I have this protagonist who has has the ability to magically nullify newtons of force when they would harm him. Functionally speaking, this makes the character immune from most forms of damage, both external and internal. While extremes of temperature, suffocation, and starvation can definitely kill him, the character is bullet resistant, knife proof, and can walk off a sledgehammer like it was a Wiffle bat. Furthermore, while the amount of force cancelled can be increased by concentrating, the process is largely automatic. Most people reflexively finch when they're about to get hit, this dude reflexively turns bulletproof.
I've thought of side effects of this trait, such as the character having a surprisingly devastating punch via breaking Newton's third law and possessing a lot of confidence, but there is something I am still worried about. How would this affect his physical development?
This character has been quasi-invincible since he was four and the effect is more than skin-deep, so to speak. With that in mind, what would his physique look like? He's most likely never pulled a muscle or overextended a tendon, but does this also mean he's never been able to build up muscle?
I'm interested in any physical ramifications of this character's kinetic-cancelling in terms of his appearance and physique, but any psychological ramifications would be appreciated as well.
Possibly relevant detail: This character's kinetic-cancelling defense can be overwhelmed with sufficient force and takes time to "recharge". Hit him hard enough (or very frequently very fast), the shield will break, and he becomes a normal human for an hour or so.