Tang Ziyi's research on physique strengthening
Actually, Ling Qingyu's worries were too superfluous. The chances of her guards suffering these damages were close to zero with Tang Ziyi equipping them prior. Especially, the materials were very lightweight.
In fact, the real damage came from long-term strain on the bodies, wearing body armor for so long during the patrol and sentry mission. A similar effect could be seen in Olympic athletes, where many grew older than they were supposed to and their bodies' capability dropped after using their full potential.
Likewise, the long-term burden of weight from above would have impacts on the bodies, creating hidden diseases for the soldiers. As seen in astronauts, living in outer space for a longer period of time, they had problems with their backs and spines.