction type would increase the length or the H Zone?

A) Concentric contraction

B) Eccentric contraction

C) Neither

The answer here is eccentric contraction because the H-Zone is the area on the sarcomere that only contains myosin (no actin). The H-Zone lengthens when the muscle lengthens eccentrically, and it shortens when the muscle is contracted concentrically. 

#3 When released from the sarcoplasmic reticulum, calcium will bind to which of the following?

A) Tropomyosin

B) Actin

C) Troponin

 The answer here is troponin because calcium binds directly with troponin, which then binds with the rope-like structure, tropomyosin, wrapped around the actin filament. The calcium does not bind directly with the actin; only myosin does this. 

Support & Courses Available

Ready for more support to help you prep for the CSCS exam? Join our Facebook Group, “Strength and Conditioning Study Group,” here. Ready for even more? Our 24-module CSCS Prep Course has muscle structure & function completely laid out for you with even more content than what we’ve provided here, plus chapter quizzes to help you pass the NSCA exam; click the link here to check it out.