During the upward phase of a biceps curl, the biceps brachii is primarily performing a:

During the upward phase of a biceps curl, the biceps brachii is primarily involved in a concentric contraction. This type of contraction occurs when the muscle fibers shorten while generating force, which is exactly what happens when the biceps muscle contracts to lift the weight during the curl.

As the forearm is raised, the biceps brachii actively shortens to overcome the load of the dumbbell, resulting in the flexion of the elbow joint. The primary purpose of this action is to generate force that moves the weight against gravity, which is key in the lifting movement of the exercise.

The other types of contractions mentioned do not apply to this phase of the exercise: eccentric contractions involve lengthening while under tension, isometric contractions occur when the muscle remains the same length while generating force, and isokinetic contractions would require a specific speed of movement against a variable load, which is not the case in a standard biceps curl. Thus, in this upward motion, the correct description of the biceps activity is indeed concentric contraction.