All forms of weight-bearing exercise, including the hatha yoga postures included in the Bikram hot yoga series, can tone your muscles when you practice consistently with sufficient intensity. Muscles begin to atrophy -- or lose tone and mass -- over time if you fail to use them, which is why people who are not overweight can nonetheless grow flabby and weak.
"Hot yoga" classes derive from Bikram yoga, a style introduced to the U.S. in the 1970s by founder Bikram Choudhury. Choudhury conducted his classes in rooms heated to about 105 degrees Fahrenheit with elevated humidity -- conditions that mimic the climate of his native India and, according to Choudhury, facilitate both deep stretching and systemic detoxification through sweat. Choudhury trademarked his strict 26-posture series in 2002; studios that use the Bikram name must be licensed by Bikram headquarters. Other yoga studios, however, teach variations on the Bikram series under the label "hot yoga," often using different postures or less heat.
Like other physically challenging yoga styles, such as Ashtanga, Bikram hot yoga is a vigorous series of postures, at least half of which require yogis to balance themselves against the pull of gravity. For example, in Balancing Stick Pose -- a variation on the Warrior III pose used in other yoga styles -- practitioners must balance on a single, locked-out leg, then pivot forward from the hip with their arms extended overhead while simultaneously raising the opposite leg behind. Bikram instructors tell students to "make a perfect letter T -- no broken umbrellas." Although students hold Balancing Stick for a mere 10 seconds, successful execution requires they contract virtually every muscle they can command, making it arguably one of the most demanding poses in the series.
As hot yogis build and tone their muscles in a Bikram yoga class, the heat and humidity help them burn calories as well. Official Bikram yoga classes are always 90 minutes long, and a 150-pound student can burn up to 715 calories in that time, according to HealthStatus. Since one pound equals 3,500 calories, you can potentially burn off a pound of fat per week by practicing five times weekly, helping to reveal your increasingly toned musculature.
Consult your healthcare provider before beginning hot yoga practice, particularly if your muscles are very weak, you are pregnant or you have a pre-existing medical condition. For your first class, treat yourself gently and sit down or rest in Savasana -- Corpse Pose, flat on your back -- if you feel exhausted, dizzy or nauseated. The teacher likely will remind you that your only goal is to remain in the heated room for the duration of class. Drink plenty of fluid before and after class to replenish what you lose through sweat.