are used by many competitors. These often specify that the speed of muscle contraction during weight training should be the same as that of the particular sport. Strength training can substantially prevent sports injuries. History Until the 20th century, the history of strength training was very similar to the history of weight training. With the advent of modern technology, materials and knowledge, the methods that can be used for strength training have multiplied significantly. Hippocrates explained the principle behind strength training when he wrote "that which is used develops, and that which is not used wastes away", referring to muscular hypertrophy and atrophy. Progressive resistance training dates back at least to Ancient Greece, when legend has it that wrestler Milo of Croton trained by carrying a newborn calf on his back every day until it was fully grown. Another Greek, the physician Galen, described strength training exercises using the halteres (an early form of dumbbell) in the 2nd century. Ancient Persians used the meels, which became popular during the 19th century as the Indian club, and has recently made a comeback in the form of the clubbell. The dumbbell was joined by the barbell in the latter half of the 19th century. Early barbells had hollow globes that could be filled with sand or lead shot, but by the end of the century these were replaced by the plate-loading barbell commonly used today. Strength training with isometric exercise was popularised by Charles Atlas from the 1930s onwards. The 1960s saw the gradual introduction of exercise machines into the still-rare strength training gyms of the time. Strength training became increasingly popular in the 1980s following the release of the bodybuilding movie Pumping Iron and the subsequent popularity of Arnold Schwarzenegger. Subpopulations Sex differences Men and women have similar reactions to resistance training with comparable effect sizes for hypertrophy and lower body strength, although some studies have found that women experience a greater relative increase in upper-body strength. Because of their greater starting strength and muscle mass, absolute gains are higher in men. In older adults, a systematic review found that women experienced a larger increase in lower-body strength. Safety concerns related to children Orthopaedic specialists used to recommend that children avoid weight training because the growth plates on their bones might be at risk. The very rare reports of growth plate fractures in children who trained with weights occurred as a