What's the best way to get fit? It all depends on whether you just want to look a little younger or shed some of that bank holiday weight. Dan Roberts gets advice from the fitness experts

What is the single best exercise? Obviously, the ideal one for a fit, strapping 20-year-old will not suit an overweight, middle-aged man. You may want to burn fat, improve muscle tone, bulk up, boost cardiovascular fitness, build core strength, develop sport-specific fitness... you get the picture.

Although it is tempting to seek the holy grail of fitness: one exercise or activity superior to all others, it's neither realistic nor helpful. Instead, let's quiz a range of experts in different fields to see what they nominate – then you can select the one that suits your age, fitness level, health goals and that you think you'll be able to stick to.

First, a word of warning: if you have any history of injury, especially back or joint problems, consult your GP before attempting the more vigorous of these exercises. That said, let's start with a bang. Matt Roberts is one of the Britain's leading personal trainers, having honed the physiques of A-listers such as Madonna and Sting (not to mention David Cameron).

"If I had to choose one thing that covers as many bases as possible, it would be trail running," he says.

"That's because it confronts you with a vast array of challenges. There's endurance work, stamina built by the interval training – as you run up and down hills – and strength, coordination and nervous-response training because of the varied terrain."

Although Roberts admits that it's lower body-orientated and won't give you the pure strength built by weight training, trail running will make you "extremely fit, lean and healthy". And this kind of running is proven to be superior to "steady state" jogging, in which you plod around the park for an hour, because it works your cardiovascular system much more effectively. Another celebrity trainer, Greg Brookes, opts for a tried-and-trusted weight-lifting move.

"The human body is only designed to move in a few fundamental ways, so replicating these natural movements ensures that the body functions safely and optimally for maximum power, strength and speed," he says. "The movement that demands the most muscle activation and energy consumption is the deadlift."

Brookes champions the deadlift because it activates virtually every muscle in the body and says the more muscles we activate the more energy required and so calories burnt. It's also ideal for the deskbound, as it works the muscles at the back of your body which are weakened by sitting all day.

"You can perform it with anything from dumbbells to barbells, but I think the kettlebell is best," adds Brookes. "Swinging a kettlebell between the legs encourages even more muscle activation, as the muscles have to work harder to decelerate the movement."

If you don't have access to a kettlebell, or indeed a gym, "resistance training" using only your body weight has many advocates in the fitness industry. With a little determination (plus gravity), exercises such as press-ups and dips are a highly-effective way to build long, lean muscles. Performed slowly and correctly, there's also zero risk of injury, as long as you don't overdo it.

Perhaps best of all, though, is the simple squat. It not only activates the buttocks, back and legs, but is easy to do. Again, make sure your movements are slow and controlled; fold your arms across your chest, bend your knees and lower your trunk until your thighs are parallel with the floor. Repeat 20 times for three sets. Simple.

Also free – and with a host of health benefits – is walking. One 15-year study found that middle-aged women who walked for at least an hour a day maintained the same weight over that period, while those who didn't got heavier. Hardly news for Tom Franklin, chief executive of Ramblers UK. "The great thing about walking is that it's suitable for almost everyone, regardless of their age or ability," he explains. "Walking for just 30 minutes a day, five times a week can produce endless benefits: it helps to combat osteoarthritis, strengthens joints and muscles and helps older people gain strength and balance to reduce bone fractures. Walking can also dramatically cut diabetes and cardiovascular disease, as well as reducing levels of breast and colon cancer."

Franklin recommends brisk walking, but adds that "regular walks at any speed will soon reveal benefits for your health by boosting your immune system, helping to maintain your weight and elevate your mood".

This last point is key. Study after study has found that regular walks – especially in somewhere green and pleasant – are an effective method of combating stress, anxiety and depression. And unlike some forms of exercise, walking provides social benefits by giving you the opportunity to spend time with friends and family.

The final two forms of exercise on our list offer optimal protection against the bane of many a Briton: back pain. This peculiarly modern complaint is a combination of our sedentary lifestyles, poor posture, too much sitting – which our musculoskeletal system was not designed for – and soaring stress levels, which cause tight, stiff muscles.

While certain muscles become inflexible, others grow weak through under-use, so osteopath Danny Williams nominates the pelvic tilt for his must-do exercise. "It not only alleviates any back pain you may have by decompressing the spine, but strengthens the muscles that stabilise the lower back and stretches the hamstrings, psoas and quadriceps."

To do a pelvic tilt, lie on the floor with your knees up and feet flat: the soles of your feet, backside, mid/upper back, shoulders and head should be touching the floor. You should also be able to slide your hand between your lower back and the floor. Inhale, then as you exhale tilt the bottom of your pelvis up so your low back gently stretches and reaches in the direction of the floor.

As you inhale, allow the spine and pelvis to return to the original position. This may all seem a bit low-key, but remember that some of the best exercises are very subtle: more explosive exercises with poor form cause injury.

Finally, another great back-preserver: Pilates. This gets the nod because, as well as reducing back pain, it corrects a host of other physical problems stemming from poor core stability, including a slouchy posture, neck pain, tight hamstrings and a protruding belly. Jane Wrafter, who runs Pilates and exercise classes in north London, explains why it's so effective: "Pilates addicts will testify that it has transformed their lives by freeing them from back pain and giving them a flat tummy to boot. All the exercises are low-impact and are performed slowly, with an emphasis on engaging the core – this basically means pulling in or hollowing the deep lower-abdominal muscle called the transverse abdominus, or TVA."

To do this, says Wrafter, imagine you're trying to fasten trousers that are too tight and notice how your belly button presses back towards your spine.

"This is your TVA switching on," she says. Although Pilates exercises strengthen muscles throughout, there is a particular focus on abs, back, shoulders, glutes (buttocks) and legs – which is why devotees have those long, lean, sculpted physiques rather than the bulked-up bods from pumping iron.

So which is best? You choose.

Matt Roberts: www.mattroberts.co.uk;Greg Brookes: www.gbpersonaltraining.com; Jane Wrafter: jcwfitness.co.uk; Danny Williams: thetravellingosteopath.com; Ramblers UK is organising Get Walking Day on 14-15 May: see www.ramblers.org.uk for details

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Walking and cycling are great for heart health, but resistance training – such as squats, press-ups or weight training – are better to maintain the muscle mass that decreases with age. Weight-bearing exercise, such as running, dancing or aerobics, is necessary to improve bone density.

Weight loss

Combining high-intensity exercise with endurance training is the best way to burn calories and decrease fat, but numerous studies have shown that regular, brisk walking helps keep weight down. Muscle-building resistance training, such as press-ups or weight-lifting will also keep the pounds off, because muscle burns more calories than fat.

Cardiovascular benefits

Intense workouts help improve and maintain cardiovascular fitness, but moderate-intensity exercise such as brisk walking is very effective, too. Interval training – short bursts of vigorous exercise, alternated with low-intensity jogging or walking – are equally effective as longer, moderate-intensity workouts.