You are controlling the movement of the can down. To control the movement requires muscular force. The biceps muscle is contracting, but at the same time it is getting longer. Whenever you are putting something down, or you are controlling how fast something is lowered to the ground, it is done so by eccentric-isotonic contraction of the working muscles. By definition, isokinetic contraction is force applied to a movable object with the speed of movement held constant.
Isokinetic contraction is a relatively new concept in training.
It requires the utilization of a special machine. Most isokinetic devices are used by orthopedic rehabilitation facilities.