• Introduction
  • The basic mechanisms of strength
  • How are muscles activated?
  • Muscle fiber types 
  • Muscle recruitment
  • Muscle hypertrophy 
  • Satellite cells
  • Growth factors
  • Inflammatory response
  • Other factors affecting strength
  • How to increase strength through training?
  • Benefits of strength in sports
  • Final thoughts
  • Sources
  • Actin: an important contributor to the contractile property of a muscle.
  • Motor unit: a motor neuron and all the muscle fibers it innervates.
  • Muscular hypertrophy: increased growth of muscle cells.
  • Muscular strength: the muscles' ability to produce maximal force.
  • Myosin: motor proteins interact with actin filaments to contract a muscle.
  • Power: ability to produce as much force as quickly as possible.
  • Sarcomere: the basic contractile unit for both striated and cardiac muscle.


Strength is often considered one of the cornerstones of athletic training. And, for good reason. In fact, several studies have shown strength to have a direct correlation to better performance in nearly every sports imaginable. However, the benefits do not end here. Increased strength has also proven beneficial in endurance performance, injury prevention, and healthy body composition.

This post explain the basic mechanisms of strength, how muscles are recruited, how strength can be improved.

The basic mechanisms of strength

Strength can be described as the ability to produce as much force as possible in a single movement, making it one of the cornerstones of athletic training. Strength can be divided into two different components; muscular strength, and muscular hypertrophy.

Muscular strength (maximum strength) describes your ability to recruit as many motor units as possible to produce the highest amount of force possible. It is often trained with low repetitions and very heavy resistance. This type of training causes neural adaptations that increase the activation of muscle fibers.

Muscular hypertrophy describes the size and strength properties of the muscle tissue itself. Training for muscle hypertrophy aims to increase the amount of contractable muscle mass, creating a stronger contraction.

Although muscular strength and muscular hypertrophy can be seen as two separate components of strength, they support and rely on each other. Training both of them helps form a foundation on which you can build more specialized and sports-specific skills.

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Muscular Strength

Relies on muscle recruitmentAnaerobic energy productionIncreases strength without adding muscle massRequires a good strength foundation to train furtherImproves rather quickly

Muscular Hypertrophy

Relies on contractable muscle massAnaerobic energy productionIncreases strength and muscle massRequires a good strength foundation to train furtherImproves rather slowly

How are muscles activated?

To understand the mechanics of strength, we must first take a look at how muscles contract. This process starts from a motor neuron located in the brain. Once stimulated, this motor neuron sends a signal to your muscle fibers, forcing them to contract. This combination of muscle fibers and a single motor neuron is known as a motor unit. 

The amount of muscle fibers innervated by a motor neuron varies greatly depending on the muscle. Small motor units consist of fewer muscle fibers which allows for accurate movements, whereas large motor units have a higher number of muscle fibers and produce broader, more powerful actions.

Thus, motor units are divided into two categories depending on which type of muscle fibers the motor neuron innervates. A single motor unit always innervates the same muscle fiber type. 

Muscle fiber types 

Skeletal muscles consist of two different muscle fiber types; slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II). The latter can be further divided into fast oxidative muscle fibers (type IIa) and fast glycolytic muscle fibers (type IIb/IIx). All muscles are comprised of both muscle fiber types.

Slow-twitch fibers are smaller in size and contain a significant amount of mitochondria (the powerhouse of the cell) and capillaries. This gives them great endurance capacity, but relatively low force production. 

Fast-twitch fibers are larger in size and contain fewer mitochondria. As a result, they have greater glycolytic capacity (ability to store and break down glucose for fuel) and force production, but lower endurance capability. 

