14.3 Movement and Locomotion
Movement and locomotion in humans and animals are powered by the coordinated action of antagonistic muscles acting on the skeleton or hydrostatic structures. In humans, movements such as the bending and straightening of limbs or walking rely on skeletal muscle pairs working in opposite directions. In animals, diverse adaptations—such as fish myotomes, bird flight muscles, earthworm body wall muscles with chaetae, and grasshopper hind legs—enable efficient swimming, flying, crawling, and jumping.
Learning Objectives
- Describe the mechanism of movement of the human forearm involving the biceps and triceps.
- Explain the mechanism of leg movement during walking involving antagonistic muscles.
- Explain the mechanism of locomotion in fish involving myotome muscles and fins.
- Describe the mechanism of flight in birds involving the pectoralis major and pectoralis minor muscles.
- Describe the mechanism of locomotion in earthworms involving alternating contractions of circular and longitudinal muscles together with chaetae.
- Explain how a grasshopper jumps or leaps using the flexor and extensor muscles of its hind legs.
Antagonistic Muscle Actions in Human Limbs and Animal Locomotion
Side-by-side illustrated comparisons of human arm bending/straightening and the mechanisms used by fish, birds, earthworms, and grasshoppers.
Human Forearm
Biceps contracts to bend the arm; triceps contracts to straighten it.
Fish
Alternating myotome contractions whip the tail side to side.
Bird
Pectoralis major pulls wings down; pectoralis minor pulls wings up.
Earthworm
Circular and longitudinal muscles work against hydrostatic pressure.
Grasshopper
Flexor folds the hind leg; extensor straightens it to jump.
Short Explanation
Antagonistic Muscles
Antagonistic muscles are a pair of muscles that work in opposite directions. Muscles can only exert force by pulling when they contract; they cannot push when they relax. Therefore, locomotion requires antagonistic pairs where one muscle reverses the action of the other.
Human Forearm Movement
During arm bending, the biceps contracts while the triceps relaxes. The pull force from the biceps is transmitted to the radius through the tendon, pulling the radius upwards and bending the arm at the elbow joint.
During arm straightening, the triceps contracts while the biceps relaxes. The pull force from the triceps is transmitted to the ulna through the tendon, pulling the ulna downwards and straightening the arm.
Leg Movement During Walking
Walking uses the antagonistic pair biceps femoris (flexor) and quadriceps femoris (extensor).
Step 1: The right calf muscle contracts, lifting the heel. The ball of the foot pushes against the ground.
Step 2: The biceps femoris contracts, bending the foot at the knee joint. The right leg is lifted.
Step 3: As the right leg leaves the ground, body weight is supported by the left leg. The quadriceps femoris contracts to straighten the leg.
Step 4: The tibialis contracts to bring down the heel. As the right heel touches the ground, body weight shifts to the right leg. The sequence then repeats for the left leg.
Fish Locomotion
The flexible vertebral column is moved side to side by the contraction and relaxation of myotome (W-shaped muscle segments).
When the myotome on the right side contracts, the left myotome relaxes, whipping the tail to the right. When the myotome on the left side contracts, the right myotome relaxes, whipping the tail to the left.
Alternating waves of contraction and relaxation pass along the body, pushing water backwards and sideways, which propels the fish forward. Fins are used to control movement and direction.
Bird Flight
Flapping of wings is driven by two large, strong antagonistic muscles attached to the sternum (breastbone):
Pectoralis major contracts to pull the wings down (pectoralis minor relaxes).
Pectoralis minor contracts to pull the wings up (pectoralis major relaxes).
Air pressure provides upthrust beneath the wings.
Earthworm Locomotion
An earthworm has an outer layer of circular muscles and an inner layer of longitudinal muscles acting antagonistically against the hydrostatic fluid in the body cavity.
Peristalsis sequence:
1. Posterior longitudinal muscle contracts and circular muscle relaxes; the posterior segment becomes shorter and thicker.
2. Chaetae at the posterior segment anchor firmly into the ground, while chaetae at the anterior segment release their hold.
3. Circular muscle at the anterior segment contracts and longitudinal muscle relaxes; the anterior segment becomes longer and thinner, extending forward.
