A person holding a dumbbell still at 90° of elbow flexion without movement is performing which type of contraction?

The correct answer is isometric contraction. In this scenario, the individual is holding a dumbbell at a fixed position with the elbow flexed at 90°. An isometric contraction occurs when a muscle generates force without changing its length. This means that while the muscles (such as the biceps in this case) are engaged and producing tension to hold the weight, there is no movement at the joint, and the length of the muscle fibers remains constant.

In contrast, isotonic contractions involve movement. With concentric contractions, muscles shorten while lifting a weight, and with eccentric contractions, muscles lengthen while controlling the weight's descent. Isokinetic contractions occur at a constant speed throughout the range of motion, which is not applicable when holding the dumbbell steady. The lack of movement and the sustained tension needed to hold the position firmly characterize the isometric contraction in this scenario.