Starting blocks are relatively stationary objects that athletes use to push off of at the start of their races. They are, in a sense, the launch pad of the human rocket.
Starting blocks are used mainly in two different sports: track and field, and swimming
Starting blocks usually have one or two long rails, and two platforms that can be adjusted along their lengths. There is also an anchoring device, so that the runner will not just push back the blocks, and fall on their face.
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This is a side view of the blocks.
Starting blocks are used by nearly all competitors, in nearly all levels of competition in the standard sprint events: 400 meter dash, 200 meter dash, 100 meter dash, 400 meter hurdles, and 100 and 110 meter hurdles
Swimming blocks are usually permanently mounted at one end of a pool. Like track blocks, one block is used per lane.
The swimming block itself looks like a small platform that is slightly tilted towards the pool. The top of the platform is usually textured to prevent slipping, since most people will be wet and barefooted already. Furthermore, they need to be able to push off of the block properly in their starting dive. The platform part of the block is used for starting races that are in the freestyle, butterfly, breastroke, or IM format. Most relays also use the platform for the start.
Most swimming starting blocks also have handles or bars on the side nearest the water. These are used for starting backstroke races and relays, where the swimmers start in the water.