Context: Although a variety of theories and studies have been cited to support the use of joint mobilization in the spine as an integral part of the treatment and rehabilitation process, information about the short-term effects of joint mobilization on acute low back injury with respect to patient pain and strength changes has been limited.Objective: To examine the short-term effects of grade 1 and 2 posteroanterior joint mobilizations at the lumbar spine on subject pain and muscle force after an episode of acute, mechanical low back pain.Design: Group (2) by time (2 or 3).Setting: Athletic training clinic.Patients or Other Participants: Male collegiate athletes (n = 19) with mechanical low back pain as assessed through a standardized evaluation were randomly assigned to a control (n = 10) or experimental (n = 9) group.Intervention(s): All subjects underwent a standardized treatment protocol of cryotherapy and stretching during data collection. Subjects completed the McGill Pain Questionnaire and a visual analog scale (the latter to assess pain levels during range-of-motion activities) and, using a handheld dynamometer, performed 3 maximum voluntary isometric contractions to determine muscle force. Grade 1 and 2 joint mobilizations were administered to the experimental group, whereas the control group was placed in a prone position of comfort for the time it took to perform the joint mobilizations.Main Outcome Measure(s): Baseline, immediate posttreatment, and 24-hour posttreatment measurements of pain and muscle force were taken.Results: Compared with the control group, the experimental group demonstrated significant decreases in the sensory subscale scores of the McGill Pain Questionnaire and in pain during lumbar extension and a significant increase in force production.Conclusions: Grade 1 and 2 joint mobilizations reduced subjects' pain and increased force production in the short-term stages of mechanical low back pain.