How muscles move bones

Flexion decreases the angle between two bones at a joint. Bending the elbow brings the forearm toward the upper arm, which is flexion. Extension straightens the joint, and abduction moves a part away from the midline.

• Extension:
• Correct: Flexion:
• Abduction:

Tendons are cords of dense regular connective tissue that attach muscle to bone. Ligaments join bone to bone, and a bursa is a fluid-filled sac that cushions where parts rub.

• A ligament:
• Correct: A tendon:
• A bursa:

A fascicle is a bundle of muscle fibers surrounded by perimysium. Each fiber inside has its own endomysium, and the epimysium wraps the whole muscle.

• A single muscle fiber:
• Correct: A bundle of muscle fibers wrapped in perimysium:
• The whole muscle wrapped in epimysium:

The belly is the thick, fleshy middle of a muscle, between the tendons that run to its origin and insertion.

• Origin: The origin is the attachment that stays still, at one end of the muscle, not its fleshy middle.
• Correct: Belly: Correct. The fleshy middle part of the muscle is its belly.
• Insertion: The insertion is the attachment on the bone that moves, at the other end of the muscle.
• Aponeurosis: An aponeurosis is a flat sheet of dense connective tissue that attaches some muscles. It is not the fleshy middle.

The insertion is the attachment on the bone that moves. When you lift the cup, the forearm (with the radius) swings up while the scapula stays put, so the radius attachment is the insertion and the scapula attachment is the origin.

• The scapula attachment is the insertion: The scapula stays still while the forearm moves, which makes the scapula attachment the origin. Being higher does not decide it.
• The radius attachment is the origin: The origin is the stable attachment. The radius moves when you lift the cup, so its attachment is the insertion.
• Both attachments are origins: A muscle has an origin at the stable end and an insertion at the moving end, not two origins. (Some muscles have several heads at their origin, but they still have an insertion.)
• Correct: The radius attachment is the insertion: Correct. The radius moves, so that attachment is the insertion.

The prime mover (agonist) does most of the work, synergists help or steady it, fixators hold the base still, and the antagonist makes the opposite movement. Roles belong to movements: the same muscle can be an antagonist in one movement and the prime mover in the next.

• Correct: The prime mover is also called the agonist: Correct. Prime mover and agonist are two names for the muscle most responsible for a movement.
• Correct: A synergist helps the prime mover or steadies the movement: Correct. Synergists add force to the same movement or keep it moving in the right direction.
• Correct: A fixator holds a bone still so the prime mover has a stable base: Correct. A fixator is a kind of synergist that stabilizes the bone the prime mover pulls from.
• An antagonist makes the same movement as the prime mover: Incorrect. An antagonist makes the opposite movement; it relaxes and lengthens, often braking, while the prime mover contracts.
• A muscle keeps the same role in every movement it takes part in: Incorrect. The muscle on the back of the arm is the antagonist when you bend the elbow and the prime mover when you straighten it.

The joint (fulcrum) sits between the effort at the back of the skull and the load of the face in front. A fulcrum in the middle makes a first-class lever, like a seesaw.

• Second class: In a second-class lever the load sits in the middle, between the fulcrum and the effort. Here the joint is in the middle.
• Third class: In a third-class lever the effort sits in the middle. Here the muscles pull behind the joint, not between the joint and the face.
• Correct: First class: Correct. Fulcrum in the middle makes a first-class lever.
• It is not a lever: The skull is a rigid bar turning about a joint under a muscle's pull, which is exactly what a lever is.

At balance, effort × effort arm = load × load arm. Effort × 5 cm = 20 N × 30 cm = 600 N·cm, so effort = 600 ÷ 5 = 120 N. The muscle pulls six times the load, because its effort arm is one sixth as long as the load arm.

• About 3 newtons: About 3 N would come from dividing the load by the six-fold ratio. With the effort arm shorter than the load arm, the muscle must pull more than the load, not less.
• 20 newtons: Pulling with the same force as the load would balance only if both arms were the same length.
• Correct: 120 newtons: Correct. 20 N × 30 cm ÷ 5 cm = 120 N.
• 600 newtons: 600 is the product of the load and its arm (600 N·cm). It still has to be divided by the 5-cm effort arm.

Short, angled fibers let more fibers pack side by side into the pennate muscle, so more cross-bridges pull at once and it is stronger. Long parallel fibers have more sarcomeres in series, so the parallel muscle shortens a longer distance, faster.

• Correct: The pennate muscle produces more force; the parallel muscle shortens farther: Correct. Pennate means more force; parallel means more range and speed.
• The parallel muscle produces more force; the pennate muscle shortens farther: This reverses both effects. More fibers side by side gives the pennate muscle more force; longer fibers give the parallel muscle more range.
• They produce the same force because their volume is the same: Force depends on how many fibers pull side by side, not on volume alone. The pennate muscle fits more fibers into the same volume.
• The pennate muscle both produces more force and shortens farther: Short fibers cannot shorten as far as long ones. The pennate muscle gives up range to gain force.

Pollex is Latin for thumb, and pollicis means "of the thumb". Flexor pollicis longus is the long muscle that flexes the thumb.

• Correct: Of the thumb: Correct. Pollicis means of the thumb.
• Of the smallest finger: The smallest finger is digiti minimi, as in abductor digiti minimi.
• Of the chest: The chest is pectoral-.
• With two heads: Two heads is biceps (-ceps = heads).