Do You Have More Slow Or Fast Twitch Muscle Fibers?

Picture from Bodyworksprime

I always knew I had some decent fast twitch muscle fibers in me. How do I know you may ask? I could always sprint at the end of a race. That's how I won most of the track races I took part in while competing in school competitions. My strategy was always to sit and wait off the leader's shoulder until the last 80-100 m and outkick whoever was there.

So I was very surprised when a newly published paper (Lievens et al, 2024) suggested that only 50 percent of elite coaches and sports scientists (out of > 400) actually know whether their athletes have slow-twitch or fast-twitch muscle fibers. Yet 90 percent of them believe that knowing the dominant muscle fiber type of an athlete is useful for optimizing training and performance.

Humans on average have a roughly 50-50 mix of slow and fast twitch fibers. There can be a huge variation around this average as some people can be mostly slow-twitch while others can be mostly fast-twitch. I have written in more detail about the 3 different types of muscle fibers we have. 

Here is a quick recap. Type I or slow twitch muscle fibers which are the smallest and produce the least amount of forces, but once trained can go all day long without fatiguing. They also help to hold and stabilize our posture.

Type II or fast twitch muscle fibers which are further divided into Type IIa and Type IIx (also known as IIb). These are larger muscle fibers and produce a greater and quicker force (than type I), but have less mitochondria, myoglobin and capillaries (compared to Type I) and are prone to fatigue quicker.

Type IIa (also known as intermediate muscle fibers) is a mixture of Type I and IIx fibers. They use both aerobic and anaerobic energy systems and fatigue slower than Type IIx. Proper training of Type IIa fibers will increase their ability to utilize aerobic energy, translating to greater endurance.

Type IIx are the largest muscle fibers and produces the most forces but are inefficient and fatigue quickly as it has low oxidative capacity and relies on anaerobic energy.

If you have read this far, you may already know more about muscle fibers than 50 percent of the coaches in the article above.

Studies have shown that those with more fast-twitch muscles have a faster last lap sprint in the 1500 m while those with more slow-twitch fibers generally do better in races when they are more evenly paced.

Another study (Swinnen et al, 2024) showed that those with slow-twitch fibers tend to have better running economy (use less energy at a given pace) since slow-twitch fibers uses less energy than fast-twitch fibers. 

The gold standard to know your fiber type is a muscle biopsy which means taking a small piece of your muscle for analysis. It is a really painful process plus who wants to give away hard earned muscle?

That is probably why the coaches surveyed in the Lievens et al (2024) study said they do not know their athletes' fiber type, especially if they coach middle distance, team sport athletes or recreational athletes who were not active previously. 

Coaches in the Lievens (2024) study relied heavily on how high you can jump (or how long you stay in the air). It is a decent proxy for fast twitch muscle fiber testing. Men with a vertical jump in excess of 50 cm (20 inches) and women 35 cm (14 inches) have predominantly more fast-twitch fibers.

Another study found that fast-twitchers can usually do 5-8 reps at a given load (80 max in the study), while slow-twitchers did 11-15 reps. About a third in the study group did 9-10 reps and they had an even mix of fast and slow twitch fibers. Those with more fast-twitch fibers tend to be stronger but note that they fatigue more quickly too.

This has implications for strength training since the 1-rep max calculations used to find how many reps to do at a lower weight will be different for fast versus slow twitch fiber types. Slow twitchers need to do more reps to get a comparable stimulus.

Perhaps a good estimate is how you fare over long versus short distances. Note that it will be affected by factors like how you have trained. This is definitely worth considering when you are deciding what races to compete in or how many reps to do while lifting. 

What muscle fibers you have is not destiny and does not have to determine your athletic goals. It may tell you the best way to get there.

Reference

Lievens E, Van de Casteele F, De Block F et al (02024). Estimating Muscle Fiber-Type Composition In Elite Athletes: A Survey On Current Practices And Perceived Merit. Int J Sp Physiol Perform. 19(11): 1197-1208. DOI: 10.1123/ijspp.2024-0043

Is High Mileage Necessary To Run Fast At Your Races?

Maroochydore, Queensland- great for long runs

My colleagues and I were discussing just how much mileage a runner can safely increase before running the risk of injury. It does seem like mileage is a tricky topic for many runners. Often many runners would ask other runners how far they are running each week. 

Most runners track it. Some believe it holds all the keys to being successful in running (more is good, less is bad).  

My first running coach in secondary school taught my teammates and I that consistency in running and intervals were necessary for fast race times (we did cross country - 4.8 km and track and field - 3000m longest event). My junior college running coach emphasized quality over quantity (no junk miles). So why do some coaches and elite runners swear by high mileage?

Running an easy kilometer is totally differerent from doing 5 x 200 m at 30 secs a rep (that's a 2:30 min  kilometer pace). Recovering from the 5 x 200 m will be different compared to running an easy kilometer at say 6:00 min per kilometer.

Perhaps we should consider training what muscle fibers we have in our bodies before deciding on mileage. We all have 3 main types of muscle fibers. Type I or slow twitch muscle fibers which are the smallest and produce the least amount of forces, but once trained can go all day long without fatiguing. They also help to hold and stabilize our posture. They are fueled by aerobic energy. 

Picture by Kurt Rawlins from Pinterest

Type II or fast twitch muscle fibers which are further divided into Type IIa and Type IIx (also known as IIb). These are larger muscle fibers and produce a greater and quicker force (than type I), but have less mitochondria, myoglobin and capillaries (compared to Type I) and are prone to fatigue quicker.

