Skeleton, Muscle and Movement

Lessons

Senior 6 · Biology

Skeleton, Muscle and Movement

4 lessons

Lesson 4 of 4

Vertebrate joints and locomotion

30 min

Weak levers, fast hands

Hold a bag of sugar in your hand with your elbow bent at a right angle. The bag weighs about 10 newtons, yet your biceps is pulling with roughly ten times that force. That sounds like poor design, until you notice what you gain: a small shortening of the biceps swings your hand through a large arc, very quickly. The joints and muscles of a vertebrate are a set of levers that trade force for speed and range. This lesson looks at how joints are built, how antagonistic muscles work them as levers, and how fish, birds and humans use them to move.

Definition

Joint (articulation): a place where two or more bones meet.

Synovial joint: a freely movable joint in which the bones are separated by a cavity filled with synovial fluid and enclosed by a capsule.

Ligament: a band of strong, slightly elastic connective tissue that joins bone to bone and holds a joint together.

Tendon: a cord of tough, almost inelastic collagen fibres that joins muscle to bone and transmits its pull.

Origin and insertion: the origin is the end of a muscle attached to the bone that stays still; the insertion is the end attached to the bone that moves.

Flexor and extensor: a flexor decreases the angle at a joint (bends it); an extensor increases it (straightens it).

Lever: a rigid bar that turns about a pivot (fulcrum); in the body the bone is the bar, the joint is the fulcrum and the muscle supplies the effort.

Kinds of joint

Joints are classified by how much they move, which depends on what joins the bones:

Fibrous (immovable): bones joined by dense fibrous tissue, for example the sutures between the bones of the skull.
Cartilaginous (slightly movable): bones joined by cartilage, for example the intervertebral discs between vertebrae and the pubic symphysis at the front of the pelvis.
Synovial (freely movable): most joints of the limbs.

Part	Function
Articular (hyaline) cartilage	Smooth, slightly compressible surface: reduces friction and absorbs shock
Synovial fluid	Lubricates the joint and nourishes the cartilage, which has no blood vessels
Synovial membrane	Secretes synovial fluid
Fibrous capsule and ligaments	Hold the bones together; ligaments allow movement in some directions and prevent dislocation

Synovial joints differ in the shape of the bone ends, which decides the movements they allow:

Type	Movement allowed	Example
Ball and socket	In all planes, and rotation	Hip, shoulder
Hinge	In one plane only (flexion and extension)	Elbow, knee, joints of the fingers
Pivot	Rotation about one axis	Between the first two vertebrae (atlas and axis), turning the head
Gliding (plane)	Small sliding movements	Between the small bones of the wrist and ankle
Saddle	Two planes, no rotation	Base of the thumb

Antagonistic muscles and levers

Each joint is worked by at least one antagonistic pair. At the elbow, the biceps has its origin on the scapula and its insertion on the radius just below the elbow; when it contracts it flexes the forearm and stretches the triceps. The triceps, inserted on the ulna behind the elbow, extends the forearm and stretches the biceps. The two never contract fully at the same time, except when the joint is being held rigid.

Every limb movement is a lever. There are three classes, depending on where the effort and the load lie relative to the fulcrum:

Class	Arrangement	Example in the body	What it gives
First	Fulcrum between effort and load	Nodding: the skull pivots on the atlas; neck muscles at the back balance the weight of the face	Balance; force or speed depending on distances
Second	Load between fulcrum and effort	Rising on tiptoe: fulcrum at the ball of the foot, body weight through the ankle, effort from the calf muscle at the heel	Force greater than the effort
Third	Effort between fulcrum and load	Flexing the elbow: fulcrum at the elbow, biceps inserted close to it, load in the hand	Speed and range of movement, at the cost of force

Worked example 1: the force in the biceps

Using Figure 2, calculate the force the biceps must exert to hold a 3.0 kg load still with the forearm horizontal. Take $g = 9.8 \text{ N kg}^{-1}$ and ignore the weight of the forearm.

Load: $W = mg = 3.0 \times 9.8 = 29.4$ N.
At balance, clockwise moment = anticlockwise moment about the elbow: $F \times 0.040 = 29.4 \times 0.35$ .
$F = \frac{29.4 \times 0.35}{0.040} = 257 \approx 260$ N (2 s.f.).

Interpretation. The biceps pulls with almost 9 times the weight of the load. In exchange, because the hand is $\frac{35}{4.0} \approx 9$ times as far from the fulcrum as the insertion, the hand moves about 9 times as far, and 9 times as fast, as the point where the biceps pulls. A third-class lever is built for speed and range, not force.

Worked example 2: rising on tiptoe

A pupil weighing 600 N stands on tiptoe on one foot. Measured horizontally from the ball of the foot (the fulcrum), the body's weight acts through the ankle 12 cm away and the Achilles tendon pulls up on the heel 17 cm away. Calculate the force in the Achilles tendon and the mechanical advantage of this lever.

Moments about the ball of the foot: $F \times 17 = 600 \times 12$ .
$F = \frac{600 \times 12}{17} = 424 \approx 420$ N (2 s.f.).
Mechanical advantage $= \frac{\text{load}}{\text{effort}} = \frac{600}{424} = 1.4$ .

Comparison. In this second-class lever the load lies between the fulcrum and the effort, so the effort arm is longer than the load arm and the muscle needs less force than the load. Compare the mechanical advantage of the elbow in Worked example 1: $\frac{29.4}{257} = 0.11$ .

