Jessica Simpson's trainer reveals she didn't adopt 'extreme diet' or 'radical exercise' to achieve 45kg weight loss

Jessica Simpson didn't adopt an "extreme diet" or take up "radical forms of exercise" to lose 100lbs in six months.

The 39-year-old singer revealed earlier this week she had ballooned to 108.8kgs (240lbs) when she was pregnant with her daughter Birdie but has managed to drop back to her pre-baby weight.

And while her trainer Harley Pasternak admitted they had their "work cut out" to help her get back into shape, he wanted her to do it by forming new habits that would stay with her in the long term.

• READ MORE: Jessica Simpson reveals her 45kg weight loss just 6 months after giving birth to baby girl

Harley said: "This particular baby was ... I think she commented to the press that she tipped the scales at 240 lbs, that's not coming from me, she mentioned that in a post. So, we had our work cut out for us and perhaps more so on this baby than the others.

"It had to be more than just getting back from her baby weight, but how do I keep whatever I am doing now forever. That's why we are not a big fan of doing any extreme diet or radical forms of exercise."

Instead, Harley - in consultation with the Dukes of Hazzard star's doctor - devised a plan which saw her walk 14,000 steps a day, which Jessica got to gradually, having started on a 6,000 steps-a-day routine.

The These Boots Are Made For Walking singer - who also has Maxwell, seven, and six-year-old Ace with her husband Eric Johnson - fit in her steps by introducing daily family walks and strolling on a treadmill while watching TV or taking telephone calls, and Harley didn't want her doing anything "too difficult, painful, hard or arduous."

The trainer didn't want to "rob" the blonde beauty of her favourite foods but helped her make meaningful changes so the three meals and two snacks a day she consumed incorporated lean proteins and vegetables.

Jessica didn't have a specific cheat day, but was allowed cheat meals throughout the week as Harley felt that made indulging "more balanced and sustainable".

He added: "You don't have to have all of your indulges in one day. It tends to be a little extreme if you do that."

Eventually, Jessica - who has worked with Harley for 12 years - stepped up to full-body workouts every other day.

He told E! News of those sessions: "We slowly ramped those up; we started really easy and slowly increased them. We started with doing a full-body workout, one set of each exercise, not really doing too much intensity per body part and gradually increasing the volume and intensity. Focusing on a few muscle groups per day. Different muscles each day of the week."

The pair kept track of Jessica's progress with daily emails that included her step count, a food diary and a screenshot of her Fitbit.

- Bang! Showbiz