You are currently exploring the Fundamentals Library, which is designed to provide a basic overview of the topics that are covered in other longer articles. This article is a part of the Physiology section.
Muscle Action
Every single movement we perform or attempt to perform is done through an interaction of the muscular and skeletal systems. The muscular system is controlled by the nervous system, which generates the motor command and transmits it to individual motor units.
When the action potential signal reaches the muscle fiber, the process of muscle action begins, known as the cross-bridge cycle.
There are two types of muscle tension - active and passive. Active tension (which requires ATP) is generated by the crossbridge cycle (actin and myosin binding).
Passive tension is generated without energy expenditure by stretching structures that resist elongation (stretching beyond resting length) - primarily the molecule titin and the extracellular matrix.
Both active and passive tension are modulated by the length at which the muscle works and the rate at which it shortens or lengthens.
Physiologically, there are a number of different modes of muscle action that have different effects and implications for exercise. The main modes are concentric, isometric and eccentric.
Muscle actions typically correlate with exercise phases (lifting & lowering). It is important to note, however, that muscle action modes refer specifically to the physiological process of generating muscle tension and not to the external result of an exercise (lifting/lowering).
Concentric
When active muscle tension is produced and the muscle shortens (flexes) - we speak of concentric muscle action. This process is mostly dependent on active tension. The calisthenics example of concentric is planche press to handstand.
Because of the way active tension works on a physiological level, concentric force production decreases as the velocity of contraction increases.
Isometric
When active muscle tension is produced and yet the muscle maintains its length (due to opposing forces producing equal torque to the muscle) - we speak of isometric mode of muscle action. This process depends mostly on active tension. The calisthenics example of isometric is the front lever.
Extrapolating from the force-velocity relationship mentioned in the concentric section, isometric action is physiologically more forceful than concentric action.
Isometric contractions can be further categorized into holding and yielding isometrics. Simply put, yielding isometrics involve contraction without the need to maintain a specific external force. An example would be pushing against the pins in a barbell bench press.
On the other hand, holding isometrics involve a scenario where we must apply and maintain a specific external force. An example would be holding the barbell in the center position of a biceps curl or any static calisthenics exercise.
These subtypes of isometric action have interesting physiological differences that are the subject to be discussed in another article.
Eccentric
When active muscle tension is produced and the muscle still elongates (due to opposing forces producing greater torques than the muscle) - we speak of eccentric muscle action. This process depends on both passive and active tension. The relative contribution of passive tension increases as the length of the muscle increases and as the rate of muscle elongation increases. The calisthenics example of eccentric is a negative one arm chin-up.
Thanks to the additional support of passive tension, eccentric action of the muscle fiber generates a greater force than isometric action (almost twice as much).
Thanks to the viscoelastic properties of titin, eccentric force production increases with velocity of elongation. This effect is attributed to the increase in passive stress, despite the simultaneous decrease in active stress (as in concentric action).
Passive Stretch
When no active muscle tension is produced and the muscle is stretched - we call this passive stretching - it is technically not an “action” because there is no active component. In this scenario, all tension is produced passively.
Passive stretching can be either static or dynamic. An example of static passive stretch is German Hang. The analogical example of dynamic passive stretch is Skin the Cat.
The passive tension of the fiber resisting stretch is lower than that of the fiber in eccentric action, because apparently the activation of the muscle fiber changes the conformation of the titin molecule and thus increases its stretch.
Single Fiber VS Muscle
It is very important to note that all of the above states occur at the level of individual muscle fibers. As we know from the process of muscle action, our muscle fibers are activated in a certain order depending on the perception of effort.
This means that when we for example perform a negative part of an exercise, some fibers will act eccentrically and some will be passively stretched. The ratio will depend on the level of perceived effort. The more difficult the task, the more fibers will be active and therefore work eccentrically.
Of course, for the sake of simplicity, we're still using these terms to describe the action of a muscle, but it's important to be aware of the physiology behind it.
For example, when we compare negative and positive training, we don't see as much of a difference in strength as we do between concentric and eccentric force production at the muscle fiber level - we're not twice as strong in the negative as we are in the positive. In fact, if that was the case, people who can barely do a two arm pull up would be able to do a one arm pull up negative).
We are stronger in the negative phase, but only about 30-40%. This is probably due to the activation deficit in the negative phase of the exercise (our body is unable to recruit as many motor units in this phase).
Residual Force Enhancement
An interesting observation in exercise physiology called Residual Force Enhancement is such that isometric performed after preceding eccentric is more forceful than isometric performed after relaxation.
To give you a calisthenics example - It is easier to perform a static hold of a one arm pull up at half ROM if we enter it through a one arm pull up negative than it is to simply stand on something and perform the isometric action to lift off. It is also much easier to enter the back lever position from an inverted hang than from a tuck position or a lift up from a German Hang.
This effect seems to be caused primarily by passive tension, which can contribute to isometric force production when eccentric precedes it. The effect is greater when the muscle is stretched over a greater distance and length.
This is probably one of the reasons why the back lever entry from the inverted hang is much easier than the planche entry from handstand. The back lever involves more muscle length than the planche, and that could make it advantageous. It is however possibly not the best comparison as the motor control is also much harder in the planche)
Isotonic & Isoload & Isoinertial?
When reading sports science or rehabilitation literature, we may come across some similar but different terms such as isotonic, isokinetic or isoinertial. Unfortunately, these terms are often confused and can mean different things depending on the author.
They all refer to exercises or tests where the task is performed dynamically, meaning on a concentric & eccentric basis.
Isotonic exercise is when the tension remains the same throughout the range of motion. This term is difficult to categorize because it is hard to say what "tension" means in this context.
On the one hand, we could have free weight or bodyweight exercises where the weight we're lifting doesn't change throughout the movement or between negative and positive phases. This type of exercise is sometimes called isoload.
Isoinertial exercise refers to exercise in which inertia is constant throughout the range of motion. Most often, this type of exercise involves a device that adjusts the resistance so that it remains the same throughout the range of motion (regardless of positive and negative phases). An example of such an exercise is the Flywheel.
In reality, because inertia is mass - isoinertial exercise is technically any free weight or body weight exercise where the mass we lift remains constant.
Therefore, it seems that isotonic exercise is a term that doesn't really make sense in exercise science, while isoload and isoinertial exercise seem to be exactly the same thing.
Going back to the flywheel, what it does is more like balancing the effects of inertia. In traditional strength training, in a positive (lifting) phase of an exercise, we have to overcome gravity AND accelerate the mass. In the lowering phase, we still have to overcome gravity, but because the weight is accelerating in the same direction as we move, the total force we produce can be less.
On a flywheel, the force we have to overcome will be the same in both phases of an exercise because the acceleration we develop in the first phase determines the acceleration we have to counter in the second phase.
As you can see, it is difficult to work with these terms, and therefore we must be very careful when using certain terminology or when reading about different forms of exercise.
Isokinetic
Isokinetic exercise refers to exercise in which the angular velocity in the joint remains the same despite the torque produced in the joint. This can be achieved using isokinetic dynamometers (such as Biodex). This form of strength testing is the gold standard in rehabilitation and sports.
Below you can see me struggling on the Biodex machine, despite the maximal effort put at every single part of the range of motion, the angular velocity stays the same.
Other Effects
The implications of muscle action modes go far beyond tension production and are also very important to understand from a programming perspective. Since they all have different fatigue effects, but also specificity factors from a strength building standpoint, we need to take the differences between them into account when designing programs.
