 Most Chinese martial art styles are classified as external styles.Internal styles () focus on the practice of such elements as awareness of the spirit, mind, qi (breath, or energy flow) and the use of relaxed leverage rather than muscular tension, which soft stylists call "brute force".While the principles that distinguish internal styles from the external were described at least as early as the 18th century by Chang Nai-chou, the modern terms distinguishing external and internal styles were first recorded by Sun Lutang; who wrote that Taijiquan, Baguazhang, and Xingyiquan were internal arts.