• Introduction
  • The basics of different muscle actions
  • Eccentric muscle actions
  • Concentric muscle actions
  • Isometric muscle actions
  • Isokinetic muscle actions
  • Isotonic muscle actions
  • Isoinertial muscle actions
  • Final thoughts
  • Sources
  • Actin: an abundant intracellular protein found in all eukaryotic cells. These thin filaments play a crucial role in muscle contraction and in cell movements.
  • Agonist muscles: the main muscle(s) responsible for a specific movement.
  • Antagonists muscles: the muscles opposing the agonist that either 1) relax to not impede with the contraction, or 2) produce tension as they lengthen to help decelerate concentric actions.
  • Fixator muscles: muscles that stabilize the bone where the agonist muscle is attached to.
  • Isoinertial muscle action: actions in which the muscle is under constant load.
  • Isokinetic muscle action: actions in which the velocity of motion is constant.
  • Isotonic muscle action: actions in which the muscle tension remains constant.
  • Myosin: a type of molecular motor and converts energy from the hydrolysis of ATP into mechanical energy.
  • Neural drive: central motor cortex and brain output in response to stimuli.
  • Synergist muscles: muscles that aid the prime mover and make the movement more efficient.
  • Tendinopathy: overuse injuries that result in pain, swelling, and impaired function.

Introduction

Skeletal muscles are responsible for all voluntary movement of the body. The process of contracting these striated muscles are therefore integral for all physical activity. However, there are still several factors that may not be fully understood by the general public.

For example, the word ”contraction” might be slightly misleading due to the fact that muscle actions can be divided into three categories; 1) eccentric (muscle lengthening), 2) concentric (muscle shortening), and 3) isometric (muscle length remains the same) muscle actions. Because muscles can only contract and/or relax relative to their resting length, they have no way to ”contract” eccentrically or isometrically. Therefore, the word ”muscle action” may be far more appropriate to describe overall muscle functionality than ”contraction”. 

This is supported by the sliding filament theory, which states that muscles shorten as adjacent actin and myosin filaments slide past one another. This shortens the distance between both ends of the muscle, which are connected to bones by tendons. As the muscle shortens, it pulls on the bone, causing movement. If a muscle is activated in a manner where the joint angle remains the same (e.g. pushing/pulling an immovable object), there is no visible change in muscle length. However, the muscle is actively trying to maintain cross-bridges cycling (repeated attachment of actin and myosin within the muscle cell) to produce more tension on the muscle. 

This post explains the basics of different muscle action types, and what role they play in various training modalities. 

The basics of different muscle actions

In order to generate voluntary movement (such as lifting a weight) the prime mover muscle (main muscle responsible for a specific movement) must shorten to produce force. This is known as a concentric muscle action. Because concentric actions rely on the shortening of a muscle, they are commonly referred to as muscle ”contractions”. Muscles can also produce tension while lengthening, such as during the lowering phase of a lift or when the muscles are unable to produce enough force to overcome the resistance. This is known as an eccentric muscle action. 

According to research, these two muscle action types produce distinct neuromuscular stimuli leading to specific training adaptations. This is also consistent with the principle of training specificity, which states that the body adapts to the specific demands placed upon it. Furthermore, research states that eccentric actions may produce greater hypertrophic gains due to higher tension and the resulting muscle damage. Although concentric muscle actions also cause muscle tissue damage, eccentric movements elicit the greatest disruptions to the muscle’s contractile, structural, and supportive elements. Whether eccentric exercises induce a higher hypertrophic response than concentric muscle actions is still somewhat inconclusive.

If the muscles are activated and producing force without a change in joint angle or muscle length, the muscles are activated isometrically. Isometric muscle actions take place when the force produced by the muscle is equal to the resistance, such as trying to lift an immovable object, or holding a weight stationary. Isometric exercises are seldom used in strength training because they do not mimic the dynamic movement patterns individuals face in their sport. However, weightlifters sometimes isometric exercises to increase the strength in joint angles where force production is the weakest. There are also three more muscle action types with an iso (meaning ”the same”) -prefix: isotonic (constant muscle tension), isokinetic (constant velocity of motion) and isoinertial (constant load). Some of these muscle action types are mostly used in clinical settings and rehabilitation and require specialized equipment.

Different muscles also fulfil different roles  during movement. Agonists are the main muscle(s) responsible for a specific movement. Antagonists are the opposing muscles that either 1) relax to not impede with the contraction, or 2) produce tension as they lengthen to help decelerate concentric actions. The vast majority of movements also require the assistance of synergists, which aid the prime mover and make the movement more efficient, and fixators, which stabilize the bone where the agonist muscle is attached to.

