How muscle contraction works and why you fatigue

November 1, 2016

by Jens Hevike

0 Comments

You might be a person that really wants to dig deep into how things work. For me, this is not only true of sports, but of everything else, too. Knowing how a muscle contraction works on a microscopic level won’t get you in shape, but it will give you a better understanding of individual effects of your workout – for example, how muscles grow and why, how and why muscles tire, and what limits they are bound by. There are three types of muscle in the human body: cardiac, smooth and skeletal. Cardiac muscle fibers are, as the name suggests, only found in the heart. Smooth muscles work as a mean of transport in organs, for example, to move fluids through the gut. Skeletal muscles connect our bones and allow us to move our bodies. The first two types are involuntary, meaning they are regulated by the autonomic nervous system and not by the conscious mind.

illu_muscle_structure — These different types of muscle work in a very similar way. For the purpose of demonstration, I will focus on skeletal muscles, which work voluntarily or consciously. Most of this type of muscle is attached to bones through tendons, hence the name skeletal muscle. Exceptions are our tongue and the muscles in the eye.

Muscle contraction

skeletal_muscle_cutout — Contractile proteins – actin and myosin – are responsible for the movement in all three types of muscle. Myosin slides along actin in a grabbing motion. This mechanism shortens the sarcomere, a segment of muscle fiber. A shortening of sarcomeres attached in sequence makes the whole muscle contract. Imagine a series of syringes glued end-to-end and then the plunger of each syringe pushing into the barrel.Lots of these “syringes” of sarcomeres bundled together make up the muscle fiber, and lots of muscle fiber packed together form a whole muscle. The strength comes from the vast amounts of bundles of muscle fibers pulling on two bones, arranged to amplify forces, like with a lever. Since muscles can only pull and not push, two or more muscles have to work in teams to move a limb in opposite directions.

Regulation of muscle contraction

actin_myosin role in muscle contraction — The proteins tropomyosin and troponin are the central players in the regulation of contraction. Tropomyosin, a kind of gatekeeper, blocks the actin binding areas (ring) where myosin (finger) binds to the actin. Only when calcium activates troponin, those “rings” are revealed and the “fingers” can grab and pull on them, sliding the two segments towards each other, thus generating force.

types of muscle contraction — The sliding movement, explained above, is called a concentric contraction. When the muscle is held in position it is called isometric contraction, and when a weight is slowly lowered it is called eccentric contraction.All this movement uses energy in the form of adenosine tri-phosphate (ATP).

ATP – Readily available energy

The body has three ways to generate ATP. First is a conversion of adenosine di-phosphate (ADP) through a creatine phosphate reaction which is quick to deliver energy but is a very limited resource. Second, anaerobic metabolism uses up the glucose as well as the vast stores of muscle glycogen (sugars) in the muscle to generate energy. And lastly, the aerobic metabolism uses glycogen, blood glucose, or fatty acids from the blood, liver and fat tissue in an oxidative process (oxygen consuming) to generate energy. The first is the fastest way to produce energy that can be used for quick short bursts, like a quick sprint, but will be depleted very quickly. The third is the slowest in terms of energy generation but lasts the longest and is suitable for endurance exercises, like a marathon.

The effects

The oxidative process takes a while to deliver energy. To avoid an energy low during the workout, a quick way to fuel the muscles is to supply the body with more readily available glucose (sugar) to the bloodstream that the anaerobic process can convert to ATP. Eat a banana or some dextrose before or during the workout to counteract the energy low. If there is not enough oxygen during strenuous muscle use, lactic acid builds up, making the muscle feel tense and tired. This creates a burning sensation that ceases after a period of rest and catching your breath. The high consumption of energy in this aerobic process is also the reason that we get out of breath. Taking deep breaths will help you catch your breath quicker. Gasping barely reaches the lungs and only fills up the larynx, which is the tube reaching down to the lungs. There is little evidence that lactic acid causes the muscle soreness that we feel even days after a workout. Muscle soreness is not well understood, but might be a result of microdamage in the muscle fibers themselves. Let me know what other muscle or health-related topics would interest you.