Type IIa (also known as intermediate muscle fibers) is a mixture of Type I and IIx fibers. They use both aerobic and anaerobic energy systems and fatigue slower than Type IIx. Proper training of Type IIa fibers will increase their ability to utilize aerobic energy, translating to greater endurance.

Type IIx are the largest muscle fibers and produces the most forces but are inefficient and fatigues quickly as it has low oxidative capacity and relies on anaerobic energy.

If you only do easy long slow runs, you utilize your slow twitch (Type I) fibers. If you run harder, your intermediate Type IIa fibers kick in to help. If you are required to do a hard 400m sprint or longer then all 3 types of fibers are used.

All the muscle fibers are fueled by 3 energy systems, one aerobic and 2 anaerobic. The Phosphagen energy system (or ATP-CP system) uses creatine phosphate to provide energy that can only last 10-15 seconds at high speed. This is what is used when you are sprinting all out.

The next anaerobic (glycolytic) system uses carbohydrates to provide another short source but fast acting energy. Medium to high intensity runs utilize this system. 

Lastly, the aerobic energy system which takes 30-40 seconds to start (since it takes time to deliver increased oxygen to your muscle fibers) and uses carbohydrates and fats to deliver long term, high volume energy for your muscles.

So, if you're training for a good timing in a 10 km race, that is at least 90 percent fueled by the aerobic system. However a fast 10 km will require help from your intermediate and fast twitch fibers. It will NOT be enough to do just low intensity, long, slow runs which targets your slow twitch fibers. You need to train your intermediate fibers as well with tempo runs (race pace you can hold), 1km intervals with short recovery, hill reps of 300-600 metres etc.

Runnning definitely damages the muscle fibers used in training and racing. The faster you run, the more fibers damaged. Your slow twitch muscle fibers starts to run out of fuel and fatigue after 60-90 minutes and you will be using intermediate fibers to compensate. That's why you recover easily when you do easy slow runs while faster and longer efforts require you to rest 2-3 days before your next demanding session.

Your bone and tendons require more time to adapt compared to muscles so do not ramp up volume and speed too quickly otherwise you could sustain a stress fracture.

As for your energy systems, your Phosphagen systems recuperates in about 3 minutes. Your aerobic system needs about a day to replenish its muscle glycogen stores. The glycolytic system requires at least 2-3 days to recover from a tough anaerobic workout.

If you are training for a marathon, you obviously have to run higher mileage compared to training for a 3000m track race or a 5 km run. 

Understand the demands of your race. Plan and schedule workouts to meet those demands. Then you can add up your mileage. So mileage is not a training plan. It's simply counting the total distance you ran for the week.

Moreover our fitness, age, physical make up and experience makes us unique or n = 1. Just because someone else in your running club is running 100 km a week does not mean you should. 

Ultimately, the mileage you're able to chalk up is limited by the recovery needed between your workouts, not so much by how far or fast you want to run.

Wouldn't it be better to identify the workouts that target the muscle fibers and energy systems required to meet your race goals than to focus on your mileage?

References

Baker JR, McCormick MC and Robergs RA (2010). Interaction Among Skeletal Muscle Metabolic Energy Systems During Intense Exercise. J Nutri Metab. DOI: 10.1155/2020/905612.

McArdleWD, Katch FI and Katch VL (2000). Essentials Of Exercise Physiology. 2nd Edition. Baltimore: LippincottWilliams and Williams.

Can Your Calf Muscle Cause Your Knee To Hurt?

Now that's some very defined soleus muscles

Really? You must be wondering how and why after looking at the title of this week's post. Well, my last patient today had knee pain caused by her soleus (or calf) muscle. She had recently been doing a lot of step ups in her gym classes and her knee pain started soon after.

Runners' knee or patellofemoral joint pain (pain under the kneecap) is very common in runners. I've written before about how this may be due to heel striking, heavy landing and hip dysfunction.

There is also some evidence where the length of the soleus muscle can influence patellofemoral joint pain (PFP). It has been suggested that in runners with PFP, there is a greater activation in the muscle compared to runners without knee pain (Piva et al, 2005).

See soleus after you cut away gastrocnemius

Your calf muscles consist of the more superficial gastrocnemius muscles and the deeper soleus muscle. If you peel off the gastrocnemius muscles, the soleus muscle lies underneath. Together they end as the Achilles tendon finishing at the heel bone (picture above).

The soleus muscle is largely thought to help with our posture as it is mostly made up of Type I slow twitch muscle fibres. The gastrocnemius muscle is made up of mainly Type II fast twitch fibres.

The fast twitch muscles of the gastrocnemius allows you to sprint. However, the gastrocnemius muscles tire easily.

The slow twitch soleus muscle is very important for your walking and running. Since they're more fatigue resistant, you use them a lot chalking up mileage whenever you run.

One of the main functions of the calf muscles is to absorb shock. If they're overused, they can't absorb shock well, your knee takes more of the load and you get knee pain.

A very simple way to take load off your soleus muscle is to take smaller steps when walking or running. Increasing your step rate, especially while running will ensure you're not over striding and heel striking. This reduces impact loading and lessens your chances of knee pain.

Reference

Piva SR, Goodnite EA et al (2005). Strength Around The Hip And Flexibility Of Soft Tissues In Individuals With And Without Patellofemoral Pain Syndrome. JOSPT. 35(2): 793-801. DOI: 10.2519/jospt.2005.35.12.793.