This is the shirt that alerts you of improper posture to help prevent neck, back, and shoulder pain. Whenever you slouch, an electronic sensor detects the increased tension in the shirt's built-in microfilaments and vibrates to let you know your posture is not optimal (vibration ceases when you correct yourself). Designed by the team physician for the Boston Celtics, the tension can be adjusted to suit your posture. The washable, unisex shirt can be worn as an undershirt and is made from a 95% cotton/5% spandex blend. Uses (and includes) three watch batteries. Fit should be snug to ensure optimal sensitivity of microfilaments; order one size smaller than you normally wear for a t-shirt. Men's sizes S (37), M (39), L (41) and XL (43); women's sizes S (4-6), M (6-10), L (12-14) and XL (16-18). (1 oz.)