Keeping fit and healthy may not require hours of physical exercise every week, research suggests.
Many say they haven't got time for the gym
Canada's McMaster University found just six minutes of intense exercise a week could be as effective as six hours of moderate activity.
The Journal of Applied Physiology study showed short bursts of very intense exercise improved muscle capacity, and improved endurance.
However, experts warn it might be too much for people not already fit.
Under current guidelines, people are recommended to take moderate aerobic exercise for 20-30 minutes three to five times a week.
The researchers compared 23 people given different three-times-a-week training regimes. All participants were reasonably fit and active.
One group cycled for two hours a day at a moderate pace, and a second cycled for 10 minutes a day in 60-second bursts, at a slightly harder pace.
A third group took part in sprint training - cycling at top speed for two minutes in 30 second bursts with four minutes rest between each sprint.
The volunteers completed an 18.6 mile cycle ride at the start of the study, and repeated it after two weeks of training.
All three groups were found to have improved to the same extent.
Analysis showed the rate at which their muscles absorbed oxygen - a key measure of fitness - was the same.
Levels of an enzyme called citrate synthase which helps the tissue to process oxygen, and helps to ward off diabetes, were also similar.
Lead researcher Professor Martin Gibala said: "Short bouts of very intense exercise improved muscle health and performance comparable to several weeks of traditional endurance training.
"Sprint training may offer an option for individuals who cite "lack of time" as a major impediment to fitness and conditioning.
"This type of training is very demanding and requires a high level of motivation, however less frequent, higher intensity exercise can indeed lead to improvements in health and fitness."
John Brewer, director of the Lucozade Sports Science Academy, told the BBC News website that short, intensive bursts of exercise were probably only sensible for people who were already fit.
"This type of exercise is going to put a strain on an individual, and I would be very wary of recommending it for the general sedentary population who want to try to get fit from scratch.
"But it might help a small proportion of reasonably fit and active people who are time short, and it is certainly true to say that any exercise is better than none."
Mr Brewer said the best way to maintain good cardiovascular health and a steady bodyweight was to follow the current guidelines.