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.
The Foundation of Movement & Exercise
The process of muscle contraction is the foundation of movement and exercise. It is how we generate torques and external forces and how we are able to move. Calisthenics, as well as exercises of any kind, can be theoretically reduced to this process. Therefore, it is very important to understand how the process works.
Initiation
Everything related to movement is initiated in the motor cortex part of the brain. The movement is first planned, and then according to that plan, the motor cortex creates and sends an electrical signal related to the movement we have decided to perform. This is often called the central motor command.
The goal of that signal is to reach the motor units that need to be activated for that movement to occur (basically, which muscle fibers need to contract and pull on the bones to produce that motion and the overall movement as a combination of those motions).
The signal sent from the motor cortex passes through neurons in thespinal cord, then through specific nerves, and reaches the neuromuscular junction, which is the connection between the neuron and the muscle fiber within a motor unit.
Motor units are recruited on an all or nothing basis, so an individual motor unit is either recruited or not. The larger the signal, the more motor units in that area will be activated - and therefore more muscle fibers.
Activation of motor units occurs according to Henneman's size principle, which states that motor units are recruited in an order, from smaller to larger units.
Cross-Bridge Cycle (Active Tension)
When the electrical signal reaches the motor unit and the muscle fiber, it passes through the cell membrane until it reaches a transverse tubule, which conducts the signal inside the cell. Here the electrical signal is converted into a chemical signal and the calcium ions are released into the cytoplasm.
The actin molecule senses the presence of calcium ions in the cell and opens its binding side so that the myosin can bind and cause contraction. When myosin heads bind to actin, they use chemical energy from the breakdown of ATP.
Thanks to this energy, they can generate a pulling force against actin filaments (known as mechanical tension), then detach and prepare to bind again. The contraction, in its very micro appearance, is therefore myosin and actin interacting in what is called the cross-bridge cycle.
Cross-bridges cause active tension. Because the sarcomeres are connected end to end throughout an entire muscle fiber, their simultaneous contraction shortens the entire structure.
Passive Tension
We distinguish two types of muscle tension - active and passive. Active tension (which requires ATP) is produced by the crossbridge cycle (actin and myosin binding) that we have already gone through.
But in addition to the active component, muscles also produce tension passively. Passive tension is generated without energy expenditure by stretching structures that resist elongation (stretching them beyond their resting length) - primarily the molecule titin and the extracellular matrix.
Passive tension, therefore, does not contribute much to the process of muscle fiber shortening (concentric contraction). It primarily contributes to the process of muscle fiber lengthening.
According to research, Titin contributes up to 70% of passive tension in single muscle fibers. It behaves like a string that unwinds as it lengthens. As the sarcomere is stretched, titin provides tension. For example, if you do a passive stretch, at the end of the movement the stiff segment of titin will provide tension and oppose the resistance.
On the other hand, if an already active fiber is stretched, we will see an immediate increase in passive force. That's because the calcium ions that are released seem to cause titin to tilt down and the bridge segment to lock onto actin.
This means that as a muscle fiber lengthens, the fibers that are not recruited will provide tension at the further lengths. Muscle fibers that lengthen as they are recruited will have both active and passive tension contributions from the start. This explains why eccentric action is much stronger than concentric action.
