Medical apparatus for the relief of pain, namely, for the treatment of back pain; medical device for therapy of chronic pain conditions, namely, for the treatment of chronic back pain; devices for electrical stimulation for relieving back pain; medical apparatus, namely, electrodes, leads, and pulse generators for electrical stimulation for use in medical treatment; medical apparatus, namely, implantable stimulation devices and associated accessories for use in medical treatment, namely, electrodes, leads, and pulse generators; devices for rehabilitating back muscles to treat pain; devices for rehabilitating the multifidus muscle to treat pain; devices for stimulating nerve tissue to rehabilitate muscles