3. Simpson’s most popular head-and-neck restraint systems utilize a chest strap. To properly size for that McAllister also measures Gulledge’s chest just below the pectoral muscles. Don’t try to puff up your chest or make muscles. McAllister says it is better to make the measurement while wearing your driving suit, but if you don’t have it handy they can account for it. Just make sure to let them know whether you were wearing your suit when you made the measurement.