bikram yoga involves 26 poses performed in a room heated to 40 °c -lrb- 105 °f -rrb- .
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 .