I have just read the article “The Truth About Bodybuilding Arm Measurements” To take accurate measurements, position your upper arm so that it’s parallel to the floor, with the elbow pointing directly ahead of you. Measure the arm at its greatest girth, which should be at the peak of the biceps. Make sure that the “circle” formed by the tape is perpendicular to the ground.
Are you supposed to tense the muscle or relax it, while taking the measurement.
Take the measurements both resting and flexed. More information is usually better and it won’t take you too much time. Its nice to know what they look like flexed, but you spend 90% of your day in the relaxed state, so that is what most people are going to see.
Arthur Jones did that test to a bunch of the competetive bodybuilders long ago. When they remarked about the small difference between the flexed and relaxed measurements, his reply was, “Well, you can’t flex fat!”