Deep breathing
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Sources
Hoopes, A. (2002). Breathing Training For Martial Artists. Shotokan Karate Magazine. (Issue 72); Generating Ki through Breathing. Shotokan Karate Magazine. (Issue 73); Stillness Training, The Basis of all Movement. Shotokan Karate Magazine. (Issue 72).
Sonnon, S. (2001). Oxygen Debt Does NOT Equal 'Cardio Training. Dvizheniye Journal July/August 2001. Available: www.amerross.com.