Turns out, it's all in their genes.
Humans have three different types of muscle fibres:
Slow-twitch (Type I) - fibres are used for aerobic activities requiring low-level force production, such as walking and maintaining your posture. To be successful as an endurance athlete, you need a large proportion of slow-twitch fibres.
Fast-twitch (Type II) - fibres are used for short, intense exercise like sprinting and weightlifting. Fast-twitch fibres can be further divided into Type II and Type IIB. Type II fibres contain both endurance and power characteristics and represent a transition between the two extremes of Type I and Type IIB muscle fibres. Type IIB fibres are very sensitive to fatigue and are only used for short anaerobic, high-force production activities, such as sprinting, jumping and lifting very heavy weights.
How do you know what type of muscle fibres you have?
A muscle biopsy is the only way to know for sure what type of muscle fibre your body has. This involves a needle being stuck into the muscle to pluck a few fibres from the body which are then looked at under a microscope. However, this is an invasive procedure and it is possible to estimate your fibre type without a biopsy.
What does this mean for my training?
Your muscle fibre type will play a major role in your training workouts. For example, someone with a greater number of fast-twitch fibres will not be able to complete as many reps while lifting weights as someone with a greater number of slow-twitch fibres. Although the type of fibre cannot be changed from one to another, a different approach to your training can change the amount of area taken up by the fibre type in the muscle.
While most people perform workouts that focus on reaching a specific goal, your training should reflect your physiology. To get the most out of your training, tailor your workouts to match your muscle composition and your body will produce better results.
References from the SIRC Collection:
1. Alev K, Kaasik P, Seene T, et al. PHYSIOLOGICAL ROLE OF MYOSIN LIGHT AND HEAVY CHAIN ISOFORMS IN FAST- AND SLOW-TWITCH MUSCLES: EFFECT OF EXERCISE. Biology Of Sport. September 2009;26(3):215-234.
2. Karp J. Twitching muscles. Athletics Weekly (Descartes Publishing Ltd.. December 8, 2011;:38-39.
3. Mealing D, Long G, McCarthy P. Vibromyographic recording from human muscles with known fibre composition differences. / Enregistrement vibromyographique a partir de muscles humains presentant des differences de composition des fibres. British Journal Of Sports Medicine. March 1996;30(1):27-31.
4. Nguyen L. Contractile activity-induced gene expression in fast- and slow-twitch muscle. Applied Physiology, Nutrition & Metabolism. April 2011;36(2):233-241.
5. WILSON J, LOENNEKE J, JO E, WILSON G, ZOURDOS M, KIM J. THE EFFECTS OF ENDURANCE, STRENGTH, AND POWER TRAINING ON MUSCLE FIBER TYPE SHIFTING. Journal Of Strength & Conditioning Research (Lippincott Williams & Wilkins). June 2012;26(6):1724-1729.