The posterior chain is pivotal in fending off injury, building strong posture, and generating strength and explosion for athletes.
As a personal trainer, it’s not often that I receive a request to train the posterior chain. Most clients want to focus on their “mirror muscles”, otherwise known as their abs, chest, and arms. While there is reason to train these muscles, the importance of training and developing the posterior muscles can’t be overlooked.
What is the posterior chain?
The posterior chain is the backside of your body. The primary muscles include the erector spinae, gluteal muscles, hamstrings, gastroc/soleus complex, trapezius, latissimus dorsi, and your posterior deltoids. These all play a pivotal role in developing a strong and resilient foundation of the human body.
Why are these muscles important?
Life is about balance, both inside and outside of the gym. Let’s use a typical 9-5 desk as an example. Many people sit for hours and hours per day often not paying attention to their posture and spinal position. After continually sitting and lead a sedentary lifestyle for an extended period, the body will start to lose its balance.
What I mean by this is that as you continually sit at your desk for extended periods of time your anterior muscles such as your quadriceps and your hip flexors start to shorten, become tight and overactive, whereas your posterior muscles are now no longer firing correctly, affecting their ability to do their job in supporting your back, spine, and pelvis.
The anterior muscles of the body will start to shift the pelvis anteriorly, otherwise known as anterior pelvic tilt. Due to this, you begin to develop lordosis in your spine, placing a lot of stress on your lower back.
Furthermore, as you start to develop this imbalance, strong quads coupled with weak glutes and hamstrings lead to an internal rotation of your femur which will then lead to the inward collapse of your knees (knee valgus), 
which in turn will lead to foot pronation at your ankle. Due to a great deal of imbalance between your anterior and posterior chain, injuries and pain become a concern.
Posture
Just as the posterior muscles of the lower body (gluteal muscles, hamstrings, and erector spinae play a crucial role in developing and maintaining a strong spinal position, the same is true about the posterior muscles that sit superiorly to the gluteal muscles, hamstrings, and erector spinae. 
The latissimus dorsi, or the lats, are one of the widest muscles in the human body, connecting at five different points. Think of the lats as one of the major muscles that pull your shoulders back and down in order to maintain proper posture. Underdeveloped lats and an overdeveloped chest will lead to an imbalance of the muscles leading to rounding of the upper back, otherwise known as kyphosis.
Stronger lats help increase shoulder retraction and stabilization which helps protect your shoulder joints and limit the risk of shoulder injuries. Start incorporating more lat and rhomboid exercises to improve overactive trapezius muscles, a forward head lean, and shoulder pain.
Oh, let’s not forget about the role the lats play in allowing you to do a pull-up or chin-up. Many people have a goal to perform a pull-up or chin-up but don’t know how to get there. Often times think the best way to accomplish this is to continually work out their biceps! In order to perform a proper pull-up or chin-up, you need to be able to properly engage your lats. The lats generate force and strength as well as stabilize and protect your shoulders during this movement.
Reflection
According to theguardian.com, low back and neck pain cost the US alone 88 BILLION a year. That’s third highest bill for any health condition!! THAT IS CRAZY! This is why this is a field I am so passionate about! Often times people convince themselves that there is something wrong with them and they will forever live with pain.
Too many people are looking for immediate relief via the way of painkillers, steroids, and injections which will only subside the pain for a short period of time, instead of assessing the whole body as a kinetic chain and paying attention to certain muscle imbalances that have developed over time. 
I truly believe exercise is the best medicine in the world.
Paying attention to the posterior chain goes a long way in fighting off pain and injuries!
Check out this article on The Kinetic Chain to help differentiate between open and closed kinetic chain exercises! Also, this article on the Transverses Abdominis and Your Abdominal Muscles is also great to help as well.