Ming-Na Wen was injured while performing stunts as Melinda May in Agents of S.H.I.E.L.D. season 4, so her real-world injury was written into the storyline for season 5.
Agents of S.H.I.E.L.D. season 5 has launched Coulson and his team into a dark dystopian future. The first episode revealed they were transported there by a Kree Monolith, but - in an intriguing twist - they didn't all materialize in the same place. Instead, the S.H.I.E.L.D. agents were scattered across a mysterious bunker, one that had survived the destruction of Earth. What's more, they didn't all emerge from their journey unscathed. Agent May found herself appearing partway through solid matter, with a steel pipe impaling her leg.
Speaking to TVLine, Ming-Na Wen explained the reason for the injury. She was injured in real life while performing stunts for season 4, and blew out her left ACL and meniscus. In April 2017, when the actress attended Wondercon, she was actually brought onto stage in a wheelchair. While the injury didn't affect production of season 4, fans were deeply concerned about whether she would be able to perform in the next season. It clearly hasn't caused any major problems, in part because the writers wrote the injury into the show. In addition, Wen credits "a really great PT, a wonderful brace, and an incredible stunt team that works their choreography to cater to my injury."
May's injury has been a recurring, and important, plot point. Four seasons on, fans are used to the idea that May is essentially unbeatable. She's faced down everything from rogue Inhumans to Asgardian berserkers, and emerged triumphant. The injury has made S.H.I.E.L.D.'s ultimate warrior unusually vulnerable. Although she's still able to handle herself - even neutralizing Kree agents due to her superior skill - May ultimately lost in battle with Sinara, Kasius's best warrior. Fans briefly believed she had actually been killed, but in reality May had been kept alive to participate in Kasius's brutal gladiator matches. Fitz saved her life by manipulating Kasius, encouraging him to send May to the surface.
Now May faces her greatest challenge yet. She's badly injured, weaker than ever before, and stranded on the surface. The atmosphere is thin, meaning she won't be able to breathe well (according to Wen, the show simulated this by using wind machines; it seems when there's little atmosphere, it's always windy). Worse still, she's now being hunted by the 'Roaches, the monstrous creatures introduced in the season 5 premiere. May's used to being the one who does the rescuing, but this time she's desperate for her friends to come to her aid.
Agents of S.H.I.E.L.D. continues with 'Together or Not At All' on Friday, January 12.