Final answer:
Several studies have examined muscle activity during various suspension training plank exercises, including studies comparing energy expenditure estimation methods and studies on how substrate stiffness regulates muscle activity.
Step-by-step explanation:
Muscle activity during various suspension training plank exercises has been examined in several studies.
In one study by JW Vezina, it compared two methods of estimating energy expenditure in resistance training activities, including plank exercises.
Another study on cultured cells plated on flexible substrates showed that substrate stiffness can regulate muscle activity, including myofibril assembly.