Muscle Fiber Type




Contraction Speed



Very Fast

Fatigue Resistance




Force Production



Very High

Mitochondria Content
(powerhouse of the cell)




Myoglobin Content
(a protein that binds iron & oxygen and gives blood its red colour)




Capillary Content
(capillaries provide muscles with oxygen and nutrients while removing unwanted byproducts)




Oxidative Capacity
(ability to use oxygen for energy production)




Glycolytic Capacity
(ability to store and break down glycogen for intense exercises)




Muscle Fiber Diameter




Muscle Fiber Color

Dark Red

Dark Red

Pale Red

Motor Neuron Size
(larger neurons provide faster activation)



Very Large

ATPase Level
(enzyme that controls glycogen breakdown and ATP synthesis)




Muscle recruitment

Skeletal muscles turn chemical energy into mechanical output in the form of muscle movement. Instead of firing all muscle fibers at the same time, muscles must be activated gradually. By altering the firing rate and activating muscle fibers with different metabolic properties, the body is able utilize energy more efficiently.

When activated by a motor neuron, all muscle fibers in a motor unit contract simultaneously. This is referred to as the all-or-none law. In short, the motor unit will always give a maximal response or none at all after the stimulus exceeds an activation threshold.

Motor units are also recruited in an order from smallest to largest. This phenomenon is called the size principle. Because slow motor units are naturally smaller in size, they have the lowest activation threshold, and are recruited first during any given movement. Larger motor units are only activated when slow motor units are unable to produce enough force. Because of this recruitment pattern, it takes a relatively long time (0,5s-2,5s) for your muscles to reach their maximum strength output. 

The maximal strength and power exerted by a muscle is ultimately determined by how many motor units are recruited (motor unit synchronization), the rates at which they discharge action potentials (rate coding), and the amount of contractable muscle mass available.

The more motor units are recruited, the more force is produced.

Muscle hypertrophy 

In addition to the recruitment of motor units, strength is highly related to muscle tissue size and its strength properties. Hypertrophy is defined as increase in cell, tissue, or organ size. According to this definition, skeletal muscle hypertrophy is a result of increased muscle fiber size. However, the number of muscle fibers does not increase (hyperplasia).

More specifically, muscle hypertrophy occurs due to increased number of myosin/actin filaments (sarcomeres) inside the muscle cell. As sarcomeres are the basic contractile units of a muscle, activating more of them leads a stronger contraction, improved strength, and increased force production.

Because fast-twitch muscle fibers (type II) are much more prone to increasing in size than slow-twitch muscle fibers, the optimal training intensity for muscle mass requires a relatively heavy load. Simply put, the stimulus must be of the right intensity to recruit the correct muscle fibers.

Muscle growth is dependent on two things: a stimulus and repair. An intense stimulus, such as weight training, produces small tears inside the muscle. To repair this trauma, your body sends a signal to various organ systems in the body. As a response, satellite cells start to proliferate to the injury location, the immune system responds with inflammation, and the endocrine system (glands that produce various hormones) starts producing testosterone, growth factors, and cortisol to start the repair process.

Inflammatory response

Muscle tissue damage consists of the disruption of sarcomeres, and is followed by inflammation and the resulting soreness muscle soreness. Inflammation is the body’s natural immune response that helps repair muscle tissue after trauma, such as the microtrauma caused by resistance training.

As the inflammatory response starts, your immune cells (white blood cells, etc.) increase blood flow to the muscle, resulting in swelling. The increased blood flow also deliver oxygen and valuable nutrients to the muscles while removing waste products. The immune response also activates satellite cells which further improves/promotes cellular repair and muscle growth.

Satellite cells

Satellite cells are mononucleated stem cells located outside the muscle fibers between the basal lamina (basement membrane) and the plasma membrane (sarcolemma). Exercise-induced disruption to muscle fibers activate these satellite cells, causing them to proliferate (increase in number) and/or differentiate (a process where unspecialized cells take on individual characteristics and reach a specific form and function).

