Joints and Movement
Senior 2 · Biology
Joints and Movement
Lesson 2 of 3
22 min
Let's begin
When you lift a bucket of water, do a push-up or raise your hand in class, muscles move your bones. But there is a surprising rule: a muscle can only pull, it can never push. So how does your arm bend and straighten? The answer is that muscles work in pairs.
Worked example
Hold a heavy bag in your right hand and place your left hand on the front of your right upper arm. Slowly bend your elbow. Then press your right palm down hard on a desk, with your left hand on the back of your right upper arm.
Conclusion: a different muscle causes each movement, because each muscle can pull in only one direction.
The muscles that move bones are skeletal muscles. You control them consciously, so they are called voluntary muscles, and under a microscope their fibres look striped. When a skeletal muscle contracts, it becomes shorter and pulls on the bone it is attached to. When it relaxes, it does nothing by itself; it has to be stretched back to its original length by another muscle.
Two muscles that produce opposite movements at the same joint form an antagonistic pair. When one contracts, the other relaxes and is stretched. The muscle causing a movement is the agonist (or prime mover); the opposing muscle is the antagonist. The roles swap when the movement reverses.
| Movement | Biceps | Triceps | Result |
|---|---|---|---|
| Bending the arm (flexion) | contracts | relaxes | forearm pulled up |
| Straightening the arm (extension) | relaxes | contracts | forearm pulled down |
Tendons are tough cords that join muscle to bone. Unlike ligaments, they are not elastic. If they stretched, some of the muscle's pull would be wasted in stretching the tendon; because they do not, the whole pull is passed to the bone.
The forearm works as a lever. The elbow joint is the pivot (fulcrum), the biceps provides the effort, and the object in your hand is the load. Using moments: effort × its distance from the pivot = load × its distance from the pivot.
Worked example
The biceps is attached 4 cm from the elbow. A student holds a 50 N load in the hand, 32 cm from the elbow. What force must the biceps exert to hold the forearm still? Solution: effort × 4 = 50 × 32, so effort = 1600 ÷ 4 = 400 N. The biceps must pull eight times harder than the load, but in return a small shortening of the muscle moves the hand a long way, quickly.
Tip
Biceps Bends; Triceps sTraightens. Muscles pull, never push.
For full marks, describe both muscles: "the biceps contracts and the triceps relaxes". Say "contracts" or "relaxes"; avoid "expands" or "pushes", which are wrong.
Explain why muscles must work in antagonistic pairs.
Solution: A muscle can only pull when it contracts; it cannot push. A second muscle pulling the opposite way is needed to return the bone and stretch the first muscle again.
Describe what the biceps and triceps do as you push yourself up during a push-up.
Solution: The elbows straighten (extension), so the triceps contracts and pulls on the ulna while the biceps relaxes and is stretched.
The biceps is attached 5 cm from the elbow and holds a 20 N load 30 cm from the elbow. Calculate the force the biceps exerts.
Solution: effort × 5 = 20 × 30 = 600, so effort = 600 ÷ 5 = 120 N.
Why is it important that tendons do not stretch?
Solution: All of the muscle's pull is then passed straight to the bone, so movement is efficient and precise.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
