Jennifer Garner nails 33-inch box jump

If you're looking for a dose of motivation this weekend, look no further than Jennifer Garner's Instagram account. Garner posted a video of her trying to land a 33-inch box jump — and succeeding. (Read more: Watching Jennifer Garner Do This Plyometric Workout Will Make You Feel Tired and Impressed)

In the clip, the 50-year-old actress begins with a quick dance before attempting to jump on top of a stack of three plyometric jump boxes totaling 33 inches high (that's nearly three feet tall!). Garner lands on her knees, not quite sticking the landing on her first jump. "That's very tall," says Garner in the clip, looking back at the camera with a laugh and shaking it off before she tries again.

The video includes two more clips of Garner jumping onto the stacked boxes. Both of these times, she completely nails it, striking a pose and proudly standing on top of her 33-inch boxes.

"That was her first time doing the 33-inch [jump]," Beth Nicely, Garner's trainer and the founder of The Limit, tells Shape. "There's a fear you have to get over [with box jumps], and it's just as much mental as physical work; it's pretty awesome seeing her go for it."

Related: Try One of Jennifer Garner's Workouts, Courtesy of Her Trainer

Nicely has been working with Garner for over two years. When they first started training together, Garner's fitness routine included dance cardio, ground plyometrics (aka jump training), strength training, and high-intensity interval training (HIIT). "Jennifer is super strong, and I'm always trying to find different ways to elevate her heart rate," explains Nicely.

About a year and a half ago, Nicely introduced Garner to Pilates boxes, which are usually about one foot tall. They started focusing on single-leg power and stability moves before upping the ante to 20-inch tall plyometric boxes. "It was a matter of proper progression," says Nicely, who now has Garner on box jumps two to three times a week.

Box jumps are a form of plyometrics, which are jump-focused exercises that "are amazing tools to increase speed and power," explains trainer Keoni Hudoba, creator of COREntine and co-founder and creator of Cyc Fitness. Burpees, speed skaters, and pop squats are a few other plyometric movements worthy of incorporating into your workout routine, he adds. "Movements such as these utilize a combination of muscle groups at once and require a great deal of strength, flexibility, and focus," he says.

Box jumps specifically target your quads, glutes, hamstrings, and core, explains Nicely. Plus, they require you to incorporate momentum with your arms, all while putting healthy stress on your joints.

"A lot of people worry about the impact on their joints and fear that it is dangerous to jump as they age," Nicely previously told Shape. "As a NASM-certified trainer and corrective exercise specialist, I can assure you that jumping is essential to bone health. Bones need to be stressed with impact to build density, which makes you less likely to break a bone or develop osteoporosis."

Related: This Plyometric Workout for Beginners Will Get You to Enjoy Jumping Exercises

Box jumps are usually done in a circuit with complementary exercises that warm up and prepare the body for 45 seconds of continuous jumping, explains Nicely. For instance, if she is training Garner on a shorter box (around 18 inches tall), then the actress will perform straight power jumps for multiple reps, which helps her conditioning for higher box jumps, such as her recent 33-inch accomplishment.

As with any exercise, Garner worked her way up to this incredible 33-inch feat. However, if you're just starting to experiment with plyometric movements, consider following a Tabata-style plyometric set with 20 seconds of explosive movement followed by 10 seconds of rest, suggests Hudoba. "It's a great way to start building your stamina and making your machine — your body — as strong as possible for the life ahead of you."

As for beginners looking to try box jump training specifically, you can try doing step-ups, and as you feel comfortable, keep increasing the surface height, advises Nicely. "When you want to start jumping, go back down to the lower surface. Jump up, land, and step back down," she adds, suggesting true newbies start with a three-inch surface height. Loaded squats are also a great way to get started. Just be sure to stand up really powerfully, says Nicely.