Muscles are made up of thousands of fibres which flex and extend to provide movement. The major Superficial muscles are shown in the diagram above and are provided as a reference. Each muscle originates from a stable part of the skeleton to which it is attached. ( see skeletal link above) Most muscles work in pairs or groups . To enable the horse to move, one muscle must shorten (contract) while its opposing muscles lengthens (relaxes). When muscles work in this way they are known as 'antagonistic pairs'