Muscle relaxants are a group of medications that act in the brain (not just in muscles) and relax the whole body. They are often prescribed by the doctor to treat back pain, muscle spasms and spasticity (carisoprodole, cyclobenzaprine, diazepam,baclofen,tizanidine, mataxalone, benzodiazepine, etc.).
Skeletal muscle relaxants (SMR) are another group of relaxants that is used in intubations and surgery.
Muscle relaxants reduce stiffness, relax muscles, relieve pain. In addition to the reduction of contractility (they block the transmission of nerve impulses), muscle relaxants may sometimes cause sedation though.