Muscles are arranged in pairs of opposites and have two main functions - to contract and release.  The contraction process is generated, the release process is not.  It is in the release process that motion problems can develop.

Every move produces its greatest stress upon a specific point.  A muscles that cannot accommodate the movement placed upon it will pull or even tear.  Thus, a movement within the safe context becomes unsafe when shortening and spasms are present.  Pressure causes discomfort and pain and the horse responds to this by resisting, refusing or not performing certain movements