This is an excerpt from Strength Zone Training by Nick Tumminello.
Biceps exercises in the lengthened-to-midrange strength zone generate the most mechanical tension on the biceps closer to the bottom of a biceps curl, when your elbow is closer to being straight. As an example, see the EZ-bar preacher curl in figure 7.1. Whereas biceps exercises in the shortened-to-midrange strength zone create the most mechanical tension on your biceps closer to the top of the biceps curl, when your elbow is closer to being bent.
During any style of biceps curl, the point at which the musculature is maximally loaded is when the lever arm is at its longest. This occurs when your forearm is at a 90-degree angle to the line of force.
If you’re using free weights, gravity is the line of force. Therefore, the point of maximal mechanical tension on your biceps is when your elbow reaches 90 degrees of flexion or when your forearm is parallel to the floor. As an example, see the EZ-bar biceps curl in figure 7.2. If you’re doing biceps curls using a cable column, the cable itself is the line of force. In this case, the lever arm is at its longest when your forearm makes a 90-degree angle with the cable.
Although biceps exercises in each strength zone category involve the same motion of elbow flexion, they each train an important complementary aspect of biceps strength that is neglected by the other. Therefore, using the two strength zones as your biceps exercise checklist ensures you make smart exercise choices and get a better, more well-rounded biceps workout.