MMA guys, along with most combative sports, walk around with Neanderthal like posture. Because we try to have hunch back posture (shoulders elevated and rounded, chin tucked, rounded upper back) when we box and kick box, we should pay extra attention to our posture outside of training. Having performed the Functional Movement Screen on a lot of MMA and BJJ athletes, I have seen many common limiting factors. Many of these being in the thoracic spine and glenohumeral area.
Other commonalities I have seen with these athletes include:
Poor ankle mobility
Weak hip stabilizers along with poor hip mobility
Poor core/rotary stability
Poor scapular stability
Poor shoulder mobility (commonly a problem because of the mobility problems in the thoracic spine)
Let's face it. The demands of our sport are vicious at times (and some of us aren't as young as we once were). It is in our body's best interest to stay healthy. There is a reason why your shoulder keeps hurting, why your low back keeps hurting and why that left knee keeps irritating you. Take some time to learn some corrective exercises that are for you and then take the time to apply them. Oh yeah, and stop playing rubber guard! Unless you are already hyper-mobile like Eddie Bravo or BJ Penn, stay away from the rubber guard. Your knees will thank you.