Myoblasts, the progeny cells of satellite cells, are able to fuse together to form new myofibers, fuse to an existing muscle fiber, or return to the quiescent (inactive) state. After fusing with the muscle fiber, myoblasts donate their nuclei to support the growth of the muscle fiber. This helps the muscle fiber synthesize new proteins, especially actin and myosin filaments, which are the active structures responsible for muscular contraction. As a result, the myofibrils (long filaments found in striated muscle cells) inside a muscle cell increase in both thickness and number. Thus, leading to increases in the muscle’s cross-sectional area, also known as hypertrophy.

Interestingly, more satellite cells are found associated with slow-twitch muscle fibers within the same muscle. This is because these fibers are activated more often during daily activities.

Growth factors

Growth factors are naturally occurring substances that stimulate satellite cells to repair muscle fibers. Growth factors also play a key role in regulating the amount muscle growth and promote changes in the DNA for protein synthesis.

Additionally, insulin is known to stimulate muscle growth in two ways; enhancing/increasing protein synthesis, and facilitating/enhancing the entry of glucose into cells. Because satellite cells use glucose for fuel, they facilitate/enable/aid cells in their growth process. Glucose is also used as a significant energy source by the muscles.

Growth hormone also has a significant impact on muscle growth. It is released as a result of intense (especially resistance) training. The amount of secreted growth hormone is related to exercise intensity. Growth hormone helps your body use fat for fuel during the muscle building process, as well as enables/stimulates the incorporation of new amino acids in skeletal muscle.

Testosterone has a specifically big impact on muscle hypertrophy. It enhances both protein synthesis and cellular amino acid uptake. In short, it speeds up the muscle rebuilding process and activates tissue growth.

Having more contractable muscle mass results in a stronger contraction.

Other factors affecting strength

Training background has a tremendous effect on strength development. Not only does it increase strength, but it also improves coordination and body mechanics, leading to better performance. In addition to training history, there are several other factors that have an impact on force production. These include muscle fiber proportion, sex, age, and joint angle.

The proportions of type I and type II muscle fibers found in a specific muscle or muscle group can significantly affect muscular strength. Although most people have approximately the same amount of each muscle fiber type, some elite athletes can have up to 70-80% of a specific muscle fiber type. This is important because each fiber type has different metabolic properties and training responses. Similarly, type II muscle fibers seem more prone to age-related loss of muscle mass (sarcopenia), leading to reduced performance and overall quality of life. Males also tend to have more muscle mass due to increased amounts of testosterone, which drives growth in both muscle and bone mass.

Interestingly, the joint angle also has a significant impact on force production. It is generally considered that muscular length changes depend on the joint angles that trigger changes in muscular strength. Therefore, each body joint should have an optimal joint angle, where muscles have the correct length to exert maximum force.

How to increase strength through training?

Maximum strength training utilizes very heavy weights that are performed close to your one-repetition maximum (1RM). A traditional muscular strength training routine consist of 5-6 sets of 1-3 repetitions at 80-100% of your 1RM. The rest period between sets range between 2-3mins.

Heavy resistance training teaches your neuromuscular system to recruit as many motor units (muscles and their connecting nerves) as possible. The resulting enhanced neuromuscular connection helps you create a stronger muscle contraction with less effort, which is also why beginners can see great results in the first few weeks of strength training.

Muscular hypertrophy training utilizes slightly less resistance than maximum strength training with a few more repetitions. A common way to train is performing 3-6 sets of 6-12 repetitions at 60-85% of your 1RM. This aims to overload the muscles and cause micro-tears inside the muscle tissue itself. As the muscle repairs the damaged muscle tissue, it becomes bigger and stronger, leading to better strength output.

Benefits of strength in sports

Increased strength comes with a lot of benefits strength for our overall health; it improves athletic performance, maintains muscle mass and bone health, promotes healthy body composition, improves your cardiovascular fitness and helps prevent injuries in many ways. It even has a positive effect on your mental health.



