Strength training or resistance training involves the performance of physical exercises that are designed to improve strength and endurance. It is often associated with the lifting of weights. It can also incorporate a variety of training techniques such as bodyweight exercises, isometrics, and plyometrics. Training works by progressively increasing the force output of the muscles and uses a variety of exercises and types of equipment. Strength training is primarily an anaerobic activity, although circuit training also is a form of aerobic exercise. Strength training can increase muscle, tendon, and ligament strength as well as bone density, metabolism, and the lactate threshold; improve joint and cardiac function; and reduce the risk of injury in athletes and the elderly. For many sports and physical activities, strength training is central or is used as part of their training regimen. Principles and training methods The basic principles of strength training involve repeated overloading of a group of muscles, typically by contracting the muscles under heavy resistance and returning to the start position for several repetitions until failure. The basic method of resistance training uses the principle of progressive overload, in which the muscles are overloaded by working against as high resistance as they are capable of. They respond by growing larger and stronger. Beginning strength-trainers are in the process of training the neurological aspects of strength, the ability of the brain to generate a rate of neuronal action potentials that will produce a muscular contraction that is close to the maximum of the muscle's potential. Intensity, volume, and frequency Three important variables of strength training are intensity, volume, and frequency. Intensity is the amount of work required to achieve the activity and is often measured by the percentage of an individual's one-repetition maximum (1RM). The intensity limits the number of repetitions that can be carried out in one set, and is correlated with the repetition ranges chosen, traditionally divided as follows: 1 to 5 repetitions per set at 80% to 100% of 1RM—heavy, may have an advantage in strength development 8 to 12 repetitions per set with 60% to 80% of 1RM—moderate, traditionally thought to maximize hypertrophy, although more recent research has shown that hypertrophy can be maximized with a wide range loading schemes equal or greater than 30% 1RM of assuming the set is taken to failure. 15+ repetitions per set with loads below 60% of 1RM—light, traditionally recommended to increase endurance Volume