You watch Sports Martial Arts Fights and Sparring and you can see people train to brush off kicks to the legs, to the abdomen, and cover their heads with their arms creating a human helmet. Training the body to be resilient by increasing muscle size to protect the bones, training the shin bones to withstand a kick etc. However since it is a sports fighting many of the lethal elements are extracted.
Lethal elements in the technical application sense could just be that instead of a round house kick to the lower body, or to the head, it would be done with the ball of the foot to the temple or the ball of the foot to the groin.
Lethal elements from the tactical application sense could be that followed with the first kick to a well muscle padded area, using that split second distraction and pause the defender is ready his muscles for the impact and the sense of little pain confirm that the strike is over, to the attacker's advantage followed up with a Knock Out strike.
If hands strikes were use, the lethal elements could be the use of a sharp or blunt object that will penetrate any muscle tissue and vital organs. So sometimes in the dark or with the surprise element it is hard to see what is coming at you except the sense of the speed of the motion directed at you.
When Imrich Lichtenfeld the creator of Krav Maga used to say the goal of Krav Maga Trained Individual is not to get hurt - he did not take it lightly. Do not get hurt!