Locomotion in three vertebrates

Fish: swimming. A bony fish's trunk muscles are arranged in W-shaped blocks called myotomes, attached to a flexible backbone. Myotomes contract one after another down each side, and alternately on the left and right, sending an S-shaped wave along the body towards the tail. The tail and body push sideways and backwards against the water, and the water pushes the fish forwards. The fins steer and stabilise: the dorsal and anal fins stop the fish rolling and yawing (swinging from side to side), and the paired pectoral and pelvic fins control pitching (tipping nose-up or nose-down) and act as brakes. Many bony fish adjust their depth with a gas-filled swim bladder.

Birds: flight. Flight muscles may make up a large share of a flying bird's body mass. The large pectoralis muscle runs from the deep keel of the sternum to the underside of the humerus and pulls the wing down in the power stroke. The smaller supracoracoideus also lies on the sternum, beneath the pectoralis, but its tendon passes up through a canal at the shoulder, like a rope over a pulley, to the top of the humerus, so it raises the wing. A rigid, fused skeleton, hollow bones braced inside by struts, and the downstroke pushing air downwards and backwards together make flight possible.

Humans: walking on two legs. Walking is a repeated cycle in which each leg has a stance phase, carrying the body weight, and a swing phase, moving forward. In stance, the calf muscles extend the ankle and push off from the toes, while the quadriceps keeps the knee from buckling. In swing, the hip flexors and hamstrings swing the leg forward and bend the knee to clear the ground. The gluteal muscles of the hip stop the pelvis from dropping on the side of the swinging leg. Because the body vaults over a stiff leg in each stance phase, walking uses little energy for the distance it covers.

Warning

Common mistakes

Mixing up ligaments and tendons. Ligament: bone to bone, slightly elastic. Tendon: muscle to bone, inelastic, so that the whole of the muscle's pull reaches the bone.
"The triceps pushes the forearm down." The triceps pulls on the ulna behind the elbow, which straightens the arm. No muscle pushes.
"A lever with a mechanical advantage below 1 is badly designed." It gives speed and range of movement; most limb levers are third class for this reason.
Safety. A suspected dislocated or broken joint should be kept still in the position found and medical help sought; do not try to push it back into place.

CrashCourse: "Joints" (Crash Course Anatomy and Physiology #20). Watch for the three structural types of joint and the movements each kind of synovial joint allows.

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Practice: joints and locomotion

Give one example each of a fibrous, a cartilaginous and a synovial joint.
State the function of (a) synovial fluid, (b) articular cartilage, (c) ligaments.
Explain why a tendon must be inelastic.
Name the type of synovial joint at (a) the hip, (b) the knee, (c) the joint that lets you shake your head "no".
Describe how the biceps and triceps act as an antagonistic pair to bend and straighten the arm.
When you nod, the weight of the head (50 N) acts 2.0 cm in front of the atlas joint, and the neck muscles pull 5.0 cm behind it. Calculate the force needed in the neck muscles to hold the head level, and name the class of lever.
Explain how the dorsal fin and the pectoral fins help a fish to swim in a straight line at a steady depth.
Explain how a muscle on the underside of a bird's wing, the supracoracoideus, is able to raise the wing.

Show answersHide answers

Fibrous: sutures of the skull. Cartilaginous: between vertebrae (intervertebral discs) or the pubic symphysis. Synovial: any of hip, shoulder, knee, elbow, fingers.
(a) Lubricates the joint and nourishes the cartilage. (b) Reduces friction and absorbs shock. (c) Hold bone to bone, keeping the joint stable and preventing dislocation.
So that all of the muscle's contraction is passed to the bone; a stretchy tendon would absorb the pull and the bone would move less, and less precisely.
(a) Ball and socket. (b) Hinge. (c) Pivot.
Biceps contracts (and triceps relaxes and is stretched): the forearm is pulled up, flexing the elbow. Triceps contracts (biceps relaxes and is stretched): the forearm is pulled down, extending the elbow.
$F \times 5.0 = 50 \times 2.0$ , so $F = 20$ N. First-class lever (fulcrum between effort and load).
The dorsal fin, like a keel, resists rolling and yawing, so the fish keeps upright and straight. The paired pectoral fins can be angled to control pitching, so the fish stays level, and they act as brakes.
Its tendon runs up over the shoulder through a canal, like a rope over a pulley, and attaches to the top of the humerus; when the muscle shortens, it pulls the top of the humerus and lifts the wing.

Key points

Joints may be fibrous (immovable), cartilaginous (slightly movable) or synovial (freely movable).
Synovial joint: articular cartilage, synovial fluid and membrane, fibrous capsule, ligaments. Types: ball and socket, hinge, pivot, gliding, saddle.
Tendons join muscle to bone; ligaments join bone to bone. Antagonistic pairs (flexor and extensor) move each joint.
Limbs are levers: most are third class, trading force for speed and range. Use moments (force × distance from fulcrum) to find muscle forces.
Fish swim by waves of myotome contraction, with fins for stability; birds fly with the pectoralis (down) and supracoracoideus (up); humans walk in stance and swing phases.

1Weak levers, fast hands

2Kinds of joint

3Antagonistic muscles and levers

4Locomotion in three vertebrates

Weak levers, fast hands

Kinds of joint

Antagonistic muscles and levers

Locomotion in three vertebrates