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.
Relative Strength
When we talk about bodyweight strength training / calisthenics, there is really one most important quality that is responsible for our progress, and that is relative strength. Relative strength itself has two components, one component is body weight and the second is strength.
In addition, many movements will include the elements of flexibility and coordination, and some of them will include speed. A helpful way to look at these qualities is to think of them as potentiators of relative strength. In some cases they will be absolute requirements, for example, you can’t really do a stalder press to handstand without a good flexibility. In most cases however, while not mandatory, they will help us achieve a certain level of relative strength.
Our body weight depends on many different factors, and it is generally highly related to strength. Taller people tend to be stronger on an absolute basis and shorter people tend to be stronger on a relative basis. However, the most important aspect is body composition. How much of the body weight we carry is the muscle tissue that contributes to force production, and how much is other tissue (like adipose).
Strength
Looking at strength, there are a number of neurological and morphological long-term physiological adaptations that cause athletes to become stronger. As the names suggest, neurological adaptations include adaptations within the nervous system and morphological adaptations include structural changes to our muscular system.
Motor Unit Recruitment Increases (Voluntary Activation)
The first neurological mechanism of increasing strength is to increase motor unit recruitment. Based on the process of muscle contraction, we know that when we perform a movement, a certain number of motor units within a muscle region are activated, and therefore a certain number of muscle fibers attempt to contract.
Motor unit recruitment is proportional to our perception of effort, meaning that the more difficult the task feels, the more motor units are activated. However, we are not always able to reach all the motor units in the muscle. When we start doing certain exercises, even if we exert maximum effort, we still don't reach the maximum potential of muscle force because some fibers are not activated despite our maximum effort.
This phenomenon is measured by what's called voluntary activation. The force we are able to produce with our will is compared to the force produced by contraction stimulated by extrinsic electrical impulse. If there is a difference, it means that our voluntary activation is not 100% and we are not activating all the motor units that could be activated, there is a recruitment deficit.
If we increase that voluntary activation from, let's say, 90% to 98%, we will get stronger simply by actively contracting previously relaxed muscle fibers. Improvements in motor unit recruitment are specific to the muscle we are working. At the same time, however, it's very transferable between exercises that involve that muscle. If we increase our motor unit recruitment of a muscle in one exercise, it's likely that some of that adaptation will transfer to another exercise that involves that muscle, and we'll get stronger at that exercise as well.
Coordination Increases
The second mechanism is coordination. Coordination is an umbrella term that is quite difficult to define in a simple way. One of the functions of coordination is to improve our strength in a given context. Strength is contextual, and its display depends on the rules we set for ourselves. Becoming more coordinated in a movement has two important effects on our strength.
First, becoming more proficient in a particular pattern will open a door to improving the internal coordination between our muscles. Becoming more coordinated allows us to decrease the activation of antagonists and increase the activation of synergists and agonists that we require in a given exercise.
In various studies that monitor the adaptations of resistance training, one of the adaptations is called antagonist coactivation. It is basically the level of activation we achieve in the muscle that opposes the movement we are making. If we decrease this activation, the net torque we are able to generate increases.
Aside from internal coordination between muscles, becoming more coordinated could also be seen as becoming more efficient at performing a particular task - which can be called external coordination. This efficiency can be achieved by manipulating the technique of the exercise to match our personal strengths and weaknesses, as well as general things that make the task easier.
Of course, all of this must be congruent with the technical standards of the exercise we are following. And obviously, the rules and standards can vary depending on the context. In calisthenics, there is no universal system that would define what the “correct execution” is.
The important thing is that each athlete has their strengths and weaknesses, which can be due to anthropometry or uneven strength proportions between body parts. So while there are some basic rules to make the exercise efficient, there are no universal guidelines that would be very precise.
Coordination is specific to the movement pattern, its speed, resistance and all other components of exercise specificity. Therefore, it is a mechanism that has the potential to make us stronger in a specific exercise, or at most in a variation of that exercise that has a similar pattern.
Hypertrophy & Hyperplasia
Now let's talk about morphological adaptations. The first such mechanism is the increase in muscle size. While muscle size is obviously not the only thing responsible for strength improvements, it is probably one of the most important contributors.
Muscle size can be increased in many ways. If you look at the muscle fiber, it is made up of myofibrils and sarcoplasm. Myofibrils are the structures directly responsible for contraction, because they are made up of sarcomeres. With this in mind, we can distinguish the ways in which a muscle grows.
The two main ways we can distinguish are hyperplasia, which is an increase in the number of muscle fibers, and hypertrophy, which is an increase in the size of the muscle fibers. Hyperplasia is currently only a speculation in research, as there are not really many human studies that would document this process. However, there are several animal studies that give us reason for a possibility of hyperplasia occurring under some circumstances.
Hypertrophy, on the other hand, is a well studied mechanism. However, it is also not fully understood in many aspects. Hypertrophy can have several subtypes. The first factor is the proportional contribution of growth.
