How muscles work
Muscles control movement, allowing us to do everything from jumping to chewing our food. They are attached to bones by tendons, sections of connective tissue that can resist high levels of tensile force. Muscles work in antagonistic pairs, shortening and lengthening in a cyclical way.
MUSCLE contraction
While under tension, muscles can change length (known as isotonic contraction) or remain the same (known as isometric contraction). Isotonic contractions can be either concentric or eccentric. In a concentric contraction, such as a bicep curl, the muscle shortens while generating force or overcoming resistance. In an eccentric contraction, such as lowering the body in a pull-up, the muscle lengthens while generating force. Eccentric contractions can be voluntary or involuntary.
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ECCENTRIC CONTRACTION
During eccentric contraction, the muscle is lengthening and generating force. Eccentric contraction is stretching under tension that works to “brake” or decelerate movements. Here, the biceps brachii works eccentrically to “brake” the downward movement of the dumbbell.
Antagonist
The biceps brachii allows the extension of the arm
Agonist
The triceps brachii drives the extension of the arm
Extension
Angle of joint increases
Synergist
The brachialis and brachioradialis muscles assist both stages of the arm curl
Concentric Contraction
During concentric contraction, a muscle creates tension while its muscle fibres shorten. As the muscle shortens, it generates enough force to move an object or weight. Here, the biceps brachii contracts concentrically to flex the elbow and lift the dumbbell.
Agonist
The biceps brachii drives the flexion stage
Antagonist
The triceps brachii allows the flexion of the elbow
Synergist
The brachialis and brachioradialis muscles assist both stages of the arm curl
Flexion
Angle of joint decreases
ISOMETRIC CONTRACTION
During isometric contraction, a muscle creates tension without any change in its length. Holding positions involve such contractions. For example, you engage abdominal muscles to stabilize your core, so you can focus on the target muscles of an exercise.
How muscles work together
In an antagonistic muscle pair, one muscle contracts as the other relaxes or lengthens. The muscle that is contracting is called the agonist and the muscle that is relaxing or lengthening is called the antagonist. For example, when you perform a bicep curl, the biceps will be the agonist as it contracts to produce the movement, while the triceps will be the antagonist as it relaxes to allow the movement to occur.
Refining movements
Muscle coactivation is a neuromuscular response. It occurs when agonist and antagonist muscles are activated simultaneously. This type of coactivation kicks in when you are new to training, as the body tries to improve joint stability and movement accuracy. Because of this, your movements may not be the most coordinated or smooth at first. With continued practice you can train your body to “lift” move in a more coordinated way.