Athletic performance

Increases maximal strength, maximal speed, coordination, acceleration, agility. Also helps generate more efficient and powerful movements – all of which provide benefits for a wide range of athletic skills.

Injury risk

Strength is important for all movement; balance, coordination and injury prevention. Strong muscles support healthy tendons and joints.

Mobility and flexibility

Utilizing the joint’s full range of motion during strength exercises increases their range of motion over time.

Mental health

Strength training has known to improve positive self-image and promote good mental health.

Body composition

Strength training is linked to increased lean muscle mass, elevated metabolism, lower BMI, and healthier body composition.

Quality of life

High levels of strength is linked to reduced risk of cardiovascular diseases, diabetes, cancer, osteoporosis, and cognitive decline. In short, an increased life expectancy.

Due to the vast number of performance benefits that comes with added strength, it is highly recommended that you incorporate resistance training into your routine regardless of the sport you participate in.

Progressive overload is fundamental for muscle growth.

Final thoughts

Whether you are a professional athlete or strive to be one, strength is one of the main components you need to take care of if you want to improve your performance. Since strength helps create a foundation for physical activities, it may even be the most important component for athletes looking to improve their other athletic qualities.

Not only does strength hold a central role in athletic performance, it can also keep you free of injuries. This ensures that you can maintain progress in your performance without significant setbacks.

If you want to have any meaningful effect on your performance, you must remember the ”holy trinity” of training – nutrition, exercise and rest. To continue developing as an athlete, all three of these factors must be in balance.

Did you learn anything new about strength in sports? Let us know in the comments below.


