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The kettlebell or girya (Russian: ги́ря) is a cast-iron or steel weight (resembling a cannonball with a handle) used to perform ballistic exercises that combine cardiovascular, strength and flexibility training. They are also the primary equipment used in the weight lifting sport of girevoy sport. Russian kettlebells are traditionally measured in weight by pood, which (rounded to metric units) is defined as 16 kilograms (35 lb). Even when kettlebells in Western gyms are labelled in kg, their sizes are based on units of whole and half poods (illus.).
Kettlebells were developed in Russia in the 1700s, primarily for weighing crops. It is said that these farmers became stronger and found them useful for showing off their strength during festivals. The Soviet army used them as part of their physical training and conditioning programs in the 20th century . They had been used for competition and sports throughout Russia and Europe since the 1940s. Though kettlebells had been in the United States in some form since the 1960s or earlier, Dragon Door Publications and Pavel Tsatsouline developed the first instructor certification program in the USA in 2001.
Unlike traditional dumbbells, the kettlebell's center of mass is extended beyond the hand, similar to Indian clubs or ishi sashi. This facilitates ballistic and swinging movements. Variants of the kettlebell include bags filled with sand, water, or steel shot. The kettlebell allows for swing movements and release moves with added safety and added grip, wrist, arm and core strengthening. The unique shape of the kettlebell provides the "unstable force" for handling - key for the effectiveness of the kettlebell exercises. 
By their nature, typical kettlebell exercises build strength and endurance, particularly in the lower back, legs, and shoulders, and increase grip strength. The basic movements, such as the swing, snatch, and the clean and jerk, engage the entire body at once, and in a way that mimics real world activities such as shoveling or farm work.
Unlike the exercises with dumbbells or barbells, kettlebell exercises often involve large numbers of repetitions. Kettlebell exercises are in their nature holistic; therefore they work several muscles simultaneously and may be repeated continuously for several minutes or with short breaks. This combination makes the exercise partially aerobic and more similar to high-intensity interval training rather than to traditional weight lifting. In one study, kettlebell enthusiasts performing a 20 minute snatch workout were measured to burn, on average, 13.6 calories/minute aerobically and 6.6 calories/minute anaerobically during the entire workout - "equivalent to running a 6-minute mile pace". Because of their high repetitions, kettlebell progression should start out slowly to build muscle endurance, support the joints and prevent injury.
The movements used in kettlebell exercise can be dangerous to those who have back or shoulder problems, or a weak core.[dead link] However, if done properly they can also be very beneficial to health. They offer improved mobility, range of motion and increased strength. 
Kettlebell Swing: The kettlebell swing is a basic kettlebell exercise that is used in training programs and gyms for improving the posterior chain muscles. The key to a good kettlebell swing is effectively hinging at the hips, creating stability through the frontal plane. Variations of kettlebell swings include Russian swings (kettlebell goes to chest level), American swings (kettlebell goes to overhead), and one-armed swings.
Turkish Get-up: A kettlebell exercise that combines the lunge, bridge and side plank to build strength. With a vertically-extended arm, the athlete transitions from laying supine on the floor to standing.
Contemporary kettlebell training is represented basically by four styles.
Hardstyle has its roots in powerlifting and Gōjū-ryū karate training, particularly hojo undō concepts. With emphasis on the "hard" component and borrowing the concept of kime, the Hardstyle focuses on strength and duality of relaxation and tension.
Crossfit kettlebell refers to implementation of kettlebell training as in CrossFit curricula, often with significant modifications to preceding styles (eg. American Swing vs. conventional swing, placing the kettlebell down between snatchs).
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