Body Shaping , a program largely about weight training, has frequent appearances by an aerobics instructor, who frequently, during squatting or lunging exercises, has followed the accurate statement about not projecting your knees beyond your toes, with a comment about avoiding knee hyperextension. That is a proper statement. Unfortunately, projecting one's knees beyond their toes is not hyperextension. Hyper-extension of the knee is an unnatural movement characterized by the knee bending backward, as from an impact like a football tackle. A knee that is normally bent is flexed, not extended.
Fitness Beach is a largely aerobic exercise program, which occasionally dabbles in weight training.
There are specific exercises to isolate training to the front, middle, or rear deltoid, but all are just different parts of one muscle. The deltoid muscles have no heads. The deltoids are single muscles, without heads. None of these issues is a big deal, but they are not precisely accurate, but now you know the real details. This information is neither complete nor exhaustive and should not be used as a substitute for a consultation with your physician or other health care provider.