Been training your biceps to get big arms? You should probably consider training your triceps instead! 1. Triceps make up 2/3 of your arms Triceps are the largest muscle group in your arms, running from the elbow to the shoulder. With a main role to straighten the elbow, triceps are made of three bundles of muscles - which means they have more potential to grow. The most prominent are the lateral and long heads which run from the shoulder to the elbow. The last head, known as