There are many reasons why you may fall “out of shape”. From injury to low motivation, it’s natural for your fitness to fluctuate. The good news is that whether you’ve always been an athlete or can never stick to a workout routine, there are proven methods to help you get back in shape.

What does “being fit” really mean?

Physiologically speaking, being physically fit involves looking holistically at several factors: cardiorespiratory fitness, muscle strength, mobility (in this context, flexibility and range of motion) and neuromuscular control (i.e. i.e. balance and agility), says Heather Milton, a counselor. board-certified clinical exercise physiologist at the Sports Performance Center at NYU Langone.

Anecdotally, being physically fit will look and feel different to everyone, but can often mean that you have good energy, feel strong, are able to perform daily tasks without pain, that you have mental clarity, and that you generally feel healthy and happy.

This subjectivity can mean that fitness will include different goals for different people. “Does being ‘fit’ mean you are able to walk all day for your job and still have energy to play with your children, or does it mean you are able to smash a new mountain bike trail without hurting yourself?” says Jacqueline Crockford, an ACE certified personal trainer. Understanding your reason will help you maintain the motivation needed to achieve your long-term goals.

The good news is that there are steps most people can take to improve their overall fitness. Here’s what to expect.

How fast can you lose shape?

To understand how to get back into shape effectively, it helps to know how quickly your early results can fade. You can actually lose your cardio endurance and muscle strength with two weeks of complete rest, Milton says. That doesn’t mean that in two weeks you’ll have lost all of your gains, but that’s when you can expect the decline to begin, she says. In general, cardio endurance depletes at a slower rate than muscular strength and endurance, which decline quite quickly when you stop training, Crockford adds.

Loss of fitness can lead to increased blood pressure, decreased blood oxygen levels, decreased neuromuscular efficiency and heart strength, reduced lung capacity, and even changes in your resting heart rate, says Milton.

Age is also an important factor to consider, says Crockford. According to a 2009 study published in the European Journal of Aging and Physical Activity.

How fast can you get back in shape?

On average, if you strictly follow an evidence-based, strategically designed fitness program – which means you’ve done your homework and adhere to a specific plan – you can expect to regain your fitness by 16 weeks, says Milton. Muscle strength can begin to improve in four to six weeks with noticeable results in 12 weeks. Improvements in your cardio also follow a linear pattern, with small changes progressing gradually over time, she says.

“However, it is in a vacuum,” warns Milton. This does not take into account lifestyle factors such as changes in weight, diet, health conditions or hydration, for example, all of which can have a huge impact on how long it takes you to get you back in shape.

“The rate at which a person regains their fitness level, both in muscle and cardiorespiratory measures, depends on several factors, including programming, previous fitness levels and exercise experience, as well as the age,” says Crockford. How long you stopped exercising also matters, says Milton. If it’s been a few weeks or even a few months, that’s a big difference than a few years. “If it’s less than a year, you go back to maybe 50% of where you started and slowly pick up from there,” she says.

How to get back in shape after a break

The first step is to set a clear and, ideally, measurable goal. Just saying you want to “get in shape” could lead to a haphazard approach to training that will ultimately take you longer to reach your goals, if you’re not frustrated and throwing back to class. road, said Milton.

The most important thing to keep in mind when getting back into a workout routine is to pace yourself. This is especially true if you’ve been largely inactive (rather than just choosing lower-intensity modalities), if you’re coming back from an injury, or if you’re older.

Older adults have more years of training experience under their belts, so they may be smarter to adopt a renewed routine, says Milton, but vice versa, if you also want to avoid doing the same grueling workouts as you did when you were younger. Bodies change over time, and it’s okay if your version of fit is different in your 40s and 20s, she says.

Steady progression in difficulty will ensure you stay on track while avoiding injury or burnout, says Milton. Cardio exercise can be increased by the total training volume, which means that if you managed to run three hours a week, you can increase the total weekly time spent running. Strength-based training progression can look like more reps using the same weight or grabbing a higher weight while doing the same reps, Milton adds. Max-rep bodyweight tests — think about how many push-ups you can do in a minute — are also a universally good strength-building tactic. Ultimately, you may get back in shape, but it probably won’t happen overnight. Small, smart steps over time will lead to the results you’re looking for, so practicing patience is a necessary part.

This story was originally featured on Fortune.com

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