Jennifer Garner
Credit: Bauer-Griffin/GC Images

InStyle’s October cover girl Jennifer Garner may have spent a few months filming her upcoming Disney family comedy Alexander and the Terrible, Horrible, No Good, Very Bad Day. But she’s spent the last 15 years doing Intense, Challenging, So Tough, Very Hard Workouts. “Her work ethic is just incredible,” says Valerie Waters, her trainer for the last decade and a half, a relationship that dates all the way back to Garner’s Alias days.

Four or five days each week, the two meet at the actress’s L.A. home well before dawn—Garner calls Waters her “sunrise sister”—and bang out 45 minutes to an hour of circuits (depending on when the kids wake up, or whether Garner’s hubby, Ben Affleck, crashes the workout). Here Waters breaks down their exact routine so you, too, can get that butt-kicking bod. “Consistency is what yields results in the long run,” advises Waters. “Even if you only have a half hour, maximize the time. The idea of getting on a treadmill for 30 minutes is daunting. When you do strength training circuits like this, it keeps you more engaged.”

Warm Up

“Prepare your body for movement,” says Waters. “Do some dynamic stretching, like leg swings, plié squats, lunges and downward dogs. Notice where your sticky points are and breathe into those areas. This is your chance to check in with yourself: what am I feeling? What hurts? What should I be aware of?” When you’re limber, you’re ready for the circuits, two groups of exercises that are each performed three times in succession.

Circuit 1

Perform these exercises with no rest between each.

1. Band side step. Place a stretch band loop around your ankles and step side to side 20 times in each direction. You’ll feel this in the glutes.

2. Body-weight squats. Keep your weight in the heels as you shift your rear back into a seated position, then return to standing. 20 reps.

3. Reverse lunge with forward kicks. Step back into a reverse lunge then bring that leg forward and kick. Repeat with the other leg. Do 20 reps for each leg.

4. Row. You can do this with dumbbells or stretch bands. Focus on the middle part of the back (this is great if you work at a computer all day!). 20 reps.

5. Pushups. Lower your knees if necessary but try for a full plank start. 10–12 reps.

6. Body saw. Jen likes to use Waters’ Valslides (pictured below), a clever, portable take on a slide board, for these . With your feet on the slides (or improvise the same principle with towels underfoot if you have hardwood or tile floors), assume a forearm plank position. Push your feet to travel away, then pull yourself back for—you guessed it—20 reps. You control your range of motion and difficulty.

Rest for 30 to 60 seconds, then repeat the entire Circuit 1 sequence a total of three times.

Rest for one minute, then begin Circuit 2, which continues below the photo.

Credit: Courtesy Photo

Circuit 2

Perform these exercises with no rest between each.

1. Single leg step-ups. Start with both feet on the ground, then step up onto a stair or low bench and step back down. Repeat with the other leg and so on for 20 reps each leg. The higher the step, the harder it is (remember to tighten your core!).

2. Fire hydrant with a kick. On your hands and knees, lift bent leg out to the side and return. Change legs and repeat, 20 reps each.

3. Donkey kicks. On hands and knees, kick leg back, pushing through the heel and squeezing the glute, 20 times. Change legs.

4. Single-leg bridge. Lay on your back, feet on the floor. Lift pelvis, then extend one leg toward the sky. Raise and lower pelvis with raised leg 20 times. Change legs and repeat.

5. Side plank. With feet stacked (optional: place them on a Valslide) and body lifted, shift hip down to the floor then back up for a 2-second hold at top; do 10 reps each side.

6. Mountain climber. Assume a plank position, then bring knee toward chest and return to extended position, 20 times each leg (optional: place feet on slides).

7. Upside down snow angel. Place feet on slides and assume plank position (with either extended arms or on forearms). Keep legs extended and without bending knees or shifting body (yes, it’s hard!), open and close your legs 20 times.

8. Bicep curls. 20 reps with 8- to 10-pound weights.

9. Dips with body weight. Sit on a bench and place hands on either side of body, tops of fingers facing forward. Scoot butt off the bench and, using the triceps, lower and raise your body 20 times.

Rest for 30 to 60 seconds, then repeat the entire Circuit 2 sequence a total of three times. For the last exercise, reach around and pat yourself on the back, because you're done!

See more from Waters on YouTube and follow her on Instagram.