Introduction
Intensity is the most important aspect of our exercise program. And while have already touched on the topic of intensity in our blog post Intensity safety quadrant, we will dive a bit deeper in this post [1].
In different spheres and fields of physical activity, intensity is expressed through the amount of weights used, duration of the activity, the amount of heat or sweat produced, or through the technical difficulty of a particular activity. While these variables all contribute to explaining intensity, it is not the correct angle or explanation. For that reason, we present another explanation of intensity.
Here we argue that intensity has a relatively simple and more objective measure. It is simply a matter of your effort. And we present the biological concepts that explain what that effort really represents.
Defining Exercise Intensity
As a general term, Intensity can be applied to many fields of study, such as physics, optics and acoustics. In Exercise, Intensity is more defined as a level of effort required to overcome a particular load or stress that was put on our muscles. Importantly, intensity is always measured in relation to our momentary capabilities.
If we can lift something very easily, that action is not intense. If we can lift a resistance very fast, that action is not intense as well. It is only when we struggle with the resistance and the movement is slow (even though we are trying to move as fast as possible), that we can call the particular action intense [2].
Summarizing this section, exercise intensity tells us how much of our own effort is required to overcome resistance.
The Basics of Muscle Physiology
Before we dive into intensity through the lens of biology and physiology, let’s first overview how our muscles contract. The most core concept we need should grasp is the concept of motor units.
A motor unit is a connection between a motor neuron (a specific type of brain cell) and a collection of muscle fibers [3]. Each motor neuron is connected to many muscle fibers through its axon terminals. This means that this motor neuron controls those muscle fibers. Muscle fibers controlled by this motor neuron are of the same type [4].
A collection of all motor units that intervene a single muscle is called a motor pool. In that respect a motor pool coordinates the contraction of a particular muscle. The more motor units there are in a motor pool, the more control we have over the force output of the corresponding muscle. Motor units differ how many muscle fibres they contain.
Concluding both paragraphs, motor units can be of different fiber types and of different sizes.
Different Shades of Motor Units
As mentioned above, a single motor neuron can intervene in multiple muscle fibers, which however are of the same type. Correspondingly, a motor unit can only be of one type. In general, we categorize muscle fibers based on their ability to resist fatigue.
In that regard, we have a binary category of slow fatiguing – Type 1 and fast fatiguing – Type 2 fiber types. Type 2 fibers are also divided based on which energy resource they use, where we have Type 2a using mostly fat while Type 2b mostly glycogen i.e. starch stored in our muscles for energy.
From the perspective of size, we categorize motor units into smaller or larger ones. Smaller motor units are used for more control and skill work, while the larger motor units are activated when higher force output is required.
If we look at the anatomy, we see that muscles used for precision work (eye coordination, wrist flexors, and extensors) have smaller motor units, compared to bigger muscles used for overcoming resistance (legs, chest, finger flexors). In that regard, precision muscles are more densely covered with neurons in relation to the total amount of muscle fibers.
Motor Unit Activation - The Biological Marker of Intensity
Speaking from the biological perspective, Intensity can be described as the number of motor units being activated for overcoming resistance.
When a muscle is overcoming a resistance, motor units are being activated based on the force requirements, sequentially. This means that the amount of muscle fibers is being activated progressively if there is a requirement to do so.
If a certain amount of muscle fibers can deal with the resistance, no further muscle fibers will be activated. And the amount of muscle fibers activated does not change with an increase in speed of movement [2]. Additionally, muscle fiber in the motor unit is active in an alternate fashion. At a given moment, one set of fibers is recovering, while another set is resting.
Only when all the fibers at a given level are insufficient to overcome the resistance a new batch of motor units is being activated by the brain to help to deal with the physiological stress [5]. When motor units fire and activate they can do that for a certain amount of time. Eventually, if persisted, their ability to produce force diminishes i.e. they fatigue.
Inroad - The Effect of Intense Motor Unit Activity
Through the build-up of fatigue, muscle fibers have a diminished ability to overcome the target resistance or, to put it in another way, their ability to produce force is diminished. Inside a motor unit, some fibers are active while some are resting. If other active muscle fibers in the motor unit can not recover fast enough, new motor units will be activated (via our brain) in order to help with overcoming the resistance.
Biologically speaking, we reach the point of momentary muscular failure, when despite all motor units being activated, we still can’t continue to overcome the resistance. At that point, most of our motor units have become fatigued, and we are not able to continue the exercise. This process of the steady build-up of fatigue is called an inroad. When we activate and fatigue motor units, different anabolic stimuli are excreted to promote positive adaptive changes. Because of the different properties of particular fibers, adaptive responses differ based on which muscle fibers were active.
If only the low threshold motor units (units that get activated at the beginning of the fatiguing process) then only those motor units will be stimulated for a change. These motor units are of type 1 which in principle don’t have much potential for an increase in strength or size. In that regard, no concrete life quality-altering improvements can be achieved, if only the lower threshold motor units are being improved.
On the other hand, if we reach a point of momentary muscular failure, we have activated the vast array of our available motor units. In that regard, we get positive improvements in the low as well as high threshold motor units (type 2). And it is the improvements in the Type 2 muscle fibers that lead to longevity and health-boosting benefits related to increased strength and muscle size. It is for that reason that training to (or close to) the point of momentary muscular failure is so beneficial and crucial.
Conclusions
In this blog post we have described and explained the basic mechanism in play that encodes exercise intensity.
As we have argued and others have confirmed, exercise intensity is not related to the speed of movement, total weight lifted, or any other external marker. Exercise intensity is solely related to the effort one is required to put into overcoming the resistance. And the effort is coded in the number of motor units required to be active.
The main point of this article follows, that a high level of motor unit recruitment is only possible if we are reaching the point of momentary muscular failure. And so when we are talking about intense exercise, we should focus on how close we go to the point of muscular failure, and not look at parameters such as speed of movement, the total amount of weight lifted, the complexity of the movement pattern used or how much sweat we produced.
