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As endurance athletes, we sometimes hear our muscles described as “slow twitch.” We’re certainly not going to take ownership over being slow! As Ricky Bobby would say, “I wanna go FAST,” and that is what we do, so what gives with this inaccurate description? Or are we just being self-conscious?
First, let’s clear up one fact that is often misinterpreted. All human beings have muscles. All human beings sometimes move slowly, and sometimes they move fast, or at least fast-ish. It should then come as no surprise that all human beings have BOTH slow twitch and fast twitch muscle fibers.
Ah, we have something else to clarify. Our individual muscles do not have just one type of muscle fibers to be classified as “fast” or “slow.” Rather, each individual muscle is composed of numerous muscle fibers that can be fast, slow, or somewhere in between. Muscles are composed of the individual pairs of contractile proteins, myosin and actin; a whole muscle fiber, which is a group of actin and myosin proteins, also known as a myofibril; a group of myofibrils is called the muscle fiber or muscle cell; a group of muscle fibers is called a fascicle; and finally, all of the fascicles together forms the whole muscle.
What makes a muscle “fast” or “slow?” I, for one, take issue with the name “slow twitch” – it should be called slow fatiguing. You see, fiber type has pretty much everything to do with how the muscle produces energy and very little with how fast it contracts, which is the way most people interpret the “twitch” part of the nomenclature. It is true that fast twitch cancontract faster, but it has more to do with force production than movement speed.
Slow twitch muscles produce energy slowly but in large quantities. Therefore, slow twitch type muscle fibers are the primary type of muscle fiber for endurance type exercise. However, that doesn’t mean that fast twitch fibers are not used at all. If, during a long run, you run a little faster for a stretch to make a pass or you have to run uphill and try to maintain your pace, fast twitch fibers take over, and you can definitely tell that this occurs due to lactic acid buildup.
Slow twitch fibers are easily able to completely metabolize glucose down to its final end product, carbon dioxide, which is expired by breathing. However, fast twitch fibers try to produce energy so fast that the entire energy production system cannot maintain the pace. About halfway through the energy production process, only 6% of the total energy yield of a glucose molecule is acquired, but this is about as far as fast twitch fibers can go and the pyruvate made from the original glucose molecule is converted to lactate and never makes it to CO2 – its dream transformation. This is enough energy for a short while, but as anyone who has tried to sprint a distance greater than 50-100 meters knows, the pace cannot be maintained, and slow twitch fibers take the brunt of the load again after a few minutes.
For more on energy systems, read or watch our video on how the body fuels endurance exercise. Fast twitch are type 2 fibers and primarily use the anaerobic glycolytic pathway, while type 1 slow twitch fibers primarily use the krebs cycle.
In an undergraduate exercise physiology course, my professor once said, “your parents or someone else you know has probably told you, at some time or another, that you can be whatever you want. Well, they were wrong.” He was a beacon of inspiration. The lecture that followed focused on muscle fiber type and the reason he crushed all of our dreams was to highlight that there is very little conversion between slow twitch and fast twitch muscle fibers. That means, if you are born with a ton of slow twitch fibers, there is not much hope for you as a sprinter or power lifter. Vice versa, if you’re chock full of fast twitch, marathons are run for participation purposes only!
Some interconversion of muscle fibers is possible, but not a dramatic shift from slow to fast or from fast to slow. There are multiple subtypes of type 2 fast twitch fibers, which range from “kinda sorta almost slow twitch fibers” all the way up to “I saw a mitochondrion once!” Now, let me explain that joke. You probably still won’t laugh, but it will at least make sense. Part of the reason slow twitch fibers are so effective for endurance type exercise is because they have a lot of mitochondria, but fast twitch fibers do not have quite as many. Mitochondria are where the krebs cycle occurs and the overproduction of lactate can be avoided, as lactate is produced when there is more pyruvate outside the mitochondria than the mitochondria can accept.
Okay, we know that slow twitch fibers have a lot of mitochondria and other enzymes involved in krebs cycle metabolism (how we get the most ATP [energy] for each molecule of fuel [usually glucose]) and this helps them resist fatigue. This fiber type is also typically small (although it has a large capacity for growth because of the mitochondria) and has a great blood supply. However, slow twitch fibers have small motor units, which along with their small size, makes them less than ideal for power or strength. This is where fast twitch fibers come in. They are large, and they are innervated by large motor units. Basically you can think of slow twitch, fast twitch, and somewhere in between as:
Here’s a nice summary table!
|Slow Twitch||Between||Fast Twitch|