TL;DR: Kinesiology is the study of human movement and it teaches us that there are two different types of muscle fibers: slow and fast-twitch as well as two different energy systems in the muscles: aerobic and anaerobic.
Intro
Some guys can put up 315 on the bench with seemingly no problem. Others struggle to put up the bar. Some can run a 10k without even breaking a sweat, whereas others are doubling over and gasping for air after running a mere 400m. How is it that some humans can perform drastically better than others in an athletic sense? What is going on internally in the body that allows them to do so much more? The field of kinesiology seeks to answer these questions.
This article will provide an overview of what Kinesiology is, as well as some helpful findings from it that can assist those looking to perform better athletically.
What is Kinesiology?
Kinesiology is the scientific study of human or non-human body movement. Kinesiology addresses physiological, biomechanical, and psychological dynamic principles and mechanisms of movement [1]. In short, anything to do with how we move falls under the umbrella of kinesiology.
The nine main objectives of kinesiology [2] are as follows:
- Relaxation
- Correcting the alignment and posture of the body (covered here)
- Increasing joints mobility
- Increasing muscle strength (covered here)
- Increasing muscular endurance
- Coordination, control, balance
- Exercise training
- Respiratory re-education
- Sensory re-education
Naturally, you will find many of these objectives covered in other body articles here on UpRiver. For the sake of relevance and reducing redundancy the scope of this article will focus on internal body mechanisms that affect athletic performance.
Slow vs Fast Twitch Muscle
Muscles in your body are made up of many individual muscle fibers. There are two main types of these fibers: slow-twitch and fast-twitch [3]. It is easy to keep the two straight as, in essence, slow-twitch muscles operate slowly, whereas fast-twitch muscles operate quickly.
Slow-Twitch (Type I)
Slow-twitch muscles are primarily used for endurance activities, such as long-distance running. They are smaller in size than fast twitch muscles. They have high concentrations of mitochondria and myoglobin and are surrounded by more capillaries compared to fast-twitch muscles. This means that they will fatigue much slower, making them suited to prolonged use.
Slow-twitch muscles produce less force than fast-twitch muscles and are slower to produce maximum muscle tension, however they are able to maintain contraction for much longer, making them very useful for maintaining posture and stability.
In summary, slow-twitch muscles fire slowly and produce low force, yet they fatigue slowly, making them ideal for prolonged use during endurance activities.
Fast-Twitch (Type II)
Fast-twitch muscles are primarily used for quick, explosive activities such as powerlifting, sprinting and jumping. They are larger in size than slow-twitch muscles, yet they have lower concentrations of mitochondria, myoglobin and capillaries, leading them to fatigue much faster, making them only suitable for short-term usage.
Fast-twitch muscles produce more force than their slow-twitch counterparts and are much quicker to produce maximum tension.
There are two different types of Fast-twitch muscles: Type IIX and Type IIA. Type IIX muscles are pure fast-twitch muscles, generating the most amount of force, but are also by far the most inefficient, leading to very quick fatigue. Type IIX muscles, often called intermediate muscles, are made up of a combination of both type I (slow-twitch) and type IIX (fast-twitch). Although they generate less force than type IIX muscles, they have comparable tension while fatiguing slower.
In summary, fast-twitch muscles fire quickly and produce high amounts of force, yet they fatigue quickly, making them ideal for quick, explosive activities.
Comparison
The National Academy of Sports Medicine [3] provides a handy chart comparing the different muscle fibers. It has been pasted below for ease of reference:
Body Composition
All muscles in the body are made up of a combination of both slow and fast-twitch muscles. Training and age will affect the composition of your muscles.
An average, untrained person will have a pretty even split between the two types, with muscles being made up of about 50% slow-twitch muscle and 50% fast-twitch muscle. A competitive sprinter would have about 70-75% fast-twitch muscles, whereas a marathon runner would have about 70-80% slow-twitch muscles.
Having more fast-twitch muscle fibers will make you stronger, yet have less endurance, whereas having more slow-twitch muscle fibers will make you have more endurance, yet weaker.
Slow-twitch muscles can be trained with low resistance, high repetition training or low intensity, long duration activities, such as long distance running, walking, biking or swimming.
Fast-twitch muscles can be trained with strength training. For more information on strength training check out our article here.
As you age you generally begin to lose lean muscle mass. This causes a reduction in fast-twitch muscle fibers. This makes resistance training an essential practice as one gets older, to help reduce the natural loss with aging.
Aerobic vs Anaerobic Exercise
There are two different energy systems that your body can use to exercise: Aerobic and Anaerobic.
Aerobic Exercise
Aerobic means “with oxygen”. You probably know aerobic exercise as the dreaded cardio. Aerobic exercise is low intensity, long duration exercise such as walking, jogging, running, biking or swimming. For the most part, slow-twitch muscles are primarily used for aerobic exercise.
During aerobic exercise, oxygenated blood is pumped to your muscles. The oxygen is used by your muscles to break down amino acids, carbs and fats into adenosine triphosphate (ATP), the fuel source needed to move your body [4].
Aerobic exercise has been shown to play a role in preventing and reversing cardiovascular disease, one of the leading causes of death in the developed world.
Anaerobic Exercise
Similarly to aerobic, anaerobic means “without oxygen”. Anaerobic exercise is high intensity, bursty exercise such as sprinting, jumping, weightlifting or high intensity interval training (HIIT). Most anaerobic exercise will utilize your fast-twitch muscles.
During anaerobic exercise your muscles break down glucose, either from your blood or from stores in your muscles, without the use of oxygen, to generate the ATP required to move your body[4]. This process produces less ATP than aerobic exercise and leads to the build-up of lactic acid. Eventually this build up of lactic acid will reach a point known as the lactic threshold, causing muscle fatigue, forcing you to stop your exercise [5]. Most people can only sustain Anaerobic exercise for a maximum of 2 minutes, before fatigue forces them to stop.
Anaerobic exercise is great for increasing muscle mass and improving bone density. It has also been shown to be quite effective at aiding in fat loss as well, especially when used in conjunction with aerobic exercise [4]
Conclusion
Kinesiology is the study of human movement and thus it is a useful starting point when looking into the body and exercise. When starting to exercise, it is useful to know the differences between slow and fast twitch muscles as well as the differences between the aerobic and anaerobic energy systems as it gives us a better understanding of what is actually going on inside our muscles when we perform an exercise. From this understanding you will be able to make better and more educated choices when planning your exercise routine.
References:
[1] https://en.wikipedia.org/wiki/Kinesiology
[2] https://www.mastersportal.com/articles/2734/what-is-kinesiology-and-why-study-a-kinesiology-degree-in-2021.html#:~:text=Kinesiology%20studies%20the%20mechanics%20of,the%20physical%20mobility%20of%20patients.
[3] https://blog.nasm.org/fitness/fast-twitch-vs-slow-twitch
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5329739/
[5] https://trufitathleticclubs.com/texas/blog/aerobic-exercise-anaerobic-exercise-and-target-heart-rates/
