"She works so hard, and Ive never really seen anything like it," he tells Women's Health. "You constantly, as a coach, have to stay on your toes and make it challenging."

Clearly, her effort in the gym is paying offher sculpted arm and back muscles are all the proof you need. Here are the rules Jen and her trainer swear by for a strong upper body.

1. She always starts with mobility work.

"You cant just hop into an explosive workout," Azubuike says. "Its very important that you get your mobility work in, and [work on] your range of motion" before you start to go hard.

"Thats where a lot of people get it mistaken, because they feel like if theyre sweaty, and their skin is warm, that theyre actually warmed up," Azubuike says. So before Jen starts a tough upper-body workout, she stretches out with 15 minutes of yoga-based moves and then foam rolls before turning up the heat with cardio.

Here are her go-to warmup moves:

Cat Cow

How to: Start in plank position, holding dumbbells in either hand on the ground. Pull right elbow toward the ceiling until right wrist is near ribs, then lower it down. Repeat on opposite side.

3. She focuses on one move at a time.

While it may be tempting to do a bunch of similar arm moves like triceps dips and triceps extensions back to back, Azubuike says that he breaks his upper-body workouts with Jen into separate sets so she doesn't tire out before she can get all her reps in.

"We want you to feel the burn at the top of the set, like the 12th or 15th rep, but you should always be able to complete it," he says.

4. She sneaks abs into every workout.

Azubuike never finishes an upper body workout (or any workout, for that matter) without throwing in some core-focused moves. "Say its a Monday, were doing triceps. We might slow it down and work some triceps, and then well put a core activity on the end of that," he says.

As for the core moves Jen loves? You guessed it: more planks. "We love to incorporate all the various types of planksforearm plank, low plank, high plank, side plank," he says. "That's one of those exercises that has all the abdominal regions firing together."

5. She challenges herself.

Jen is always looking for ways to level up her upper body workouts. "She's working so hard sometimes, she starts to change the move," Azubuike says. "We might be in a traditional forearm plank, and she might start to do in and outs with her legs. Shes like, 'It's too easy!'"

Even if you don't have plans to hit a red carpet anytime soon, Azubuike says it's important to push yourself wherever you are in your fitness journey. "What a lot of people don't realize is that its not necessarily how often youre working out, but what your actual intensity level is," he says. "Its all about gauging where you are as an individual as opposed to confining yourself to, 'Oh, I can only do this on Monday, Wednesday, Friday.'"

But it's still fair to say that Jennifer Aniston is basically superhuman, according to her trainer: "She's a very, very special athlete."

Jennifer Aniston isn't just driven in the gym. Watch this video to see how she's used that talent to see serious gains on the big and small screens too.