Muscle can grow by adding myofibrils or by increasing the sarcoplasmic content. Usually they occur while maintaining proportion, and this is what we would call conventional hypertrophy. In either case, however, the process doesn't have to be proportional. It can also be dominated by one of the growth elements.
If sarcoplasmic content growth dominates, we speak of sarcoplasmic hypertrophy. When myofibril content growth dominates, we speak of myofibril packing.
Hypertrophy also differs in the direction of growth and the area of growth. The addition of sarcomeres can occur either in the proximal or distal part of the muscle.
In addition, we have been talking about the addition of sarcomeres in a parallel direction, but this addition can also occur in a longitudinal direction. This process is then called longitudinal hypertrophy or sarcomerogenesis.
In research, hypertrophy is usually measured by increases in lean body mass, increases in cross-sectional area and muscle volume, or changes in muscle fiber diameter. Specifically, longitudinal hypertrophy is usually measured by changes in fascicle length.
Muscle size is an important contributor to strength. The more sarcomeres in parallel, the more force a fiber can produce, so when a fiber is actually activated by motor unit recruitment, the force it produces is greater. But in addition to the mere addition of contractile material, there is a second reason why larger muscle makes a positive contribution to strength.
As muscle grows, the moment arm of its pull on the bone increases due to changes in the direction of its vector. So even though it produces the same force, it will produce more torque, and torque is what really matters in this context.
Lateral Force Transmission Improvement
However, muscle size is not the only morphological mechanism of strength improvement. The next important adaptation that is not really well studied is the increase in lateral force transmission.Costamers are the structures that connect the sarcomere to the muscle cell membrane. Interestingly, most of the transmission of contractile force that occurs in the sarcomere occurs laterally through costamers.
Most of us would think that it just happens longitudinally (sarcomere to sarcomere till reaching the tendon). Most sarcomeres however don't transmit force to another sarcomere, they transmit force to the endomysium, and that's how the force is transmitted all the way to the tendon. One of the adaptations that probably makes the fiber force greater is to increase the number of costameres.
This hypothetically allows more of the actual force produced to be transmitted to the endomysium and tendon. If you think about it, this mechanism would increase the actual force produced by the muscle without significantly increasing its size. Unfortunately, as mentioned above, the mechanism is not very well studied at the moment. Therefore, increasing the lateral force transmission by increasing the number of costameres is rather a hypothesis that seems likely.
Tendon Stiffness Increase
The last mechanism that is also rarely talked about is the increase in tendon stiffness. Tendons adapt to strength training by getting bigger and stiffer. If a tendon is not stiff it will elongate along with the contraction of muscle fibers.
Therefore muscle fibers will contract at a faster speed than the speed determined by a rotational velocity. Muscle fibers can’t produce as much force when they shorten fast as the force-velocity curve describes.
Increasing the stiffness of the tendon slows down the relative speed of muscle shortening for the rotational velocity and by this interesting feature, it likely increases strength. Obviously, this mechanism will be relevant in specific conditions (not in static exercises).
The Time Frame of Mechanisms
Each of these mechanisms will contribute to an overall improvement as the adaptations occur. However, the proportion of contribution to strength gains from each of these mechanisms will vary based on the relative time frame of an individual's training journey and based on the exercise.
When we train a particular exercise, we may see a lot of improvement in the first few weeks and months. The strength we gain is mainly due to the improvement in our coordination specific to that exercise.
That's because coordination, unlike other mechanisms, improves very quickly. There are many exercises that depend heavily on coordination, and for these exercises, the period of getting better by improving coordination may be much longer.
For example, when we learn the front lever, there is an aspect of coordination that needs to be developed, but since the exercise is not that difficult in terms of motor control, very soon after we have to start gaining strength through other mechanisms.
That's not the case with something like a human flag, where some additional things make that movement much more difficult technically. In the flag, we can improve through coordination for much longer before other mechanisms become more important.
The second mechanism that is used quickly is motor unit recruitment. For most athletes, this mechanism will be very effective during the first month of training.
After this initial phase, morphological adaptations such as tendon stiffness, lateral force transmission, and of course hypertrophy will begin to contribute to the majority of our improvements.
Within the types of hypertrophy themselves, longitudinal hypertrophy seems to be a much more rapid process that plateaus faster, but transverse hypertrophy is a more slow, prolonged process.
This doesn't mean that they don't occur in the first few months of training, but that their contribution to coordination and motor unit recruitment is proportionally smaller to other adaptations.
The reason it's difficult to create a clear model of what happens first and what happens next is that people come to calisthenics from different backgrounds. For example, a bodybuilder may be at the peak of his development in terms of hypertrophy, but completely lacking in the coordination element.
On the other hand, someone who has trained in acrobatics may have mastered the technique of many elements and be generally skilled, but he lacks the muscle mass and development through other mechanisms that would allow him to achieve a higher level of strength in the specific exercises.
