Subject
Female; Humans; Male; Adult; Middle Aged; Double-Blind Method; Cross-Over Studies; beta-Endorphin/blood; Biomarkers of Pain; Acute Pain/blood/physiopathology; Analgesia/methods; Chronic Pain/blood/physiopathology; Low Back Pain/blood/physiopathology; Naloxone/pharmacology; Narcotic Antagonists/pharmacology; Pain Measurement/drug effects; Pain Threshold/drug effects/physiology; Physical Stimulation