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Types of Muscle Actions

Eccentric muscle action (muscle lengthens)Concentric muscle action (muscle shortens)Isometric muscle action (muscle length remains the same)Isotonic muscle action (muscle tension remains the same)Isokinetic muscle action (movement velocity remains the same)Isoinertial muscle action (resistance remains the same)

Eccentric muscle actions

Eccentric (muscle lengthening) muscle actions occur when the force applied to the muscle exceeds the amount of force the muscle produces. This results in the lengthening of the muscle-tendon system while the muscle contracts. This occurs as the body supports its weight against gravity (e.g. the lowering phase of a lift), when the muscles absorb impact, and when muscles store elastic energy in preparation for concentric contractions (stretch-shortening cycle). Thus, eccentric muscle actions are sometimes referred to as ”negative work” as opposed to the ”positive work” of concentric muscle actions. 

Eccentric muscle actions produce the greatest amount of tension in the muscle (20-50% higher maximal strength than that of concentric contractions). During contraction, this tension stretches the weaker sarcomeres, and places stress on adjacent myofibers, and thus, the entire sarcomere. It has been suggested that mechanical factors (e.g. repeated muscle fiber stress) lead to sarcomere damage during eccentric exercise, and that muscle damage is related to the strain experienced by the muscle – not the amount of force produced. The increased tension and subsequent stress are also why eccentric exercises are often associated with delayed onset muscle soreness (DOMS). To date, the most useful strategy to prevent delayed onset muscle soreness consists of repeating sessions of submaximal eccentric contractions, which progressively increase in intensity over time. 

Eccentric muscle actions also require less motor unit activation and consume less oxygen and energy than concentric contractions. This makes the metabolic cost of eccentric muscle actions approximately four times lower than performing the same exercise concentrically. Due to its ”high force-low energy cost” characteristics, eccentric muscle actions have been of great interest in several scientific fields. Studies suggest that eccentric training can improve neural drive (central motor cortex and brain output in response to stimuli), and thus impact muscular strength, power, and coordination during an eccentric task. Eccentric exercise is also widely used in sports rehabilitation and treatment of tendinopathies (overuse injuries that result in pain, swelling, and impaired function). Finally, eccentric exercises are often prescribed to older individuals with cardiorespiratory problems, sarcopenia, diabetes, and musculoskeletal diseases. Muscle-lengthening exercises have shown positive effects on muscle function, metabolism, insulin resistance and blood lipid profile.

Overall, it is well established that eccentric muscle actions produce significant changes in muscle structure and function.

Concentric muscle actions

Concentric muscle actions refer to movements where the muscle shortens throughout the length of the muscle and produces force. This occurs as the muscle contracts according to the sliding filament theory, which also alters the angle of the joints the muscle is attached to. During positive (muscle produces enough tension to shorten) and negative work, some muscles are in an eccentric phase and others are in a concentric phase (agonist/antagonist).

For example, bending an arm at the elbow requires the concentric contraction of the biceps brachii, which is the prime mover, or agonist muscle for this movement. The antagonist muscle for this movement is the triceps brachii, which lengthens in an eccentric action. The agonist muscle is not always the one contracting concentrically. For example, in a bicep curl, the bicep is the prime mover on the way up (concentric muscle action) and on the way down (eccentric muscle action). 

Although concentric movements have proven effective in increasing strength and muscle mass, muscle-shortening exercises alone do not produce optimal training adaptations. In fact, one must perform twice as many repetitions to have the same training stimulus as movements including both concentric and eccentric muscle actions. It is recommended that the majority of strength training consist of exercises including both muscle action types (e.g. includes a lowering and lifting phases). These are also what human movements naturally consist of.

Isometric muscle actions

Isometric muscle actions describe contractions where the muscle remains at a constant length. This can occur in two ways; 1) when the resistance is equal to the force that the muscle produces (maximal effort), or 2) when the weight is held in a static position. This is also why isometric actions are sometimes called static muscle work. Interestingly, some stabilizing muscles (e.g. postural muscles) mainly perform isometric actions to maintain posture.

Isometric exercises have been shown to significantly increase the tension of the muscle. Because the force production of a muscle changes throughout its range of motion (strength curve), training methods such as supramaximal and functional isometrics can be used to strengthen the muscle at its weakest point (”sticking point”) of the strength curve. Isometric exercises can be especially beneficial for enhancing stabilization, because muscles often contract isometrically to aid in stabilization. Due to their low-impact nature, isometric exercises are also well-suited for rehabilitation purposes as well as for people with osteoarthritis. Isometric exercises also produce little delayed onset muscle soreness delayed onset muscle soreness because the majority of training-induced muscle trauma occurs during the eccentric (muscle lengthening) phase of a movement.

