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.

Antagonist

The biceps brachii allows the extension of the arm

Agonist

The triceps brachii drives the extension of the arm

Synergist

The brachialis and brachioradialis muscles assist both stages of the arm curl

Extension

Angle of joint increases

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.

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

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.

DKRefining 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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Muscle

Bundle of fascicles

Fascia

Thin casing of connective tissue

Epimysium

Sheath of tissue around a muscle

Perimysium

Sheath of connective tissue surrounding a fascicle

Fascicle

Bundle of fibers (muscle cells) that make up a muscle

Endomysium

Fine tissue layer surrounding a muscle fiber

Muscle fiber

Formed by many muscle cells merged together, ranging from a few millimeters to a few inches in length

Capillary

Brings oxygen-rich blood to the muscle cells

Satellite cell

A muscle stem cell that’s key to repair and growth

Sarcolemma

Plasma membrane surrounding a muscle fiber

Sarcoplasm

Cytoplasm of muscle cell with many nuclei

Sarcoplasmic reticulum

Complex network of tubules involved in storing calcium ions

Myofibril

Rod-like fiber containing filaments of contractile proteins; its arrangement of thin and thick filaments gives a striped appearance

Sarcomere

Basic functional unit of contraction of a muscle fiber; it extends from one Z band to the next

Myofilaments

The contractile proteins in the myofibrils arranged into groups (the thin and thick filaments)

Z band

Anchors the thin filaments and marks the junctions of sarcomeres

M line

Connects the thick filaments

Thin filament

Mainly comprises the protein actin

Tropomyosin

Actin-bonding protein

Thick filament

Comprises the protein myosin

Myosin head

Forms cross bridges with actin during contraction

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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HOW SLOW- AND FAST-TWITCH MUSCLES COMPARE

DK 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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RELAXED MUSCLE

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CONTRACTED MUSCLE

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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