Devotees endure searing heat, holding their bodies in challenging poses leaving them dripping with sweat but exhilarated at the culmination of a tough workout. Bikram yoga improves mindfulness, flexibility, strength, muscle tone and general fitness, advocates boast. But a new study has warned the practise, which involves performing 26 poses, can raise a person's body temperature and heart rate to dangerous levels. A typical session lasts for 90 minutes and is performed in a room heated to 40°C (105°F), with 40 per cent humidity. Scroll down for video . Scientists at the University of Wisconsin-La Crosse have warned that Bikram yoga, which involves 26 poses performed in a room heated to 40°C (105°F), raises the risk of participants suffering heat stroke . A team of researchers at the University of Wisconsin-La Crosse has found hot yoga can place pressure cause a person's heart rate to fluctuate and their core body temperature to reach potentially dangerous levels. Emily Quandt, who led the study, working under John Porcari, said: 'The dramatic increases in heart rate and core temperature are alarming when you consider that there is very little movement, and therefore little cardiovascular training, going on during class.' Twenty apparently healthy volunteers - seven men and 13 women - ranging in age from 28 to 67 took part in the study. All regularly practised Bikram yoga and as such were familiar with the 26 standard poses. They were also all acclimatised to the hot, humid environment. Before taking part in a session each volunteer swallowed a core body temperature sensor, and was given a heart-rate monitor to wear during the class. Body temperature was recorded before the class started, as well as at 10-minute intervals throughout the session. Meanwhile heart rate was recorded every minute. The researchers found heart rate fluctuated throughout the class, depending on the difficulty of the pose being performed. But body temperature was found to steadily increase throughout the 90-minute class, in both men and women. The average body temperature for men was 39°C (103°F) and 38°C (102°F) for women. Scientists advise that the standard 90-minute class is reduced to just an hour to reduce risks . A male participant recorded the highest core temperature of 40°C (104°F), while seven volunteers' temperature reached 39°C (103°F), the scientists said. They noted that while none of their volunteers exhibited signs or symptoms of heat intolerance, body temperatures such as these can pose a danger for some people. They said the risk for heat-related illness, such as heat stroke, increases at 40°C (104°F). Dr Quandt said while the excessive perspiration experienced by people doing Bikram yoga is often cited as a benefit because it releases toxins, the results of the new study show this sweating was insufficient to cool down the body. She advises three ways to improve the safety of Bikram yoga classes - duration, temperature and hydration. Dr Quandt noted that  participants started to experience dangerous core temperatures around 60 minutes into the session. Therefore, shortening the length of the typical class to just an hour could help minimise the risk of heat intolerance. In addition lowering the temperature of the room may appear obvious, but would help reduce the risk Dr Quandt said. Furthermore, hydration lies at the heart of the controversy surrounding Bikram yoga, she said. In many classes, participants are designated just one water break. Though, many teachers do acknowledge students should hydrate as required throughout their class. Some Bikram enthusiasts believe that by minimising their water intake they augment the mindful aspects of the practise, with fewer water breaks decreasing the potential for mental distraction. Dr Porcari, said: 'Nothing is gained from withholding water in any setting. 'Exercise leaders must actively encourage hydration, particularly when classes take place in extreme environments like those seen in Bikram yoga classes.' He said Bikram yoga teachers must be familiar with essential science principles surrounding exercise, including a clear understanding of the physiology of the thermo-regulation. 'Knowing the risks associated with things like blood pooling and vasodilation, as well as the signs and symptoms of heat-related illness, is absolutely essential,' he added. And the researchers said it would be wise to reduce the temperatures in classes, while ensuring all participants are properly hydrated. One male study volunteer recorded a core temperature of 40°C (104°F) WHAT ARE THE SIGNS OF HEAT EXHAUSTION? WHAT ARE THE SYMPTOMS OF HEAT STROKE? Suspected heat stroke should always be treated as a medical emergency, health experts warn, advising an ambulance is called in extreme cases. Dr Cedric Bryant, chief science officer at the American Council on Exercise, which commissioned the study, agreed. He said: 'Bikram teachers should recognise that participants' thermoregulatory systems will be challenged in this environment. 'It is essential that they are aware of the early warning signs of heat intolerance.' The signs include cramps, headache, dizziness and general weakness. It is important that teachers respond appropriately and are trained in what to do in the event of an emergency to keep the participant's safety as the top priority, Dr Bryant added. The researchers concluded: 'Exercising in hot and humid environments - whether inside a studio while practising Bikram yoga, or outside running during the warm months - can place participants at risk for heat-related illness, especially if those individuals do not adequately hydrate before, during and after exercise. 'Not only does it take time for the body to acclimatise to activity performed in hot and humid environments, proper hydration is crucial.' 'Given the popularity of Bikram yoga and its proven benefits, it is likely here to stay,' said Dr Bryant. 'It is our job to make sure it is done safely.'
Bikram yoga involves 26 poses performed in a room heated to 40°C (105°F)
Typical sessions last 90 minutes and advocates say it improves mindfulness, flexibility, strength, muscle tone and general fitness .
20 volunteers took part in a new study to monitor body temperatures .
Experts warn one man's temperature reached 40°C/105°F putting him at risk of heat exhaustion and heat stroke .