Jennie Garth on September 11, 2012 in Los Angeles, California. Credit:

Great Garth!

On Tuesday in Los Angeles, Jennie Garth appeared to be relishing her morning workout -- listening to some tunes and sweating it out, makeup-free, in her workout clothes.

Wearing a baseball cap, blue sweats and a lime-green sports top, the 40-year-old actress showed off her taut, lean physique -- particularly her enviable flat, rock-hard mid-section -- as she exercised near the home she shares with her three daughters Luca, 15, Lola, 9, and Fiona, 5.

The former Beverly Hills, 90210 actress, 40, has definitely bounced back after her 16-year relationship with husband Peter Facinelli crumbled this past spring --channelling her heartbreak into a body makeover.

"Your body changes with your emotional state of mind," explained the 5-foot-5 star, who is down to a size-26 pants and lost 20 pounds in three months.

The sunny blonde achieved the new shape through hard work. "She did a lot of outdoor activities: running, hiking, and riding her horse," a pal recently told Us Weekly.

She also hit local gym Cold Iron Barbell thrice weekly. "We don't use machines here. It's all free weights," co-owner Shannon Stewart tells Us. "We like to do circuits: She'd push over a tractor tire, then do push-ups, then jump rope, do deadlifts and run for a bit."

Adds Stewart: "She loves how her new body looks and she feels healthier, which is most important."