Hatha yoga is the oldest and most systematic style of yoga. In fect, it would be correct to say that hatha yoga is more an umbrella term and base of all popular yoga systems or trends in the world today. If someone wants to learn any style, the first thing is to build the correct base by learning hatha yoga teacher training course under the able guidance of a true master or guru.
There are many reliable texts available on hatha yoga but there are four major ones- Hatha yoga pradipika by yogi swatmarama, Goraksha samhita by Gorakhnath, Gherand samhita by sage Gherand and Hatharatnavali by Srinivasabhatta Mahayogindra. All these texts were written between 6th and 15th centuries A.D. Even there are references to hatha yoga in Upnishads and puranas which date back around 6th century B.C. The actual reference in texts to hatha yoga began around 6th century B.C. and it proves the authenticity of this great system Hatha yoga and it gives us the glimpse of the historical aspect of Hatha yoga.
In ancient times, hatha yoga used to be practiced for the purpose of achieving higher state of consciousness and reach the stage of Samadhi. But , today this great science has been misunderstood completely and only it is now being understood and utilized in very limited sense, just as physical exercise or just as to demonstrate the flexibility of the body. This is the high time to correct this misconception because hatha yoga is a very important and helpful science in today’s world and humanity.
Source: Hatha Yoga TTC India
Traditional Yoga India 
Yoga is really about creating balance in the body through developing both strength and flexibility.
Cheers!
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