The health benefits of regular exercise are hard to ignore.

Physical activity, at any level, plays a critical role in controlling weight, strengthening muscles and bones and improving mood and mental wellbeing. But what a lot of us don’t know is what happens to our bodies – and our hard-earned fitness – should our regular dose of training grind to a halt.

Whether it’s a lack of motivation or an injury-induced lull, here’s what you need to be aware of - remember the impact will vary from person to person.

[Read more: 10 exercise rules to get the most from your workout]

Set the tone

One of the biggest misconceptions, says fitness consultant Laurel Alper, is that muscle turns into fat and vice versa when you stop exercising or ‘detrain’.

“If you started off happy with your weight and then you stopped working out but ate the same food, you’ll look bigger because the muscle tone will be lost,” she explains.

“You know the saying: if you don't use it, you lose it. You are only as good as your last workout.”

Flexing your muscles

Muscle mass will start to decline in as little as two weeks, but it will take a good few months for all of your muscle to completely disappear and, better still, the effects are reversible.

“Muscles have a memory,” clarifies Laurel. “That's why if you don't ride a bike for 20 years, you may be a bit wobbly at first but your muscles will remember.”

Breathe easy

VO2 max, the maximum oxygen you can uptake and employ, will plunge around 12 days after you take your foot off the gas.

Studies show your capacity will continue to decrease, albeit at a slower rate, for the first three months after ceasing activity, but again this will depend on how well conditioned your body is.

Under pressure

If your prescribed activity has a positive effect on your blood pressure, you’re going to suffer the consequences immediately – and within a month or so, the benefits you encountered will be entirely lost. But, Laurel stresses, it’s an “ongoing discipline to achieve the overall result of lowering your risk of cardiovascular disease, diabetes and living a healthy lifestyle”. “It's about moving around and not being slumped in one place.”

[Read more: Why you're never too old to start running: Training tips for over 50s]

Carb control

Exercise has a profound effect on the way our bodies store and use carbohydrates – and in layman’s terms, those who up their exercise digest their food better and are less prone to storing fat. It’s all to do with our GLUT4s: insulin-regulated glucose transporters found in tissue and muscle. So if you want to avoid the love handles – get moving. 

Calorie counting

The more active you are, the more calories you burn, so if you’re slacking on the fitness front, your metabolism – the rate at which your body gets its energy via food – will decline within a week. If you lack get-up-and-go, be sure to choose an exercise you enjoy. “People ask me, ‘What's the best exercise?’ and I say, ‘Well, it's the one that you're going to do!” Laurel says.

Give it a rest

While you may worry about the downsides that come as a result of pausing your schedule, Laurel admits it’s important to take time off too. “Rest is also recovery and it’s beneficial. You've got to exercise more than you have time off, of course, but generally speaking, it takes about the same amount of time to get your fitness back.”

Find out more about Laurel Alper’s work at her website

Have you ever stopped exercising? Why, and what happened to your body? Let us know in the Comments section below.