Fitness trackers are able to track heart rate accurately but not the number of calories burned, new research suggests.

A team from Stanford University asked 29 men and 31 women to wear several fitness trackers on their wrists and run, cycle and walk, in order to test out the accuracy of the wearable devices.

Seven fitness trackers were used – the Apple Watch, Samsung Gear S2, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Basis Peak – and apart from the Samsung Gear S2, they all had error rates below 5 per cent for heart rate measurements. The Gear S2 had an error rate of 6.8 per cent.

However, the error rates were drastically different with regards to measuring energy expenditure. The most accurate was the Fitbit Surge (27.4 per cent error rate), while the PulseOn fitness tracker performed worst with an error rate of 92.6 per cent, although there was no data for the Gear S2 on calories burned.

Euan Ashley, study co-author, said: “We were pleasantly surprised at how well the heart rate did. Under many circumstances for most of the devices, they actually did really quite well.

“At the same time we were unpleasantly surprised at how poor the calorie estimates were for the devices. They were really all over the map.”

Ashley and his team stressed that the number of calories fitness trackers say we have burned should be taken “with more than a pinch of salt”.

“People need to know that on energy expenditure, they give rough estimates. If you go to the gym and you think you’ve lost 400 calories, you might feel you’ve got 400 calories to play with.

Energy expenditure differs greatly from person to person, affected by factors such as height, weight, gender, body fat percentage and the type and intensity of exercise.

The findings were published in the Journal of Personalised Medicine.

Your risk of suffering a stroke can be significantly reduced by sleeping for between seven and eight hours each night, and exercising for 30 to 60 minutes, three to six times a week, according to research.

However, if you sleep for less than seven hours, or more than eight hours, the risk of a stroke increases, the findings indicate.

The research was carried out by a team from the New York University School of Medicine, who looked at data taken from 288,888 adults between 2004 and 2013.

The analysis took into account factors such as age, ethnicity, lifestyle and health, as well as exercise levels.

People who slept for the average amount of time (seven to eight hours per night) had a 25 per cent reduced risk of suffering from a stroke.

However, those who slept more than eight hours per night had a 146 per cent increased risk of experiencing a stroke. For those who slept for less than seven hours per night, they had a 22 per cent increased risk of a stroke.

For the greatest protection against stroke, the researchers concluded that you should complete at least half an hour of exercise, a minimum of three times a week.

Behind heart disease and cancer, strokes are the third most common cause of death in England, with approximately 110,000 people suffering from a stroke each year.

The team presented its findings in Los Angeles at the American Stroke Association’s International Stroke Conference.

A new study suggests that the use of fitness gadgets, such as Fitbits, makes teenagers less likely to exercise.

There was a significant increase in the number of Fitbits sold to teenagers last year, however a study from Brunel University has found that wearing one of these devices for eight weeks actually made teenagers less likely to exercise.

The study looked into the effects of Fitbits on 84 Year 9 pupils form both the South-East and North-West of England.

The researchers found that wearing a fitness device made the teenagers feel less confident about their abilities, as well as being less connected to their peers. Many of the participants in the study also felt that they had less choice in when and how they could exercise.

Many of the participants saw a short-term boost in their activity levels, but many became bored shortly afterwards, with levels of physical activity declining after a month.

“I did it for four weeks, and then the last couple of weeks I just sat at home all day,” one participant said.

Fitbits enable the user to set up ‘friendly competitions’ between themselves and other users, with this intended to motivate the user into exercising more. However, the Brunel study found that, while many of the participants used this feature, it meant that they felt less motivated to exercise and added to their guilt over failing to do so.

Charlotte Kerner, who conducted the research, said that peer comparison was a key factor in undermining pupils’ motivation. “There wasn’t a desire for our participants to be more active for themselves and their own goals, or for fun,” she said. “It was simply because they wanted to beat their mates.”

She said that the Fitbit’s predefined daily target of 10,000 steps was also demotivating: “They strived to achieve it, but would often fall short. This made them feel really bad about themselves, and put them off exercise.”

The researchers said that they believe it would be more effective if the digital technology was to be used by PE teachers to enable them to provide specific goals for individual students, rather than the students using them for a generic exercise plan.

New research suggests that spending just one minute each day exercising can have a positive impact on your health.

A study, conducted by the Universities of Exeter and Leicester, found that women who undertake 60-120 seconds of high-intensity exercise each day have four per cent better bone density than those who do less than a minute.

It was also discovered that the benefits are increased for those who exercise more, with women who do over two minutes of exercise a day having a six per cent higher bone density than those who do under a minute.

The higher a person’s bone density, the lower the chance of them developing osteoporosis or suffer bone fractures in old age.

Over 2,500 women took part in the study, making the research particularly important due to the higher risk of osteoporosis in women.

According to the International Osteoporosis Foundation, a tenth of women aged 60 are affected by osteoporosis, and this rises to two-thirds of women aged 90.

In the research, women were asked to wear devices to monitor their activity, with the data collected over a week then compared to measurements of their bone health.

“We wanted to make every second count in our analysis, because short snippets of high-intensity activity are more beneficial to bone health than longer, continuous periods,” said lead author Dr Victoria Stiles.

“But there’s a clear link between this kind of high-intensity, weight-bearing exercise and better bone health in women.”

“We were careful not to ignore short bursts of activity throughout the day.”

“We don’t yet know whether it’s better to accumulate this small amount of exercise in bits throughout each day or all at once, and also whether a slightly longer bout of exercise on one or two days per week is just as good as one to two minutes a day.”

“But there’s a clear link between this kind of high-intensity, weight-bearing exercise and better bone health in women.”