To build bigger arms you need to be sure you are focusing on building bigger triceps. It is said that the triceps occupy two thirds of the arms. Breaking it down even further, the long head of the triceps specifically is responsible for almost two thirds of the size of the triceps. This means that the tricep long head is critical to optimizing the overall size of your arms.
In the following video, Jeff from Athlean-X YouTube channel show you how to increase the size of the long head of the tricep by targeting it both in the stretch position with heavier weights and in the contracted position with lighter weights. Contrary to what some would advise you, you do not only want to train the triceps in positions of stretch for the long head activation. While this is a good differentiator between the function of the long head vs the lateral and medial heads of the triceps it does nothing to place the muscle in a fully contracted state.
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In order to fully contract the triceps you will need to get the upper arm into extension behind the body. This is because the long head is attached to the scapula and crosses the shoulder joint (unlike the other two heads of the triceps). Full activation and contraction of the muscle demands that the arm get into extension behind the body at the end of the movement.
We can do this with the right selection of exercises. Now the important part of this concept is that while you are able to achieve a full contraction in this state, the strength of that contraction will not by optimal. This is due to something called active insufficiency. When a muscle is fully shortened across all of the joints that it crosses (as the triceps is when you straighten the elbow and extend the shoulder behind the torso), then the force production of that muscle is less.
This doesn’t mean that you shouldn’t be trying to include these type of exercises in your training. Again, I am going to argue that they are not only necessary but are needed to complement the heavier exercises that you are doing to take advantage of the stretch reflex that occurs when doing the triceps movements with your arms over your head.
So let’s get a game plan for how you are going to use this information to build bigger arms. First you want to use heavier weights and use exercises for your triceps that will help you to build size by getting your arms over your head. This includes, lying dumbbell triceps extensions, bodyweight tricep extensions, overhead plate extensions and rope pushaways. Each of these movements gets the humerus up and away from the point of attachment of the long head of the triceps on the scapula leading to a stretch on the muscle.
The lighter weights can be reserved for the tricep kickbacks, cobra pushups, rowing tricep pushdowns among others. The key here is again to realize that while you may feel strong in the beginning of these exercises your strength will be limited when you approach complete contraction. Plan on using a weight that is going to still allow you to fully contract as you get towards the end range of motion.
The combination of these two approaches will not only help you to focus your efforts on the long head of the triceps but to build much bigger arms. Because, as mentioned in the beginning of this video, the majority of tricep arm size comes from the development of this head of the tricep muscle you want to be sure you target it as best you can.