Improving the body’s ability to recruit motor units requires slow-twitch muscle training.

Fiber type predicts, to a large extent, the athletic activity for which you may be best suited, bearing in mind that many activities demand characteristics of both fiber types. Humans have a combination of two types of fibers, but one may predominate.

Overview

Human skeletal muscle is made up predominantly of two types of muscle fiber: red fiber and white fiber. Red fibers (type 1) are also known as slow-twitch fibers, and white (type 2) are called fast-twitch fibers. White, fast fibers can also be broken into two types—2A and 2B. 2A fibers sit in between the slower red fibers and the ultimate fast 2B white fibers.

An Olympic sprinter, for example, may have around 80% fast-twitch, white fibers, and a good marathoner the reverse. Fiber type propensity may also determine to some extent your ability to lift heavy weights with speed and power.

Facts About Muscle

There are some quick facts you can use to compare the two types of muscle fiber.

Red

• Slow twitch

• Contract slowly

• Contract for longer period of time

White

• Fast twitch

• Contract with greater force

• Only used for short bursts of effort

• Muscle exists as 3 types: cardiac muscle, smooth muscle, and skeletal muscle. Skeletal muscle is the target of strength and conditioning training.
• Smooth muscle makes up blood vessels and certain organs.
• Skeletal muscle makes up approximately 45% of total body weight.
• Skeletal muscle attaches to two bones and crosses a joint between them.
• Muscle cells are elongated and cylindrical in shape and are called fibers. Muscle cells and fibers are synonymous.
• Muscles can contract and shorten, thus creating a pulling force on bones and the attachments to bones (tendons and ligaments)
• Muscles are organs, which means they have more than one type of tissue. Muscle contains muscle and fibrous connective tissue (fascia).
• Muscles also incorporate blood vessels and nerves.
• The nerves process messages from the central nervous system to the muscle, triggering contraction. Blood vessels supply nutrients and the energy required for movement and remove waste products.
• A motor unit consists of a motor neuron (nerve cell) and the muscle fibers that it controls. Motor units are often referenced in relation to muscle activation in weight training.

Fiber Types and Resistance Training

Fast-twitch fibers favor speed and power activities like sprints and throwing events that take only tens of seconds at most. Slow-twitch fibers favor endurance competitors like marathoners and triathletes. Having some transition fibers like the moderately fast and moderately enduring 2A fibers can be useful for middle distance runners where speed and endurance are useful.

When lifting weights, 2B fibers help you lift heavy with great power. 2B, fast-twitch fibers drive explosive power when doing 1RM or sets of low, heavy repetitions. Type 1, slow-twitch fibers are more suited to muscle endurance training, for example, sets of 20-30 repetitions.

Can fiber types be converted? The short answer is no, they cannot. However, you may be able to "train up" the fibers you have of a particular type. For example, if you have 70% slow fibers and 30% fast fibers, there is some evidence that training heavy, at 5-8 RM, for example, will theoretically boost the cross-section size of the 30% of type 2B fibers, if not the number.

The reverse may also be true. For example, a predominantly fast-twitcher, a sprinter, may be able to emphasize his slow fibers by running regularly for an hour or more in order to compete in long-distance races or by doing sets with a high number of reps in the gym.

Regular full-body weight training in the range of 10 to 15 reps per set is likely to hit your type 2A intermediate fibers.

In summary, if you're a gym lover, being blessed with white, fast fibers (2B and 2A) will probably give you a lifting edge in total weight lifted. If you have slow, type 1 fibers predominantly, you may not win a lifting competition anytime soon, although there is no reason why you should not be able to bulk up substantially.

By Paul Rogers
Paul Rogers is a personal trainer with experience in a wide range of sports, including track, triathlon, marathon, hockey, tennis, and baseball.

Thanks for your feedback!

We cyclists ask a lot of our muscles. We expect them to push the pedals continuously for hours on end, to be ready to surge over climbs and sprint at maximum intensity, and to recover and do it all over again tomorrow.

But not all muscle is the same, and different muscle types function in different ways. Furthermore, many cyclists don’t realize that directly addressing muscular strength can also improve endurance and efficiency. Let’s examine muscle types and consider how building strength can make you faster. 

For more on strength training check out Ask A Cycling Coach Ep 277.

Muscle Fibers And Motor Units

The basic structural elements of muscles are muscle fibers. Fibers of each type cluster into bunches called motor units, which bunch together to form a muscle. Because of their distinct fiber composition, different motor units have different strengths. This makes your muscles more versatile; by selectively recruiting motor units, the same muscle can pedal aerobically for hours, and also generate the anaerobic surge of a sprint.

Human muscle fibers have traditionally been divided into 3 types, differentiated by how they are fueled and how they output force. Although recent research reveals a broader range of minor fiber types and subtypes, the 3-type model is still commonly used and useful in its simplicity.

Muscle Fiber Types

1. Slow Twitch Muscle Fibers (Type 1)
   These fibers specialize in low-intensity sustained activity, such as riding at an easy endurance pace. They contain lots of mitochondria, which they use to efficiently fuel themselves through aerobic metabolism. If properly fueled they can sustain almost indefinitely, but they can not exert high force. 
2. Fast Oxidative Glycolitic Fibers (Type 2A)
   These fibers have lower mitochondrial density than Type I fibers, and are fueled by a mixture of aerobic and anaerobic metabolism. They have a higher force output but fatigue more quickly. These fibers are recruited as intensity increases.
3. Fast Glycolitic Muscle Fibers (Type 2B or 2X)
   These powerful fibers exert the highest force for the shortest time. They rely exclusively on stored ATP and anaerobic metabolism and take a long time to recover, and are the fibers responsible for generating the peak power of a sprint. Note that while these have traditionally been referred to as type IIb fibers, some studies label them IIx in humans.

