The upper arm is made up of elbow flexors and extensors. The elbow flexors (biceps) bend your arm while the elbow extensors (triceps) straighten your arm. When you climb, you are constantly pulling and overworking the biceps muscles in the front of your arms. Over time, the biceps muscles overdevelop and the opposing triceps muscles become weak, creating an imbalance. This imbalance can lead to overuse and micro-tears in the triceps muscle group. You should be aware of dangerous movements that can increase the stress on the triceps and eventually lead to pain and injury. These movements include the repetitive use of gastons, mantles and the act of pulling a thick rope through a belay device.