There’s a perfect answer to this question in Vincent Bugliosi’s Reclaiming History: The Assassination of President John F. Kennedy which is the definitive book (over 1,600 pages long and including a CD full of footnotes) about the Kennedy Assassination and answer almost every question or conspiracy concerning JFK’s assassination.
From Reclaiming History:
Dr. John Lattimer, who studied the assassination for years, researched the entire brace issue and concluded it may have been responsible for Kennedy’s death. He writes that Kennedy had “bound himself firmly in a rather wide corset, with metal stays and a stiff plastic pad over the sacral area, which was tightly laced to his body. The corset was then bound even more firmly to his torso and hips by a six-inch-wide knitted elastic bandage, which he had wrapped in a figure eight between his legs and around his waist, over large thick pads, to encase himself tightly…He apparently adopted this type of tight binding as a consequence of the painful loosening of his joints around the sacroiliac area, probably a result of his long-continued cortisone therapy.” The result? When he and Connally were hit by the same bullet, the “corset prevented him from crumpling down out of the line of fire, as Governor Connally did. Because the president remained upright, with his head exposed, Oswald was able to draw a careful bead on the back of his head.” (Lattimer, Kennedy and Lincoln, p. 171; Lattimer, “Additional Data on the Shooting of President Kennedy”, p.1546)
Since the first bullet that struck Kennedy passed through soft tissue and did not penetrate any organ of the body, it was the opinion of Dr. Perry, Kennedy’s chief attending surgeon, that “barring the advent of complications, this wound was tolerable, and I think he would have survived it” (3 H 372). Writer James Reston Jr. captioned his article on this issue, “That ‘Damned Girdle’: The Hidden Factor That Might Have Killed Kennedy (Los Angeles Times, November 22, 2004, p.B9). If this is true, the Japanese destroyer that sunk Kennedy’s PT boat in World War II and killed two of his crewmates, only injuring Kennedy’s already fragile back when he was hurled backwards onto the deck (Leaming, Jack Kennedy, p.139; O’Donnell and Powers with McCarthy, Johnny, We Hardly Knew Ye, p.48), finally claimed Kennedy as its third victim twenty years later.
If there is no certainty as to the role the president’s back brace played in his death, there is something closer to certainty that caused his death and which he himself was responsible for. As indicated earlier, President Kennedy did not want Secret Service agents riding on the steps attached to the right and left rear bumper of the presidential limousine…
The likelihood is high that if Kennedy had not been opposed to Secret Service agents riding on the back of his car — the agent standing on the right rear step would have blocked Oswald’s sight on Kennedy’s head.