Types of joint, articulating bones, main agonists and antagonists, types…

The idea

At a joint, bones meet and movement occurs about an axis. A synovial joint like the elbow is a hinge joint: the humerus, radius and ulna articulate there, allowing flexion and extension. At the shoulder, a ball-and-socket joint, the humerus articulates with the scapula. During a movement, the agonist contracts to produce the action, while the antagonist relaxes to allow it. Muscle contraction is isotonic (concentric or eccentric) or isometric. In the upward phase of a biceps curl, the biceps brachii is the agonist working concentrically. In the downward phase, the biceps brachii remains the agonist, working eccentrically to control the descent against gravity, while the triceps brachii remains the antagonist.

What earns the marks

• Identifies a named joint type, such as hinge at the elbow or ball-and-socket at the shoulder, and links it to the movement it permits.
• Names the articulating bones at a stated joint, for example humerus, radius and ulna at the elbow, or humerus and scapula at the shoulder.
• Defines the agonist as the muscle that contracts to produce or control the movement and the antagonist as the muscle that relaxes to allow it.
• Applies the agonist–antagonist pairing to a named action, such as biceps brachii as agonist and triceps brachii as antagonist during elbow flexion.
• Recognises that during the downward phase of a movement controlled by gravity, the agonist remains the same but works eccentrically.
• Distinguishes isotonic contraction (concentric and eccentric) from isometric contraction, where tension develops without joint movement.

Common mistakes and how to put them right

• Naming the antagonist as the muscle that produces the movement.

  Put it right: The antagonist relaxes and lengthens to allow the action, while the agonist contracts to produce it.

• Assuming agonist and antagonist roles reverse during the downward phase of a lift.

  Put it right: In a biceps curl's downward phase, the biceps remains the agonist working eccentrically to control gravity.

• Listing bones that do not articulate at the stated joint, such as naming the radius and ulna at the shoulder.

  Put it right: At the shoulder the humerus articulates with the scapula.

• Confusing joint type with movement.

  Put it right: A hinge joint permits flexion and extension in one plane, whereas a ball-and-socket joint permits movement in several planes.

Exam tips

• Read the stem carefully to identify which joint and which phase of movement are being asked about before choosing an option.
• For agonist–antagonist items, decide the direction of movement and the role of gravity first, then assign the controlling muscle as agonist.
• Check each option against the definition of the joint type or contraction type rather than choosing the longest or most familiar-sounding option.
• Where an option names bones, confirm that every named bone genuinely articulates at the joint given in the stem.