Victor Forge
There are two types of muscle fibers: slow-twitch (Type I) and fast-twitch (Type II). The type of muscle fiber a person has can be determined by a muscle biopsy, which is an invasive procedure. To avoid this, a muscle fiber composition test can be performed to determine the type of muscle fiber a person has. This involves performing a standardized warm-up, determining your one-repetition maximum (1RM) on a given exercise, resting for 15 minutes, and then using 80% of your 1RM to perform as many repetitions as possible in a single attempt. The number of repetitions performed can indicate whether a person has more slow-twitch or fast-twitch muscle fibers.
| Characteristics | Values |
|---|---|
| Muscle fiber types | Slow-twitch (Type I) and fast-twitch (Type II) |
| Subtypes | Type IIa and Type IIb |
| Slow-twitch characteristics | Generate less power and strength but can sustain activity for longer; high-density of capillaries to bring blood to the muscles |
| Fast-twitch characteristics | Generate more power and strength but cannot sustain activity for as long |
| Slow-twitch activities | Marathon |
| Fast-twitch activities | Football, Olympic weightlifting |
| Muscle fiber test | Determine your one repetition maximum (1RM) on a given exercise, rest for 15 minutes, then use 80% of your 1RM to perform as many repetitions as possible in a single attempt |
| Number of repetitions | Less than 7 reps: fast-twitch dominant; 7 to 9 reps: balanced; more than 9 reps: slow-twitch dominant |
| Alternative test | Muscle biopsy |
| Non-invasive alternative tests | Magnetic resonance imaging (MRI), proton magnetic resonance spectroscopy (1H-MRS), phosphorus magnetic resonance spectroscopy (31P-MRS) |
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What You'll Learn
Slow-twitch vs fast-twitch muscle fibres
Slow-twitch and fast-twitch muscle fibres are the two types of muscle fibres responsible for all your gains in the gym. Depending on your training methods and genetics, you could be slow or fast-twitch dominant. Slow-twitch muscle fibres (Type I) generate less power and strength than fast-twitch fibres but can sustain activity for longer. They are used for low-intensity, endurance activities and have a high density of capillaries, which helps bring blood to the muscles. They use an aerobic energy system, running on oxygen and glucose to produce ATP. During a marathon, you primarily use slow-twitch fibres.
Fast-twitch muscle fibres (Type II), on the other hand, provide quick bursts of power, speed, and strength. They use a lot of energy very quickly and then get tired, requiring a break. They are used for high-intensity activities that require speed and sudden movements, such as sprinting, hopping, and blinking. Fast-twitch fibres can be further categorized into Type IIa (intermediate muscle fibres) and Type IIb. Type IIb fibres fatigue more quickly and are specifically targeted during Olympic weightlifting.
To determine your muscle fibre type, you can perform a muscle fibre composition test. This involves determining your one-repetition maximum (1RM) on a given exercise, which is the maximum weight you can lift in one repetition. After a 15-minute rest, you then use 80% of your 1RM to perform as many repetitions as possible in a single attempt. The number of repetitions completed will indicate your muscle fibre type. For example, completing less than 7 reps suggests fast-twitch dominance, while completing more than 9 reps indicates slow-twitch dominance.
It is important to note that muscle fibre composition can be influenced by training. Through sprint training, the power generated by slow-twitch fibres can be improved, and through endurance training, the endurance level of fast-twitch fibres can be increased. Additionally, the composition of muscle fibres is specific to the muscle group tested and may not apply to other muscles in the body.
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Muscle biopsy method
A muscle biopsy is a procedure that involves removing a small sample of muscle tissue for testing in a laboratory. It is a relatively simple and minimally invasive procedure that is performed on an outpatient basis, meaning patients are usually free to leave on the same day as the procedure.
There are two main methods of performing a muscle biopsy: the needle biopsy and the open biopsy. The former is the most common method and involves inserting a thin needle through the patient's skin to extract a sample of muscle tissue. There are several types of needle biopsies: core needle biopsy, which uses a medium-sized needle to extract a column of tissue; fine needle biopsy, which uses a thin needle attached to a syringe to draw out fluids and cells; image-guided biopsy, which is guided by imaging procedures such as X-rays or CT scans to avoid specific areas like the lungs or liver; and vacuum-assisted biopsy, which uses suction to collect more cells. Local anaesthesia is typically administered for needle biopsies to prevent pain or discomfort.
On the other hand, an open biopsy requires making a small incision in the skin to remove a larger section of muscle tissue. The muscle selected for the biopsy depends on the location of symptoms, which may include pain or weakness. Commonly sampled muscles include the bicep, deltoid, or quadriceps.
Prior to the procedure, patients are typically asked to remove their clothing and are given a gown to wear. The skin over the biopsy site is cleaned with an antiseptic solution, and a local anaesthetic is injected to numb the area. During the procedure, patients must lie as still as possible. After the procedure, patients should monitor the surgical site for signs of infection or bleeding, although no follow-up visits are usually necessary.
Muscle biopsies are important for diagnosing diseases involving muscle tissue, particularly neuromuscular disorders, infections, and other abnormalities. They can help determine the source of the disease and ensure proper treatment.
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Non-invasive muscle testing
Electromyogram (EMG): EMG is a technique that records the electrical activity of muscles during contractions. It helps detect and classify muscle fatigue, providing valuable information for sports performance and injury management. EMG can identify the specific muscles engaged during different movements, aiding in the development of targeted training programmes.
