To build your biceps, like any muscle, you'll need to work them through their full contractile range. This means doing a variety of exercises that'll overload them in all positions: shortened, lengthened, and mid-range.
Incline dumbbell bicep curls are what most people use for the lengthened position, which makes sense at first.
Only problem is, they don't overload the lengthened position. At the bottom, the biceps are stretched but there's no lever arm. As you curl, the lever extends further away from the body increasing the amount of mechanical work being done by the biceps.
So, while you may fully lengthen the biceps into a loaded stretch, you haven't overloaded this portion of the range. The mid-range is where most of the work is being done.
The line of pull from the low cable means that the biceps are working hard out of the starting (lengthened) position.
The resistance profile of the lift also does a good job of mimicking that of the muscle throughout the entire range. At the bottom, the lengthened position is challenged. As you curl up into the mid-range (your strongest position), the lever lengthens slightly, then at the top (weaker position) the lever drops off a little.
This makes the exercise extremely effective throughout the entire range because it places optimal levels of tension at each point on the strength curve.
The one drawback of this exercise (compared to the incline dumbbell curl) is a lack of stability.
Sometimes when people first try the low-cable curl, they struggle to keep their shoulders stable. This causes them to start swinging their shoulders or shrugging up, which creates momentum and takes tension off the biceps. Try to "lock the shoulders down" to focus all of your attention on the biceps.
With incline dumbbell curls, you have a bench to drive into and provide artificial stability throughout the shoulders. This makes it easier to focus on only working the biceps. So I often teach people the incline dumbbell curl first to build a habit of putting the biceps in a lengthened position while also programming some scapular stability work. Then they progress to the low-cable curls for enhanced biceps growth once they can control the movement.
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