Updated: Jul 2, 2020
Before reading this article I recommend you to read this.
Every dynamic movement consists of three parts as it relates to the muscles involved in the movement. We have eccentric(lengthening), isometric(static), and concentric(shortening). Training these three parts individually can be incredibly beneficial for sports performance and is often referred to as Triphasic training. Today I'll discuss the second part, Isometric training.
An isometric action refers to when the joint angle remains constant. It is important to note that the muscle itself does not remain constant as it is still contracting. The tissue might look like static, but if we were to look in a microscope at each muscle fiber, the myosin head keeps on grabbing and releasing the actin. After all, it is called an isometric contraction.
The isometric phase has two physiological processes that need to be trained to maximize the force transfer from the eccentric phase to the concentric contraction. If a muscle needs to increase its level of force production, it has two options:
By increasing the number of motor units recruited(motor unit recruitment) as well as the rate they are firing at(rate coding), the athlete can produce more force in less time, which sport is all about.
Motor unit recruitment
The number one thing that dictates motor unit recruitment is the size principal - "a recruitment pattern based on the size of the motor neuron and the number of fibers it innervates." Think of the recruitment occurring from smallest to largest, and research also show that slow-twitch fiber(type 1) is recruited before the fast-twitch muscle fiber(type 2). The fast-twitch muscle fibers are the ones we want to recruit since they are the muscle fibers behind power production. As the force inflicted on a muscle increases, additional larger motor neurons are recruited. The more force we create eccentrically, the more motor units must be recruited to decelerate and stop the load. The best part about this is that it is trainable. Training isometric lets the athlete respond instantly to increased levels of force by activating larger, higher-end, fast-twitch motor units. This allows more energy to be received into the stretch reflex and the stretch-shortening cycle(SSC), the result of which will be a higher level of rate of force development.
When a muscle needs to build intramuscular tension quickly to overcome an imposed load, it increases the frequency of the signal sent to the nervous system and the motor unit's alpha motor neuron. To clarify, if the nervous system needs to increase the force of a contraction, it increases the frequency of the signal cause numerous contractions to happen quicker. All of these "mini-contractions" together will let the muscles reach their absolute peak force. Just as the motor unit recruitment, this can be trained, and a well-trained alpha motor neuron will signal a dozen of mini-contractions, and the muscle will reach maximal force production, leading to a faster energy transfer.
Imagine a soccer player that wants to dribble their opponent and quickly has to change direction. The player has minimum time to produce an isometric force great enough to completely stop the energy of the eccentric contraction, loading his quads before reaccelerating concentrically and sprinting in the other direction. This action has to occur faster than their opponents if they want to run them off.
How to apply it to your training
With any exercise that we want to apply isometric training to the athlete has to bring the eccentric load to a halt instantly. The athlete who can stop the eccentric load quicker will benefit more with an improved stretch reflex and more energy absorbed to use in the SSC. Any delay between the eccentric contraction and concentric contraction will result in lost energy from the SSC. Since it is crucial to bring the eccentric load to an instant stop, always have someone spotting you when performing isometric training. The main goal of isometric training is to teach the athlete physiological components to absorb energy instantly. Therefore, the athlete must perform the eccentric phase quickly. They must hit the isometric phase like a stone hitting the ground, bringing the weight to a complete stop, fast!
Here are some general parameters for isometric loading:
If an athlete can produce more force, he can apply more force, simple as that. However, it is vital to notice that if you train slowly you'll become slow. Therefore, always maximize the speed during the eccentric and concentric phases. Isometric training is all about energy transfer, turning that eccentric energy into explosive concentric actions. Sport is about producing more force in less time. Strength is not what sport is about. Strength has a role in sport but id ineffective if it can't be applied at high velocities. Therefore an athlete who can transfer more force and at higher speeds will have an advantage compared to someone that can't do it as well.
Example video of a 3 seconds isometric back squat.