Mechanomyogram (MMG): MMG is a non-invasive method that focuses on recording the vibration signals generated by muscle fibres during contractions. By analysing these vibrations, researchers and practitioners can assess muscle fatigue and better understand the mechanics of human movement. MMG offers insights into muscle function and can contribute to the design of more effective training and rehabilitation strategies.
Near-Infrared Spectroscopy (NIRS): NIRS is a valuable tool for monitoring muscle oxygenation changes during exercise or muscle contractions. By assessing oxygen depletion and the accumulation of metabolic by-products, NIRS helps determine optimal current intensity and provides insights into muscle fatigue. This technique is particularly relevant for endurance athletes and those seeking to improve their aerobic capacity.
Ultrasound (US): Ultrasound technology offers a non-invasive means to visualise and record muscle deformations during contractions. It aids in detecting muscle fatigue and understanding muscle mechanics. US can also help identify structural issues within muscles and guide targeted interventions for improved performance and injury prevention.
Standardised Weight-Lifting Tests: These tests involve performing specific weight-lifting exercises, such as the bench press or back squat, at a predetermined percentage of an individual's one-rep maximum (1RM). By observing the number of repetitions completed, it is possible to determine whether an individual is slow-twitch dominant, fast-twitch dominant, or balanced. This information can then guide training programmes, with slow-twitch dominant individuals focusing on endurance exercises and fast-twitch dominant individuals emphasising explosive movements.
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One-repetition maximum (1RM)
The 1RM can also be estimated indirectly using repetition testing on submaximal loads, with the Epley and Brzycki formulas being the most commonly used. For example, if a person can lift 100 pounds for 10 reps, the estimated 1RM is 133 pounds for both formulas. However, if the person only completes 6 reps, the Epley formula estimates a 1RM of 120 pounds, while Brzycki estimates 116 pounds. These formulas are generally for experienced weightlifters, as novices may find their nervous system cannot handle the stress of high weights.
The National Strength and Conditioning Association (NSCA) recommends a specific protocol for determining 1RM, which includes warming up with light resistance, resting, and then increasing the weight. For upper body lifts, weight is added in increments of 10-20 pounds or 5-10%, while for lower body lifts, 30-40 pounds or 10-20% is added. This process continues until the individual can complete one rep with proper technique.
The 1RM test is used to design resistance training programs, with different percentages of 1RM used depending on the goal, such as improving muscular strength or endurance. It is also used to determine muscle fiber type, with fast-twitch dominant individuals performing less than 7 reps at 80% 1RM, and slow-twitch dominant individuals performing more than 9 reps.
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Training for muscle fibre types
Human skeletal muscle is composed of three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. The different types of muscle fibres can be trained in different ways to improve performance.
Slow-twitch (Type I) muscle fibres generate less power and strength than fast-twitch fibres, but they can sustain activity for longer. They are commonly used in endurance sports, such as long-distance running, as they are more resistant to fatigue. To train these muscle fibres, it is recommended to perform high-rep sets or slow eccentric movements, as well as long jogs, hikes, or bike rides for cardio to improve the aerobic system. Training at a lower intensity with higher repetitions will target these muscle fibres.
Fast-twitch (Type II) muscle fibres include Type IIa and Type IIb fibres. Type IIa fibres have a fast shortening speed and can transfer energy from aerobic and anaerobic sources. Type IIb fibres have an even faster shortening speed and greater anaerobic potential. These fibres are used in high-intensity activities such as sprinting and weightlifting, as they produce greater amounts of force, power, and strength, but they fatigue faster. To train these muscle fibres, it is recommended to perform low-rep training with explosive exercises and heavy weights. Training at a higher intensity with lower repetitions will target these muscle fibres.
It is important to note that the composition of muscle fibres can be influenced by training to some extent, but it is also determined by genetics. Additionally, the specific training methods may vary depending on the individual's goals and fitness level. For example, if the goal is to increase strength, the repetition range will be lower with a higher intensity, while improving muscular endurance will require training at lower intensities with higher repetitions.
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Frequently asked questions
Muscle fibers are the two types of muscles responsible for all your gains in the gym. Depending on how you train and your genetics, you could be slow-twitch or fast-twitch dominant. Slow-twitch muscle fibers (Type I) generate less power and strength than fast-twitch fibers but can sustain activity for longer. Fast-twitch fibers (Type II) are further categorized into Type IIa and Type IIb.
To test your muscle fiber type, you can take a muscle fiber test. This involves determining your one repetition maximum (1RM) on a given exercise, resting for 15 minutes, and then using 80% of your 1RM to perform as many repetitions as possible. The number of repetitions completed will indicate whether you are fast-twitch dominant, balanced, or slow-twitch dominant.
Knowing your muscle fiber type can help you train more effectively and efficiently. If you are fast-twitch dominant, you will benefit from exercises with quick bursts of power, speed, and strength. If you are slow-twitch dominant, you will benefit from high-rep sets or slow eccentric movements, as well as cardio activities such as long jogs or bike rides.
Yes, there are a few non-invasive alternatives to the muscle biopsy method, which is considered the gold standard. These include magnetic resonance imaging (MRI), phosphorus magnetic resonance spectroscopy (31P-MRS), and proton magnetic resonance spectroscopy (1H-MRS). However, some of these methods have provided contradictory or equivocal results.
