An exercise ball is a ball constructed of soft elastic with a diameter of approximately 35 to 85 centimeters (14 to 34 inches) and filled with air. The air pressure is changed by removing a valve stem and either filling with air or letting the ball deflate. It is most often used in physical therapy, athletic training and exercise. It can also be used for weight training. The ball, while often referred to as a Swiss ball, is also known by a number of different names, including balance ball, birth ball, body ball, ball, fitness ball, gym ball, gymnastic ball, physioball, pilates ball, Pezzi ball, sports ball, stability ball, Swedish ball, therapy ball, or yoga ball.
The physical object known as a "Swiss Ball" was developed in 1963 by Aquilino Cosani, an Italian plastics manufacturer. He perfected a process for molding large puncture-resistant plastic balls. Those balls, then known as "Pezzi balls", were first used in treatment programs for newborns and infants by Mary Quinton, a British physiotherapist working in Switzerland. Later, Dr. Susanne Klein-Vogelbach, the director at the Physical Therapy School in Basel, Switzerland, integrated the use of ball exercise as physical therapy for neuro-developmental treatment. Based on the concept of "functional kinetics", Klein-Vogelbach advocated the use of ball techniques to treat adults with orthopedic or medical problems. The term "Swiss Ball" was used when American physical therapists began to use those techniques in North America after witnessing their benefits in Switzerland. From their development as physical therapy in a clinical setting, those exercises are now used in athletic training, as part of a general fitness routine  and incorporation in alternative exercises such as yoga and Pilates.
A primary benefit of exercising with an exercise ball as opposed to exercising directly on a hard flat surface is that the body responds to the instability of the ball to remain balanced, engaging many more muscles. Those muscles become stronger over time to keep balance. Most frequently, the core body muscles — the abdominal muscles and back muscles — are the focus of exercise ball fitness programs.
Other uses 
Some people recommend sitting on an exercise ball instead of a chair (for example, an office chair). This is based on the theory that the abdominal and back muscles are constantly engaged and active in order to maintain proper posture and balance on the ball. There is no scientific evidence of those benefits occurring by just sitting without additional exercises. However, some people warn against using a Swiss ball as a chair due to ergonomic considerations  or biomechanical reasons 
This large plastic ball, known as a "birth ball", can also be used during labour to aid the descent of the fetal head into the pelvis. Sitting in an upright position will also aid fetal positioning and is more comfortable for the woman. Sitting on the ball with arms placed on a bed, table or otherwise sturdy object for support and gently rocking the hips may help the woman during contractions and aid the natural physiological process of birth.
See also 
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- Flett, Maureen (2003). Swiss Ball: For Strength, Tone and Posture. Sterling Publishing Company, Inc. ISBN 1-85648-663-X.
- Klein-Vogelbach, Susanne (1990). Functional Kinetics: Observing, Analyzing, and Teaching Human Movement. Springer-Verlag. ISBN 0-387-15350-0.
- Carriere, Beate; Renate Tanzberger (1998). The Swiss Ball: Theory, Basic Exercises and Clinical Application. Springer. ISBN 3-540-61144-4.
- Hillman, Susan Kay (2005). Introduction to Athletic Training. Human Kinetics. ISBN 0-7360-5292-5.
- Milligan, James (2005). Swiss Ball For Total Fitness: A Step-by-step Guide. Sterling Publishing Company, Inc. ISBN 1-4027-1965-5.
- Mitchell, Carol (2003). Yoga on the Ball. Inner Traditions / Bear & Company. ISBN 0-89281-999-5.
- Vera-Garcia FJ, Grenier SG, McGill SM (2000) Abdominal muscle response during curl-ups on both stable and labile surfaces. Phys. Ther. 80, 564-569
- Mayo Clinic Staff (August 24, 2007). "Slide show: Core exercises with a fitness ball". Mayo Clinic. Retrieved April 2008.
- Clapp, Jane (2006). Working on the Ball: A Simple Guide to Office Fitness. Andrews McMeel Publishing. ISBN 978-0-7407-5699-3.
- Greenwood-Robinson, Maggie (2007). The Biggest Loser Fitness Program. Rodale. p. 65. ISBN 1-59486-695-3.
- Gregory DE, Dunk NM, Callaghan JP (2006). "Stability ball versus office chair: comparison of muscle activation and lumbar spine posture during prolonged sitting". Hum Factors 48 (1): 142–53. doi:10.1518/001872006776412243. PMID 16696264.
- McGill SM, Kavcic NS, Harvey E (May 2006). "Sitting on a chair or an exercise ball: various perspectives to guide decision making". Clin Biomech (Bristol, Avon) 21 (4): 353–60. doi:10.1016/j.clinbiomech.2005.11.006. PMID 16410033.
- Ergoweb, Opinion: Balls as Office Chairs a Bad Idea
- Gregory, Diane E. "The Use of Stability Balls in the Workplace in Place of the Standard Office Chair". Centre for Research Expertise for the Prevention of Muscloskeletal Disorders, University of Waterloo. Retrieved Jan, 2011.
- Wesson, Nicky (2000). Labor Pain: A Natural Approach to Easing Delivery. Inner Traditions / Bear & Company. ISBN 0-89281-895-6.