Which of the following is a common field test used by personal trainers?

The skinfold assessment is a common field test used by personal trainers to estimate body fat percentage. This technique involves using calipers to measure the thickness of skinfolds at specific body sites, which can then be used in various formulas to assess body fat levels. It is a practical method that can be performed in a gym setting with minimal equipment, making it accessible for personal trainers to use with their clients.

Other body composition methods like air displacement, hydrostatic weighing, and dual X-ray absorptiometry, while accurate, are typically laboratory-based and require specialized equipment and environments. This makes them less feasible for personal trainers who are often working in fitness facilities where space and equipment for complex tests are limited. Hence, the skinfold assessment remains a preferred choice among trainers for its convenience and efficacy in tracking body composition changes over time.