How muscles work
Muscles are attached to bones via tendons, which can stretch to help deal with forces produced by movement. Muscles often work in antagonistic pairs to control the movement around a joint, such as the arm curl shown here. They can contract in many different ways.
Types of contraction
In strength training, three types of contraction are referred to as isotonic—which is broken down into eccentric and concentric—and isometric. These names describe how a muscle is changing. For instance, isotonic involves a change in muscle length; eccentric contractions involve lengthening of a muscle, while concentric involve shortening. In an isometric movement, a muscle is activated but doesn’t cause any movement, as there is no change in the muscle’s length. (See How Training promotes muscle growth.)
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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.
Concentric Contraction
During concentric contraction, a muscle creates tension while its muscle fibers 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.
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
Muscles can only pull—they cannot push. To that end, they often work in antagonistic pairs. The prime mover, also known as the agonist, works alongside the synergist to create the joint motion. The antagonist, the muscle that opposes the prime mover, helps in controlling the movement on the other side of the joint.
Refining movements
When you first start performing strength training exercises, your nervous system tries to activate both agonist and antagonist at the same time, which results in “choppy” and less coordinated movements. Over time and with practice, your nervous system adapts (see also Strength training and your brain) and coactivation is reduced in the antagonist muscle group, resulting in a smoother and more efficient joint action, as well as more potential force production.
Unraveling a muscle’s structure
Skeletal muscle comprises cylindrical bundles of muscle fibers known as fascicles. Each muscle fiber—also a muscle cell—is constructed from contractile protein filaments that produce muscle contraction. Each muscle also has a vascular network that transports oxygen and chemical substrates for energy production and removes waste generated by muscular contractions.
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Slow- and fast-twitch muscle fibers
There are two main types of skeletal muscle fibers: slow-twitch (or type 1) and fast-twitch (type 2). Your nervous system automatically chooses the right type of fiber for the given exercise. The majority of skeletal muscles have a fairly even split of both types of fiber, allowing for the ability to perform a variety of tasks of different magnitudes and durations.
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Muscle contraction at the microscopic level
The shortening and lengthening of skeletal muscle is achieved by contractile protein filaments in the myofibril—actin and myosin. A nervous impulse triggers a cycle of events within the muscle fiber. The filaments of actin and myosin attach, bend, detach, and then reattach through a repeated sequence to pull the actin filaments toward the center of the sarcomere, creating tension within a muscle.
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THE CYCLE OF CONTRACTION
ATTACHMENT
The activated myosin head attaches to binding site on the actin filament, forming what’s known as a cross bridge between the filaments.
POWER STROKE
The myosin head pivots and bends, pulling the actin filament toward the M line and bringing the Z bands closer together.
DETACHMENT
A molecule of ATP (chemical energy) binds to the myosin head, causing it to loosen its grip on the actin filament; the cross bridge detaches.
REENERGIZING
ATP releases energy to convert the myosin head from its bent position to its upright form, ready for the next cycle of contraction to begin.
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