Functional Strength Training for Endurance Athletes

There’s a common misconception amongst cyclists that strength training immediately leads to expanding muscles and weight gain. Many riders intentionally avoid strength training as a result, which is ironic given that muscles are the driving force behind turning the pedals. Well-applied strength training can increase muscular recruitment, allowing riders to better utilize their existing muscle mass. It can also strengthen and improve the composition of existing muscles, leading to greater efficiency and increased power output. This strength training can take two forms: 

Low Force, High Velocity (LFHV)

Otherwise referred to as plyometrics, LFHV exercises use body weight (or very low additional load) and explosive movements over multiple repetitions. The goal is to increase your maximal force production, by training neuromuscular pathways to activate the muscles as quickly as possible. Examples include jumping rope, box jumps, and jump squats.

LFHV training is easy, since it requires little special equipment. It’s unlikely to cause injury because of light loads, and it is not normally associated with increases in body weight or muscle mass. However, evidence is mixed as to how effective these exercises are for actually improving cycling performance.

High Force, Low Velocity (HFLV)

These exercises use heavy loads (80% – 100% of 1 rep max) and occur over just a few reps. Research demonstrates surprisingly wide-ranging benefits of this type of strength training for endurance athletes. Compared to LFHV, these exercises elicit improvements in power output across a wide range of loads. They are shown to improve movement economy, power at VO2 max, and overall time trial performance. Studies also show this training to be effective for athletes of all experience levels, with the greatest improvements in movement efficiency demonstrated by amateurs and non-elite riders. This means for most of us, HFLV training may be low-hanging fruit for big gains. What are some specific ways this training improves cycling performance?

Adaptive Training

Get the right workout, every time with training that adapts to you.

Check Out TrainerRoad

Peak Force Improvements

A major result of addressing muscular strength through HFLV training is an increase in peak force. On the bike, this directly translates into improvements in the overall maximum torque you can generate during a sprint. However, this can have benefits for more than just your top-end power. By increasing your ability to produce peak force, you also increase your opportunity to lift your sustainable power.

The reason for this makes sense if you think about it. Metabolically, it’s a big ask to expect your muscles to sustainably operate at a high percentage of their absolute maximum. If your peak power is 600w, a 300w FTP would mean operating at 50% capacity for sustained periods. But if you raise your peak power to 1200w, that same FTP only requires 25% of your maximum. By increasing your overall strength, you open the door to potentially easier improvements in FTP.

Better Efficiency

As you pedal your bike, a complicated series of muscular interactions occurs. Some muscles activate, others turn off, and others stabilize the system. HFLV training helps to fine-tune coordination of these muscles. It does this by increasing agonist activation, strengthening the motion of the primary mover (the quads). It also improves synergist activation, the recruitment of secondary and stabilizing muscles. Finally, it decreases antagonist activation, lessening the action of muscles that resist the intended motion. You probably lubricate your bike’s drivetrain regularly, and through HFLV training you can have an analogous effect on the efficiency of your legs.

Strengthening Aerobic Fibers

Perhaps most notably, HFLV training can greatly improve cycling economy during the latter part of long rides – in other words, when the decisive moves usually happen in races. Studied riders who used this method along with their regular endurance training had lower heart rates, lower blood lactate concentrations, and stronger final sprints than athletes who only used endurance training. While strength training is typically associated with anaerobic fibers, these results suggest HFLV also strengthens Slow Twitch (Type 1) fibers. Stronger aerobic fibers are better able to handle the burden of most efforts during a hard ride. Recruitment of more easily-fatigued anaerobic fibers is delayed, saving more of their strength for the final sprint. 

In Conclusion

Muscular strength, especially when addressed through High Force, Low Velocity training, can improve more than just your final sprint. Without adding muscle mass or body weight, it can facilitate increases in FTP, make your pedal stroke more efficient, and leave you fresher for the end of your race. 

References:

• Bazyler, Caleb D. MA; Abbott, Heather A. M.Ed; Bellon, Christopher R. MA; Taber, Christopher B. MS; Stone, Michael H. PhD Strength Training for Endurance Athletes, Strength & Conditioning Journal: April 2015 – Volume 37 – Issue 2 – p 1-12 doi: 10.1519/SSC.0000000000000131
• Schiaffino, S., & Reggiani, C. (2011). Fiber Types in Mammalian Skeletal Muscles.Physiological Reviews, 91(4), 1447–1531. doi:10.1152/physrev.00031.2010 
• Scott, W, et. al, Human Skeletal Muscle Fiber Type Classifications, Physical Therapy, Volume 81, Issue 11, 1 November 2001, Pages 1810–1816, https://doi.org/10.1093/ptj/81.11.1810
• Staron, R. S. (1997). Human Skeletal Muscle Fiber Types: Delineation, Development, and Distribution.Canadian Journal of Applied Physiology, 22(4), 307–327. doi:10.1139/h97-020 
• Talbot J, Maves L. Skeletal muscle fiber type: using insights from muscle developmental biology to dissect targets for susceptibility and resistance to muscle disease. Wiley Interdiscip Rev Dev Biol. 2016;5(4):518-534. doi:10.1002/wdev.230

How can I improve my slow twitch muscles?

Is it possible to increase motor unit recruitment with exercise?

Which activities predominantly use slow twitch muscle?

Why are slow twitch muscles more beneficial?