In the martial arts world it is a well established fact that straight punches are highly effective. While they may not have the same power as say, a rear hand hook, which has all of the rotational power of the core behind it, they are the quickest way to bring a fist to an opponent's face. A straight punch that comes directly down the center has much more power than a sloppy straight punch. Perfect straight punches also seem to sap the endurance of an athlete much less. From a human movement perspective, there is a good reason for this.
Before I get into it, I will need to introduce you to four words that you may or may not have heard before, as they pertain to the shoulder joint: abduct, horizontally adduct, flex, and internally rotate.
Abduct: When the bone of the upper arm moves laterally away from the body
When a person lifts their arm out to the side, the shoulder joint is abducting.
Muscles used: supraspinatus, deltoid, upper pectoralis major (upper pec)
Horizontally adduct: When the bone of the upper arm, when held out to the side, moves toward the center line of the body.
When a person is holding a bar on their chest, and then presses it up, the shoulder joint is horizontally adducting.
Muscles used: Frontal deltoid, Pectoralis major (pecs) , coracobrachialis
Flexion (flex): When the bone of the upper arm is raised in front of the body.
When a person lifts their arm to grab a door handle, their shoulder joint is flexing
Muscles used: frontal deltoid, upper pectoralis major, coracobrachialis
Internally rotate: When the bone of the arm is rotated in, toward the body.
When a person rotates their entire arm from palm up, to palm down their shoulder joint is internally rotating.
Muscles used: Latissimus Dorsi (lats), Teres major, Pectoralis Major, subscapularis
For my purposes here the punch I will focus on is the rear hand straight and the area of the body I will focus on is the shoulder joint. Here is a short video explaining these movements in the context of a punch.
When new students are learning the rear hand straight, they often find it hard to drive their fist forward while keeping their elbow tight to their body.
They tend to abduct their shoulder joint, causing their elbow to move away from their body. Next they horizontally adduct their shoulder joint and extend their elbow. Horizontal adduction allows the student to use two shoulder girdle muscles, the serratus anterior and the pectoralis minor, and this may be one reason that they tend to punch like this, in order to get more preceived power into their punch.
The main problem with allowing the elbow to move away from the body (shoulder abduction) is that this movement does nothing to add to the power of the punch. All of the energy used to move the elbow out is wasted, and then more energy must be used to bring the elbow back toward the center line of the body (horizontal adduction). So when a student punches in this manner, the only muscles of the shoulder joint that are being used to propel the fist forward are the front deltoid, the pecs, and the coracobrachialis. The shoulder girdle muscles serratus anterior and pectoralis minor are also being used.
Now let us look at a proper rear hand straight. The first movements that occur together are elbow extension and shoulder joint flexion. The flexion propels the fist straight towards its target. As the first travels forward, the shoulder joint internally rotates. This movement causes the fist to "turn over" as well as the "shoulder to impact the jaw" two terms which I constantly hear trainers talking about. The internal rotation is important because of the affect it has on the shoulder girdle. This internal rotation allows the serratus anterior and pectoralis minor to be used toward the end of the punch.
In this way both the flexion and internal rotation of the shoulder joint lend their power to the punch, so their is no wasted movement in the shoulder joint. The muscles of the shoulder joint that are helping to propel the fist forward are the frontal deltoid, pecs, coracobrachialis, teres major, lats, and the subscapularis. The internal rotation of the shoulder is also allowing the shoulder girdle muscles, the serratus anterior and pectoralis minor to aid in the punch without wasting any movement and with perfect alignment of the bones of the arm so that the full power of the punch can be transferred to the target.
As can seen from focusing on the shoulder joint when a punch is thrown correctly, from begining to end, all of the energy being expended is used to propel the fist forward. When a straight punch is thrown incorrectly, there is a large amount of wasted energy that is being used for no purpose whatsoever. From a kinseological perspective, this is why it is important to keep your elbow tight when you throw a straight punch.