Grades 3–5 · Organisms
How can the same arm bend and straighten?
Bringing a cup toward your mouth bends your elbow; reaching the arm outward straightens it again. Both movements use muscles that pull. A skeletal muscle creates tension and passes that pull through a tough connecting band called a tendon. It cannot push a bone away to reverse the movement.
In the diagramMuscles pull on opposite sides of a jointBiceps helps bend the elbowBicepsTricepsTendonsA tendon transfers a muscle’s pull to boneOther muscles can help, steady or slow the motion.
The elbow acts as a movable meeting place between bones. The biceps, at the front of the upper arm, helps bend it. A tendon passes its pull to the forearm on one side of the joint. Because the pull acts to one side of the turning point, it changes the forearm's angle rather than simply pulling the bones straight together.
The triceps, at the back of the upper arm, pulls through a tendon on the other side of the joint. Its pull helps straighten the elbow. Different tendon routes allow two pulling actions to turn the same joint in opposite directions. Other muscles also contribute to the movements.
The pair is not a simple switch with one muscle always completely off. Muscles can work together to steady the joint or help slow a movement. Holding a cup still may also require tension even though the elbow is not moving. The visible bend or straightening depends on the coordinated pulls and the forces they act against.
About this explanation
Starting knowledge: Know that the elbow is a joint and muscles can pull.
Sources
- OpenStax — Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems