According to Crangle, some researchers have favoured a linear theory, which attempts "to interpret the origin and early development of Indian contemplative practices as a sequential growth from an Aryan genesis",[54][note 4] just like traditional Hinduism regards the Vedas to be the ultimate source of all spiritual knowledge.[55][note 5] Thomas McEvilley favors a composite model where pre-Aryan yoga prototype existed in the pre-Vedic period and its refinement began in the Vedic period.[58]
The Rigveda, however, does not describe yoga, and there is little evidence as to what the practices were.[7] Early references to practices that later became part of yoga, are made in Brihadaranyaka Upanishad, the earliest Hindu Upanishad.[67] For example, the practice of pranayama (consciously regulating breath) is mentioned in hymn 1.5.23 of Brihadaranyaka Upanishad (c. 900 BCE), and the practice of pratyahara (concentrating all of one's senses on self) is mentioned in hymn 8.15 of Chandogya Upanishad (c. 800–700 BCE).[68][note 8] The Jaiminiya Upanishad Brahmana teaches mantra repetition and control of the breath.[71]
Although muscle stimulation occurs in the gym (or home gym) when lifting weights, muscle growth occurs afterward during rest periods. Without adequate rest and sleep (6 to 8 hours), muscles do not have an opportunity to recover and grow.[citation needed] Additionally, many athletes find that a daytime nap further increases their body's ability to recover from training and build muscles. Some bodybuilders add a massage at the end of each workout to their routine as a method of recovering.[51]