Pranayama is the combination of the word "pran", life force, with "ayama", meaning expansion, extension. In this sense pranayama expands the life force though all levels of our being: physical, psychological and spiritual. Pranayama is the science for expanding and channelling the prana or life force. Prana is closely associated with breath, when the mind is clear and balanced, the breath is even and rhythmic; when the mind is nervous and tense, the breath is strained and erratic. Pranayama is a bridge between the conscious and the unconscious and serves to integrate body, mind, and spirit. Unlike other muscles which can either be controlled (like the muscles of the arms) or not (the muscles of internal organs), the diaphragm, the most important breathing muscle, is both a voluntary and an involuntary muscle, therefore links the conscious and unconscious functions of the body. A central focus of yoga is to make what is unconscious conscious, pranayama brings breath and prana into consciousness.