Despite having benefits in strength, muscle mass, and increased bone density, studies suggest they may not be as effective for athletic performance as dynamic movements. Static exercises may even result in lowered performance in explosive, dynamic movements. Isometric exercises also significantly increase blood pressure, potentially leading to blood vessel ruptures and irregular heartbeat. Therefore, isometric training is not recommended for people with high blood pressure or heart conditions.

Isokinetic muscle actions

Isokinetic muscle actions are movements in which the speed of the movement remains constant (as opposed to the constant length and constant load of isometric and isotonic exercises), but the resistance varies. Isokinetic exercises are often used in rehabilitation and require specialized equipment called isokinetic dynamometers. These machines are generally expensive and require extensive background knowledge to operate properly. This is why isokinetic dynamometers are more commonly found in healthcare facilities instead of regular gyms.

The exercise velocity of isokinetic dynamometers is often set between 30-500 degrees per second with slower speeds providing more resistance than faster speeds. The machines even come with interchangeable attachments and built-in safety features allowing for isolated exercises of various muscle groups. The accommodating resistance of isokinetic exercises means that muscles consistently experience overloading. 

Isokinetic exercises offer benefits that other training methods cannot. Because the velocity of the movement is controlled electronically (regardless of how hard the machine is pushed), isokinetic exercises allow for similar high-tension contraction to isometric exercises, with the range of motion of isotonic movements. In short, they provide maximum strength training resistance throughout the entire range of motion of a limb. Whether isokinetic training can improve how fast a person can move a limb through a range of motion is still unknown. In theory, this can be applied to some activities such as throwing.

Despite being a good way of increasing strength, the limitations of isokinetic training (knowledge, cost, convenience, etc.) make it less desirable in comparison to other training methods. Thus, more traditional resistance training methods are recommended for most individuals.

Isotonic muscle actions

Isotonic muscle actions are movements where the resistance on the remains constant throughout the entire range of motion of the exercise. Thus, the weight remains the same regardless of the movement or velocity, which is in contrast to traditional resistance training methods. In most cases, when talking about strength and power exercises, people usually refer to isotonic training or traditional weight training (free weights or machines). 

In addition to offering similar benefits to traditional strength training (muscular strength, hypertrophy, power, and local muscle endurance), isotonic exercises have been shown to improve tendon and ligament strength. Together, these can improve joint stability and help reduce potential injuries. Lifting a weight also forces the bones to support a weight they are previously not accustomed to. This causes more minerals to be stored in the bones, which increases bone density and bone strength. Finally, the increased muscle mass may also increase metabolism, which can aid in maintaining a healthy body composition.

Isoinertial muscle actions

Isoinertial muscle actions refer to movements where the resistance remains constant across the entire range of motion of the limb, and provides a maximal muscle force in every angle. This is achieved with an isoinertial device that uses the inertial force of rotating flywheels generated by the person’s movements. Due to the absence of frictional force, the resistance remains identical in both concentric and eccentric phases of the movement. This allows for a significant eccentric effort with low metabolic cost. Isoinertial exercises were initially used to mitigate neuromuscular dysfunctions and muscular atrophy of astronauts during extended periods in zero gravity.

According to research, isoinertial exercise can increase strength and power in different joint angles. Some studies have also shown positive effects in sports performance (e.g. agility). However, implementing such eccentrically-focused training during a microcycle must be carefully planned due to its effects on the neuromuscular system. These effects can last for up to several days after exercise.

Eccentric muscle actions produce greater increases in protein synthesis, anabolic signaling, and gene expression than other types of muscle actions.

Final thoughts

Human movement is a result of a complex interplay between nerves and muscles. To facilitate the myriad of voluntary movements, the muscles have different properties and specified roles. Different muscle actions include eccentric, concentric, isometric, isotonic, isokinetic, and isoinertial muscle actions. Each have their own characteristics, functions, and induce different training adaptations. 

The main function of concentric muscle action is to initiate movement by shortening the muscle. This prime mover, or agonist, muscle works in combination with the opposing antagonist muscle, which is activated  eccentrically. In addition to agonist and antagonist muscle activation, some muscles work as synergists and/or fixators. Synergists assist the work of the prime mover at a particular joint, making the movement more efficient. Fixator muscles stabilize the bone where the agonist muscle is attached to. This allows the prime mover to achieve maximum and effective contraction.

Did you learn anything new about muscle action types? Let us know in the comments.

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