This report examines associations between injuries and flexibility, stretching, warm-up, and body composition. Military studies show that either too much or too little flexibility increases injury risk. Static stretching prior to exercise does not appear to reduce the overall injury incidence, although further research is needed on some types of injuries. Static stretching also appears to reduce strength and power (explosive strength). Warm-up (low intensity activity prior to exercise or sports) appears to reduce injury risk. Body mass index (BMI; weight in kg/ height in m²) is a surrogate measure of body fat because it is highly related to laboratory measures of body fat. However, Soldiers can also have a high BMI because of higher muscle mass. If high BMI reflects a larger percentage of body fat relative to height, injury risk might be increased because the additional fat would increase the intensity of physical activity, leading to more rapid fatigue and repetitive stress on the musculoskeletal system. Low BMI could reflect a paucity of fat or muscle/ bone, or both. Low BMI may make Soldiers more susceptible to injury if they lack the muscle mass or strength in the supportive structures (ligaments, bones) required to perform certain physical tasks, and if they overexert or overuse the available muscle mass or supportive structures. Studies in basic combat training show that both high and low BMI increases injury risk. However, studies among active duty Soldiers only show that injury risk increases as BMI increases, possibly because very few active duty Soldiers have very low BMI (i.e., less than 18 kg/m²).