A special device called a dynamometer is required to perform ___ muscle actions.

The correct choice is isokinetic muscle actions. A dynamometer is specifically designed to measure the force or power output of an individual during isokinetic exercise, which occurs when the muscle length changes at a constant speed. In this scenario, the dynamometer provides resistance that adjusts automatically to accommodate the user's strength throughout the range of motion, ensuring that the speed remains constant. This allows for precise measurement of muscle performance and can help in rehabilitation settings or for athletic training where specific data on muscle function is required.

In contrast, concentric muscle actions involve the muscle shortening as it contracts, which can be measured without a dynamometer using other equipment like free weights. Isometric actions, where the muscle length does not change during contraction, also do not necessitate the use of a dynamometer for measurements. Eccentric actions, where the muscle lengthens under tension, can likewise be measured using standard weights or resistance machines. Therefore, isokinetic actions are uniquely suited to the functionality of a dynamometer, differentiating it from other types of muscle contractions.