The following three pictures are expressing what is regarded as the correct posture, and all these pictures are based on a common view in which straightening up one's back along the vertical axis is justified. As far as I know, almost all of experts engaging in medical-and-fitness-related industry appear to have almost same opinions, and I have an opinion that goes against their widely accepted opinion.
While most experts think like above, they also know that the correct alignment of spine should have four arches as you can see below. Pointing up the importance of standing along vertical axis can hinder a person to make correct S-shape on his spine as he will try to straighten his back. Since thoracic spine draws an arch rising backward, trying to straighten one's back will quite go against favorable result; a person who is directed to straighten his back will rather try to stick out his chest, which will make a reverse curve of correct one on his back.
Here are examples of correct postures that can be defined to be so from an athletic viewpoint. Their spines have well curved shape with thoracic spine making backward curve and lumbar spine making forward curve. (center is Cristiano Ronaldo)
So what is the key element to make this posture? A certain muscle is the answer of it - a muscle called psoas major muscle. Psoas major muscles work to bend the hip joint. So if your psoas major muscles contract in the situation of your legs being fixed to the ground, those muscles will cause your pelvis to tilt forward. Black people have bigger psoas muscles than other races, especially compared to Mongoloid race; Mongoloid race particularly has small psoas muscles. That is the reason why black people have hip-up posture.
Comparison of Psoas Major Muscle: Left is Jamaican sprinter and right is Japanese sprinter.
In Recent MLB scene, one-hand follow through has become common. As far as I know, this trend started in the early 1990s, triggered by some sluggers such as Frank Thomas, Juan Gonzalez, and Mark McGwire. Charlie Lau and Walt Hriniak has been deemed to be essential figures who played important roles for prevalence of this technique.
however, I want to cast a doubt on this trend. Alex Rodriguez and Albert Pujols are typical hitters who are employing it in today's MLB, so please look at the photo below. Their arms are shrinking and the wrists are rolling over at the early stage of their swings. Although, one-hand follow through, at first glance, is likely to prevent the wrists from rolling over, the results actually become opposite as you can see below. As a proof of this, by searching for their names on Google images, you will be able to find a lot of similar pictures of their swings.
For extending your arms fully in the follow through, both of the scapula should be abducted, making an arch on your back. One-hand follow through, however, makes you difficult to keep that position because pulling a bat with the bottom hand makes the same-side scapula adducted.
All of these hitters - Kyle Seager, Josh Willingham, Nelson Cruz, Matt Carpenter - always swing without releasing their top hand. This is the real extension.
So let me cite an example, a home run swing by Jack Cust, and here is the movie containing high-speed slow motion. Although his swing looks like a kind of short stroke one, when we look into the swing with a high-speed movie, we can find it to have an excellent extension as you can see below.
So now, I want to create a conceptual term "effective follow through." The term refers to an amount of the force a hitter can add to the ball in his follow through toward the fair zone, especially toward center fielder's direction.
Hitters who do the one-hand follow through seem to have a large follow through at a first glance, but it is no more than optimal illusion that comes from their finish pose. However, a part of the force they can add to the ball in their follow through is going out of the fair zone as they pull their bottom hands toward the back-stop's direction, extremely speaking. In this case, we can say that their effective follow through are not that large.
Next, watch a movie of Alex Rodriguez' tee batting. In this movie he let his wrist roll over too much early on in his swing, and he clearly have an intention of pulling the bat with his bottom hand. This movie shows us that the one-hand follow through and rolling over of the wrist in early stage of the swing are in correlation with each other.
In fact, hitters who employ one-hand follow through don't carry the ball very far even though they can hit considerable number of home runs. On the other hand, some of the hitters with double-handed follow through can carry the ball surprisingly far. Hitters who can hit really long home run are typically employing double-handed follow thorough such as Nelson Cruz, Giancarlo Stanton, Justin Upton, Cecil Fielder, Bryce Harper, and Edwin Encarnacion. Here is a movie I edited, gathering swing examples that I think have optimal double-handed follow through.
I don't mean, however, that you have to swing always with your both hands until the finish of the swing. There are two types that are the most vulnerable to the failures which I wright about in this article: hitters who use always one-hand followthrough, and hitters who intend to make a large follow through by releasing the top hand, extending the bottom hand intentionally - later type swing is particularly prone to cause excessive bottom-hand pull. In fact, the unfavorable tendencies that are shown in the swings with one-hand follow through can hardly be observed in the swings of hitters who sometimes swing with one hand, and sometimes with double hand, such as Mark Trumbo, Jose Bautista, and Prince Fielder. So I want to recommend hitters who use one-hand follow through to do some kind of drills in which they use double-hand follow through for keeping their swing healthy.
