I have few fitness pet peeves and I need to tell you about them. I'm currently studying for my Personal Training certification as part of my prep for entering a Physical Therapist Assistant program and also to enrich my knowledge base about health and wellness.
Myth 1: Lactic Acid buildup causes sore muscles. This is scientifically inaccurate and also silly to say. People latch on to the idea I think because they think "acid" and "ouch" but the explanation is more complicated and related to blood pH, ATP production from energy sources, and the body's ability to recover from exercise.
The story starts with the fact that lactic acid is the FUEL for the muscles but read for yourself.
Myth 2: Getting to the "anaerobic phase" during strenuous exercise is the only "fat burning zone." It's an inaccurate, mostly marketing way people refer to how you can burn fat calories to pitch pushing yourself too hard, or buying some hokey exercise plan, or just trying to sound cool when you are doing cardio kick boxing. You burn fat all the time when you are existing in your life, even when you are sitting on the couch. The percentage of fat burned depends on the type of exercise. Anaerobic work (like HIIT) kicks you into a mode where you burn a lot more calories in general and fat has high energy per gram (energy = calories = 9 per gram). You will burn fat as a fuel in a slightly higher percentage but you will also torch through carbs as well. It's just fat is a slightly higher percentage of calories burned at high intensity than at low intensity. But it's not the only way to burn fat, just letting you know, so that you don't end up in a puddle trying to run 10 miles thinking it's the only way to do this. Plus you CANNOT spot reduce. In fact, mixing up aerobic (not as high intensity) as well as anaerobic is the best way to train your body.
(There's more to this, that involves understanding slow and fast twitch muscles, how mitochondria get energy - ATP - from fat and carbohydrates, but hopefully there is enough info here to help you understand that the body gets energy from both molecules).
Hobson, Katherine. “The 'Fat-Burning Zone': A Fitness Myth Debunked.” U.S. News, 3 Mar. 2009, health.usnews.com/health-news/blogs/on-fitness/2009/03/03/the-fat-burning-zone-a-fitness-myth-debunked.
Kelliher, Steven. “Aerobic Versus Anaerobic Fat Burning.” LIVESTRONG.COM, Leaf Group, 11 Jan. 2014, www.livestrong.com/article/431402-aerobic-vs-anaerobic-fat-burning/.
Tremblay, MSc Sylvie. “Fat Burning Vs. Carbohydrate Burning.” LIVESTRONG.COM, Leaf Group, 18 July 2017, www.livestrong.com/article/32587-fat-burning-vs.-carbohydrate-burning/.
Myth 3: Yoga spinal twists and hot yoga detoxy the body. No no a thousand times no. First, spinal twists are actually abdominal and spinal muscle twists. They can be active (often done seated) or more passive (often supine). Any time your body is working against gravity the pose should be active to support body weight. Supine with support (blocks blankets bolsters) can be taken more passively to stretch muscles versus create tonality. But the internal organs are really not affected that much - they are too deep to be actually "squeezed" in any detoxing sense. Kidneys (and liver) filter blood - that's what they do to eliminate waste as part of their actual function (and return nutrients back to the blood). You can create a intestinal compression that might assist digestion, but toxin removal, other than a catchall and misleading term, is not part of the twist benefits. Twists can enhance mobility and done properly assist with back health.
As for hot yoga, saying that you sweat out toxins is inaccurate and perpetuates delegitimization of yoga as a viable form of health and wellness by making instructors sound scientifically ignorant. Hearing people in my profession, who are supposed to study anatomy and physiology, blindly parrot what some uninformed other teacher imparted to them is frustrating.
What really happens in hot yoga? You sweat, and you can stimulate a large increase is BPM due to your body not being able to cool down as efficiently, so your body temperature controlling centers send more sweat to the skin in a effort to "for the love of all that is holy cool this organic system down." As long as you stay hydrated (and by the way you are supposed to completely replenish the sweat "weight" you lost for optimal health), you can improve your cardiovascular and circulatory systems and feel good for having worked out. You've also consumed calories (and when you exhale the CO2 that's the byproduct of metabolism aka calorie burn). But there is no detoxification (unless you consider water and salt "toxins").
All this is said with the purpose of helping you think anatomically and physiologically correctly about how your body works, so that you prevent injury, move with attention to intention, and feel good in your skin!