The term “functional training” has been defined as everything from performing exercises that mimic real life activities to any exercise that incorporates 3 planes of motion. Some don’t consider an exercise functional unless it involves the use of balls, suspension straps, or wobble boards. When a term is interpreted differently by so many the term itself loses value. The truth is all exercises have value assuming client’s goals, limitations, and ability are considered, but understand that there are risks and benefits to every choice we make and the benefit should always outweigh the risk.
In the case of “less functional” exercises such as a leg extension, bicep curl, calf raise, etc. fewer muscles have to work together to create motion and provide stability and, therefore, there is a decreased chance and incidence of compensation and injury. If you have more planes of motion, it stands to reason that you have more muscles contributing to motion in the multiple planes. The body will get from point A to point B however possible. Therefore, when performing exercises considered “functional” such as squats, deadlifts, pullups, dips, etc. you have more opportunities to compensate, and chances are the body will do so in order to keep going resulting in an increased chance of injury.
If your take away from this is that exercises that integrate multiple joints are dangerous or bad then you’ve missed the point. The point is there is no functional vs. nonfunctional exercise or good vs. bad exercise. There are only appropriate and inappropriate choices, which are determined by the individual. Therefore, any exercise that brings you closer to your goal should be considered “functional” regardless if it looks like the activity you want to improve or is just simply more challenging.