If you ask any endurance athlete training for a competition whether it’s biking, running, swimming or all three, they will tell you muscular strength and endurance is based the amount of energy exerted with regards to intensity and length of each training session, nutrition and water intake, with water intake being one of the most important aspect of training. Heat, unless it is sauna, is not a part of their training regimen.
Bikram’s next claim is hot yoga improves strength by putting muscle tissue in optimal state for reorganization. Muscles do not reorganize, fascia reorganizes. When fascia reorganizes our musculature can change.
AM Hedley et al., conducted the study The effects of acute heat exposure on muscular strength, muscular endurance, and muscular power in the euhydrated athlete, Ten healthy weight-trained men participated in the study. The male subjects were randomized to into 2 different groups including normothermic (22.5°C, 65% humidity), and hyperthermic (65- 75°C with 15% humidity) for 30 minutes. The results showed that all study participants experience hemodynamic stress with 30 minutes of hyperthermic (sauna) exposure. Hemodynamic stress includes elevated blood pressure (124/78 to 148/60) and pulse (64 beats per min to 122 beats per min). Body temperature was measured orally and in the eardrum (tympanic membrane). Hyperthermic exposure increased by to 2.48 to 2.72°C. The study concluded that acute heat exposure is detrimental to muscular endurance and muscular strength and power showed unequivocal changes.
Dr Lawrence Armstrong’s article titled Heat and Physical Performance published by Human Kinetics discussed how heat and humidity affect exercise performance with regards to muscular strength and endurance. He reveals that elevated body temperature (hyperthermia) and dehydration are the critical factors in adverse effects vs heat and humidity since there are variable differences in exercise activities with regards to heat exposure, physical intensity and opportunity to rehydrate with fluids.
Dr Armstrong concludes that hyperthermia reduces muscular endurance with muscular strength being unaffected. Hyperthermia affects muscular endurance by causing carbohydrates (glycogen) in the skeletal muscles and liver to be consumed at a faster rate. The bodies limited carbohydrate resources can only sustain a couple of hours of intense exercise in hyperthermic conditions. In addition, the bodies thermoregulatory response to hyperthermia results in dilation of the blood vessels in the skin pooling blood in the limbs, like when your hands and feet swell while exercising in heat. Blood pooling in the limbs decreases blood volume, reduces cardiac output, increasing strain on the heart. This decreases oxygen the muscles, fatiguing the body.
He goes on to say that dehydration, irregardless of environmental temperatures, decreases blood volume and cardiac output declining in muscular endurance. Research does show that a small or moderate reduction in water loss is unlikely to affect muscular strength. However, with severe dehydration (6% or more water loss) in conjunction with sustained or repeated exercise lasting longer than 30 seconds reveals a deterioration in muscular strength conceivably due to reduced blood flow to the muscles, an increase in cortisol reducing testosterone which is responsible for muscle building. If you are becoming severely dehydrated in a 90 minute hot yoga class you may be decreasing your muscular strength and likely decreasing your muscular endurance.
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Namaste,
If you would seek health, look first to the SPINE - Socrates 
Brilliant!
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