Christian Bale, now there's a man who knows a thing or two about yo-yoing weight.
In Bale's case at least, the midriff drift is intentional as the actor man has carved out a niche for himself as the go-to guy for body transformations. He may make the transition from freaking stacked to skinny thin look seamless, but there's a lot of work goes into them weight revolutions.
When Bale played Marky Walhberg's drug-addicted older half-brother Dicky Eklund in The Fighter (for which he won an Oscar trophy), he shed the pounds big time to create an authentically emaciated aesthetic. But this was a piece of cake for Bale in comparison to the lengths he went to in preparation for 2003's The Machinist.

Image: Paramount Classics
If you've seen the film, based on the work of Dostoyevsky (he wrote books), you'll remember Bale's scarily skeletal frame, something he achieved through a diet of "water, an apple and one cup of coffee per day, with the occasional whiskey", losing 62 pounds in the process. Legend (Wikipedia) has it he wanted to go even further, only for producers to stop him because, ya know, he'd die.
However, shortly after filming The Machinist, Bale picked up the role of Batman in Batman Begins, with only six months to get into suitably bulky shape. Now a classic, it was the first Batman film since the 1997 car crash that was Batman & Robin, a film which did for Chris O'Donnell's career and is seared into my consciousness due to its stella awfulness, not to mention Arnie's 'turn' as Mr. Freeze.
With only a short amount of time to get into the caped crusader's costume, Bale took on a high-protein diet and a regime involving heavy-core, plyometrics and resistance training. Ironically, he managed to pack on a whopping 100 pounds, only for director Christopher Nolan to tell him he'd got a bit podge, ordering the actor to drop 20. Bit annoying.

Image: Paramount Classics
Adding to the irony roster was the fact that in bulking up, Bale actually incorporated intermittent fasting and fat loss, with nutrition playing just as much a part in his awe-inspiring transformation as what he got up to in the gym.
However, what he got up to in the gym played a pretty key role too, and here's a similar workout to the one Bale used to get into BatShape:
Day One: Back
Wide Grip Lateral Pull Downs - 3×10
Barbell Rows - 3×10
Shrugs - 4×25
Wide Grip Pull Ups - 3×10 (Can add weight)
Deadlifts - 10-8-5-5-3-Max
Day Two: Arms
Dumbbell Curls - 3×10
Skull Crushers - 3×10
Hammer Curls - 3×10
Tricep Overhead Extensions - 3×10
Seated Concentration Curls - 3×10
Tricep Cable (Rope) Extensions - 3×10

Image: Warner Bros
Day Three: Shoulders
Dumbbell Front Raises - 3×10
Pull-Ups - 3×10 (Can be weighted)
Dumbbell Lateral Raises - 3×10
Dumbbell Shrugs - 3×25
Presses - 10-8-5-5-3-Max
Day Four: Legs
Calf Raises - 25-25-25
Seated Leg Curls - 10-10-10
Weighted Lunges - 20-20-20
Weighted Step Ups - 20-20-20
Front Squats - 10-10-10
Squats - 10-8-5-5-3-Max
Day 5: Chest
Dumbbell Chest Press (Light) - 10-10-10
Cable Flyes - 10-10-10
Decline Cable Flyes - 10-10-10
Incline Bench Press - 10-10-10
Weighted Push-Ups - 25-25-25
Barbell Bench Press - 10-8-5-5-3-Max
Abs and Cardio
Credit: Lionsgate Films
Cardio
20 Minute HIIT (High Intensity Interval Training)
90 Sec. Walk @3MPH
30 Sec. Sprint @8-10MPH
OR
60 Sec. Walk @3MPH
60 Sec. Jog@6-7MPH
Ab Training
3 Rounds of:
50 Crunches
25 Leg Lifts
25 Flutter Kicks
15 Second Banana (Reverse Superman Hold)
OR
50 Crunches
25 Russian Twists
25 Flutter Kicks
30 Second Plank Hold
Now, I know what you're thinking. Fuck. That. Shit.
But, the good news is that unlike Bale, if you do want to follow this regime you're not constricted by time, unless that is Ben Affleck's been dropped from Batman vs Superman (a possibility) and you've been called in to replace him. Sure, it may require more time than you care to give to the gym, but think of it this way: after doing this, Bale was able to beat the shit out of Tom Hardy/Bane (sort of). Now, if that isn't motivation, I don't know what is. To the BatGym!
Words by Ronan O'Shea