  • Ahtiainen, J.P., Pakarinen, A., Kraemer, W.J. & Häkkinen, K. (2002) Acute hormonal and neuromuscu­lar re­sponses and recovery to forced vs. maximum repeti­tions multiple resistance exercises. International Journal of Sport Medicine. Volume 24, Issue (6). pp. 410-418.
  • Atha, J. (1981) Strengthening muscle. Exercise and Sports Science Reviews. American College of Sports Medicine. Volume 9, Issue (1). pp.1-74.
  • Baechle, T.E. & Earle, R.W. (2003)  Essentials of strength training and conditioning. National Strength and Conditioning Association. Human Kinetics, Champaign, IL. 
  • Connolly, D.A.J., Sayers, S. & McHugh, M.P. (2003) Treatment and Prevention of delayed onset Muscle Soreness. Journal of Strength and Conditioning Research. Volume 17, Issue (1), pp.197-208.
  • Deschenes, M.R. & Kraemer, W.J. (2002) Performance and Physiologic Adaptations to Resistance Training. American Journal of Physical Medicine & Rehabilitation. Volume 81, Issue (11), pp. S3-S6.
  • Guyton, A. C. & Hall, J.E. (2000) Textbook of Medical Physiology. Philadelphia. W. B. Saunders Company, 1-1064.
  • Hamill, J., Knutzen, K.M. & Derrick, T.R. (2014) Biomechanical Basis of Human Movement. 4th Edition. Lippincott Williams & Wilkins. Philadelphia, PA. 
  • Fleck, S.J. & Kraemer W.J. (2004) Designing resistance training programs, 2nd edition. Human Kinetics Publishers, Champaign, Illinois.
  • Häkkinen, K. (1990) Voimaharjoittelun perusteet, vaikutusmekanismit, harjoitusmenetelmät ja ohjelmointi. Gummerus kirjapaino OY, Jyväskylä, pp. 5-244.
  • Häkkinen, K. (1989) Neuromuscular and hormonal adaptations during strength and power training. A review. Journal of Sports Medicine and Physical Fitness. Volume 29, Issue (1). pp. 9-26. 
  • Häkkinen, K., Alen, M., Kraemer, W.J., Gorostiaga, E., Izquierdo, H., Rusko, H., Mikkola, J., Häkkinen, A., Valkeinen, H., Kaarakainen, E., Romu, S., Erola, V., Ahtiainen., J. & Paavolainen. (2003) Neuromuscular adaptations during concurrent strength and endurance training versus strength training. European Journal of Applied Physiology. Volume 89, Issue (1), pp. 42-52.
  • Häkkinen, K., Myllylä, E. (1990) Acute effects of muscle fatigue and recovery on force production and relaxation in endurance, power and strength athletes. Journal of Sports Medicine and Physical Fitness. Volume 30, Issue (1). pp. 5-12.
  • Häkkinen, K., Pakarinen, M., Alen, M., Kauhanen, H. & Komi, P.H. (1988) Neuromuscular and hormonal adaptations in athletes to strength training in two years. Journal of Applied Physiology. Volume 65, Issue (6). pp. 2406-2412.
  • Jones, T., Howatspon, G., Russell, M. & French D.N. (2013) Performance and neuromuscular adaptations following differing ratios of concurrent strength and endurance training. Journal of Strength and Conditioning Research. Volume 27, Issue (12). pp. 3342-3351. 
  • Kraemer, W.J., Ratamess, N.A. & French D.N. (2002) Resistance training for health and performance. Current Sports Medicine Reports. Volume 1, Issue (3), pp.165-171.
  • McArdle, W.D., Katch, F.I. & Katch, V.L. (1996) Exercise Physiology: Energy, nutrition and human performance. Williams & Wilkins, Baltimore, ML.
  • McCarthy, J.P., Myron, A., Pozniak, A. & Agre, J.C. (2002) Neuromuscular adaptations to concurrent strength and endurance training. Medicine & Science in Sports & Exercise. Volume 34, No (3). pp. 511-519. 
  • Miles, M. (1994). Exercise-induced muscle pain, soreness, and cramps. Journal of Sports Medicine and Physical Fitness. Volume 34, Issue (3). pp. 203-216.
  • Nummela, A. (1997) Energia-aineenvaihdunta. In Mero, A., Nummela, A. & Keskinen, K. (ed.) Nykyaikainen urheiluvalmennus. Mero Oy, Jyväs­kylä, pp. 119-126.
  • Ozmun, J.C., Mikesky, A.E. & Surburg, P.R. (1994) Neuromuscular adaptations following prepubescent strength training. Medicine and Science in Sports and Exercise. Volume 26, Issue (4), pp. 510-514. 
  • Siff, M. (2000) Biomechanical foundations of strength and power training. In 
  • Zatsiorsky, V. (ed.) Biomechanics in Sport: Performance Enhancement and Injury Prevention Blackwell Scientific Publications, Cambridge Uni­versity Press. pp.103-139.
  • Stone, M.H., O’Bryant, H.S., McCoy, L., Coglianese, R. Lehmkuhl, M. & Schilling, B. (2003) Power and maximum strength relationships during performance of dynamic and static weighted jumps. Journal of Strength and Conditioning Research. Volume 17, Issue (1). pp.140-147.
  • Stone, M.H., Sanborn, K., O’Bryant, H.,Hartman, M., Stone, M.E., Proulx, P., Ward, B. & Hruby, J. (2003) Maximum Strength-Power-Performance Relationships in Collegiate Throwers. Journal of Strength and Conditioning Research. Volume 17, Issue (4). pp. 739-745.
  • Tesch, P.A. (1987) Acute and long-term metabolic changes consequent to heavy-resistance exercise. In Marconnet, P. & Komi, P.V. (ed.) Muscular Function in Exercise and Training. Karger, Switzerland, pp. 67-89.
  • Tesch, P.A., Colliander, E.B. & Kaiser, P. (1986) Muscle Metabolism during intense, heavy-resistance exercise. European Journal of Applied Physiology. Volume 55, Issue (4). pp. 362-366.
  • Viru, A. & Viru, M. (1993) The specific nature of training on muscle: a review. Sports Medicine. Volume 4, Issue (1). pp. 79- 98.

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