4. Chaetae at the anterior segment anchor into the ground, while chaetae at the posterior segment release their hold.
5. The shortened posterior segment is pulled forward.
These alternating contractions and relaxations produce a wave of peristalsis along the body.
Grasshopper Jump
Antagonistic flexor and extensor muscles are attached to the inner surface of the exoskeleton of the muscular, long hind legs.
At rest: The flexor on the hind leg contracts, pulling the leg toward the body into a folded Z shape. The grasshopper is ready to jump.
Take-off: The extensor contracts rapidly, straightening the hind leg backwards.
Projection: The catapult-like ejection of the hind legs projects the grasshopper forward and up into the air.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Which muscle contracts to pull the wing of a bird downwards during flight?
What changes occur in the body shape of an earthworm when its circular muscles contract and its longitudinal muscles relax?
During the bending of the human forearm, which bone is pulled upwards and by which muscle?
Common Mistakes
- Confusing which tendon connects to which forearm bone: remembering that the biceps connects to the radius (bending) while the triceps connects to the ulna (straightening).
- Reversing the flight muscles in birds: mistakenly believing the pectoralis minor pulls the wings down because it sounds like “minor”; in fact, the pectoralis major powers the downstroke, and the pectoralis minor powers the upstroke.
- Confusing earthworm muscle actions: forgetting that circular muscle contraction narrows and lengthens the segment, while longitudinal muscle contraction thickens and shortens the segment.
- Confusing the grasshopper resting position: thinking the extensor is contracted at rest, when actually the flexor contracts to fold the leg into the resting Z-shape.
Concept Misunderstandings
Muscles can both push and pull bones or body walls.
Muscles can only pull by contracting; they cannot push. Therefore, locomotion requires antagonistic pairs where one muscle reverses the action of the other.
The fish tail moving to the right is caused by the muscles on the left side contracting.
When the myotome on the right side contracts, it pulls the tail to the right; alternating left-right contractions propel the fish forward.
In walking, the quadriceps femoris bends the leg at the knee joint.
The biceps femoris is the flexor that bends the leg at the knee; the quadriceps femoris is the extensor that straightens the leg.
Summary
- Human Forearm: Biceps contracts + triceps relaxes \(\rightarrow\) radius pulled up (arm bends). Triceps contracts + biceps relaxes \(\rightarrow\) ulna pulled down (arm straightens).
- Human Walking: Calf muscle lifts heel \(\rightarrow\) biceps femoris bends knee \(\rightarrow\) quadriceps femoris straightens leg \(\rightarrow\) tibialis brings down heel.
- Fish: Alternating contractions of left and right W-shaped myotomes whip the tail side-to-side, pushing water backwards and sideways to propel the fish forward.
- Bird: Antagonistic pectoralis muscles attached to the sternum control flight; pectoralis major pulls wings down, pectoralis minor pulls wings up.
- Earthworm: Antagonistic circular (lengthen/thin) and longitudinal (shorten/thicken) muscles work against hydrostatic pressure in waves of peristalsis assisted by chaetae anchoring.
- Grasshopper: Hind legs fold into a Z-shape when the flexor contracts; rapid contraction of the extensor straightens the leg, catapulting the grasshopper into the air.
Short Activity
Objective: test your understanding of movement and locomotion mechanisms.
A. Objective Quiz
Which of the following events occurs when the human forearm is straightened?
How do the antagonistic flight muscles of a bird move the wings upwards?
B. Fill in the Blanks
The W-shaped muscle segments in a fish that contract and relax antagonistically to whip the tail are called .
In an earthworm, when the circular muscles contract and longitudinal muscles relax, the anterior body segment becomes longer and .
C. Matching / Drag and Drop
Match each animal or movement with its correct muscle or locomotion structure. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Muscle / Structure to Its Locomotion Role
Write the correct muscle or structure next to each role.
| Locomotion Role | Muscle / Structure |
|---|---|
| 1. Contracts to pull the bird's wings down during flight | |
| 2. Anchors earthworm segments to the ground during peristalsis | |
| 3. Acts as the flexor to bend the human leg at the knee joint | |
| 4. Contracts to straighten the hind leg backwards in a jumping grasshopper |
Answer choices: Pectoralis major • Chaetae • Biceps femoris • Extensor
