In one classic study, the diamond push-up (aka triangle push-up) was found to be the most effective exercise for activating the triceps brachii!
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Diamond Push-Ups: Proper Form, Pros, & Mistakes To Avoid
6 Different Kinds of Pushups and Their Benefits
Diamond push-up exercise instructions and video
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How To: Diamond Push-Up
Diamond Push-Ups – Muscles Worked, How-To, Benefits, and Alternatives – Fitness Volt