This is an excerpt from ACSM's Body Composition Assessment With Web Resource by American College of Sports Medicine,Timothy Lohman & Laurie Milliken.
By Leslie Jerome Brandon, PhD, FACSM; Laurie A. Milliken, PhD, FACSM; Robert M. Blew, MS; and Timothy G. Lohman, PhD
For the triceps, the subject is measured standing, with the arm hanging freely at her side. The measurer stands behind the subject and places the palm of his left hand on the subject's arm above the mark, with the thumb and fingers directed inferiorly. This is a vertical measurement taken at a point midway on the posterior aspect between the lateral projection of the acromion process and the inferior margin of the olecranon process of the ulna. Using a tape measure, measure the distance between these two points along the lateral portion of the arm with the elbow flexed to 90º (see figure 4.2). The tape is placed with its zero mark on the acromion and stretched along the upper arm, extending below the elbow. Mark the midpoint on the lateral side of the arm. Have the subject straighten and relax the arm at her side, and place a second mark, level with the first, on the midline of the posterior arm. The mark should be at the crest of the skinfold, and the measurement should occur midway between the crest and base of the skinfold (see figure 4.3).
Using a professional-grade skinfold calipers, pressure is applied with the thumb to open the caliper jaws, and the opening is slipped over the skinfold roughly midway between the crest and base of the skinfold (see figure 4.4). The calipers are placed perpendicular to the long axis of the fold as pressure exerted by the thumb on the caliper is gradually released, allowing the jaws of the caliper to close on the fold until the thumb is no longer exerting any pressure. The measured value should be taken between 2 and 4 s after the caliper thumb releases pressure. If the caliper applies force for longer, fluids will begin to exit the tissues within the fold, and the measured value will decrease, resulting in inaccurate measurements. After taking the measurement, the caliper jaws should be opened and removed followed by the release of the skinfold by the left hand. Failure to perform this final procedure may result in a bruising or lacerating pinch to the participant.