Final answer:
An isometric contraction occurs when the muscle tension increases but the muscle length remains constant, with no movement at the joint, such as holding the elbow at a 90-degree angle with resistance in the hand.
Step-by-step explanation:
Holding the elbow at a 90-degree angle with resistance in the hand is known as an isometric contraction. In isometric contractions, muscle tension increases but there is no change in muscle length or the angle of the skeletal joint. Therefore, despite the increase in muscle tension, the position of the elbow remains the same as the force produced cannot overcome the load.
When the biceps muscle contracts without causing movement, which is what happens when holding the elbow at a 90-degree angle with resistance, it is an example of isometric contraction. Unlike isotonic contractions, which involve either concentric contraction (shortening muscle during a lift) or eccentric contraction (lengthening muscle during a controlled lowering), the isometric contraction involves neither shortening nor lengthening of the muscle fibers.