A Muscle Contracts How?

Picture this – you’re at the gym, watching yourself in the mirror as you complete every single rep.  Only to make sure your form is on point of course!  Or maybe you’re just admiring how taught your quads look in those flashy new LuLu’s.  Whatever the case – eyes are on the mirror, and what else do you see?  Your muscles hard at work!  Rep after rep, you start to feel the burn.  But do you ever wonder just how that process happens; I mean, how your muscles contract?

Disclaimer: I’m about to get real sciencey on you.

There are three main types of muscle tissue – cardiac muscle, smooth muscle and skeletal muscle.  We’ll focus on the latter which attaches to our skeleton via connective tissue called tendons and through the process of contraction our bones begin to move.

What exactly is a contraction, you ask?  

No, I’m not referring to the 14 hours of labor your poor mother had to endure to bring you into this beautiful world.  There are two main types, concentric and eccentric, both of which are isotonic contractions; meaning tension stays the same whether the muscle is shortened or lengthened.  Let me explain further.

Concentric refers to the shortening phase of a muscle when stimulated by tension.  When completing a dumbbell curl, flexion occurs at the elbow joint, shortening the biceps brachii.  Eccentric refers to the lengthening of a muscle against that same tension, in this case it would be the return of the Dumbbell to its starting position.


This eccentric motion is sometimes referred to as “negative work,” as it may actually allow for greater force production and less muscle fiber recruitment; therefore, creating greater microscopic tears (this is a good thing) and is usually experienced by that feeling of muscle soreness.  You know that feeling, after a Monday session with your Squat Squad, stairs and toilet seats become dreaded nouns.

Now for some microanatomy!

Skeletal muscle fibers are made up of light and dark bands creating a striped appearance.  Each of these muscle fibers can contain several thousand myofibrils, which are protein filaments that run parallel to each other throughout the length of a cell.  The dark bands (A bands) contain a protein filament called myosin, whereas the light bands (I bands) contain a protein filament called actin.  The A bands (myosin filament) have tiny projections called cross-bridges that extend out at an angle reaching for the I bands (actin filaments), and an additional Z line divides each myofibril into a series of segments called sarcomeres, which act as the contracting units of skeletal muscle.

Yay!  We’re almost there!


Okay – this is where the magic happens!

The Sliding Filament Model – it even sounds like a really cool trick, doesn’t it?!  For a fiber to contract, it requires energy or ATP (adenosine triphosphate) which in simplest terms, is synthesized from the food we consume.  With sufficient levels of ATP, the brain and spinal cord (through means of the Central Nervous System) communicate to the motor neurons to release a neurotransmitter called acetylcholine.  When the neuromuscular junction detects acetylcholine, calcium is then released to the area surrounding the fiber.


Calcium then uncovers binding sites along the I bands of the sarcomeres for the A bands to attach.  The myosin filaments then bind with the calcium receptor sites on the actin filaments and cross bridges are formed by myosin pulling the actin toward the centre, and therefore shortening the sarcomere.  If multiple muscle fibers are voluntarily stimulated at the same time, the entire muscle will contract, shortening the overall length of the muscle.  Voila!  A concentric contraction!

There you have it folks!  Now you know what a contraction is, and exactly how it occurs.  Thanks for hangin’ in there.  I’m sure you felt like throwing spitballs at me half way through, but you did it.

So the next time you’re at the gym, admiring your voluptuous badonkadonk in the mirror, take a minute to really think about all of the amazing action that’s going on deep beneath your epidermis.  The work of ATP, your CNS, the A bands with the I bands and the Z bands in between.  Your body truly is an amazing machine.robot