My glutes are on fire just watching her!

Alexa, play "Rocky" theme tune - because Jessica Biel is one helluva champ.

The mom-of-one shared a snippet of her fitness routine on her Instagram account and wait 'til you get a load of her lower body workout. 

Half pistol squat, half weighted arm raises, and throw in a balance challenge for good measure: this is not a move for the faint hearted.

While her husband, Justin Timberlake, might be known for his prowess as a dancer, it seems that Biel's talents aren't any less admirable - she frequently posts videos of her aerial skills (flips and head stands) much to the amusement of her eight million followers.

Celebrity trainer Ben Bruno, who regularly puts Biel and others like Chelsea Handler and Kate Upton through their paces, commended the 36-year-old actress for mastering the task at hand.

So, why consider adding single-leg squats, aka pistol squats, into your routine? According to Shape Magazine, this elite strength move requires balance, mobility, and co-ordination.

"This move requires a ton of hip, knee, and ankle mobility," trainer Rachel Mariotti said. "It demands core stability and balance while "building unilateral strength in the hips, glutes, quads, and hamstrings, which makes it more acrobatic than any other standard single-leg exercise."  

In order to nail it yourself, Mariotti recommends working on your forward lunges, reverse lunges, and side lunges to build up strength in your legs.

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