HORTON, J. F., D. M. LINDSAY, and B. R. MACINTOSH. Abdominal muscle activation of elite male golfers with chronic low back pain. Med. Sci. Sports Exerc., Vol. 33, No. 10, 2001, pp. 1647–1654. Purpose: The purpose of this study was twofold: 1) to determine whether elite male golfers with chronic low back pain (CLBP) exhibit different abdominal muscle activity patterns during the golf swing than asymptomatic control (AC) golfers and 2) to determine whether elite male golfers with CLBP experience greater fatigue in the abdominal muscles than AC golfers after a typical practice session. Methods: Surface EMG data were collected bilaterally from the rectus abdominis (RA), external oblique (EO), and internal oblique (IO) muscles. Muscle activity during the golf swing was measured using the root mean square (RMS) of the EMG signal in various phases of the golf swing. Fatigue was assessed using the median frequency (MF) and RMS of the EMG signal during a 10-s submaximal isometric contraction. Low back pain was quantified with the McGill Pain Questionnaire before and after the practice session. Results: No differences in the RMS of abdominal muscle activity were noted during the golf swing between AC and CLBP subjects. However, EO (lead) onset times were significantly delayed with respect to the start of the backswing in CLBP subjects. Low back pain in CLBP golfers increased significantly after the practice session. Abdominal muscle fatigue, as measured with MF or RMS, was not evident after the practice session for either AC or CLBP subjects. Conclusion: Abdominal muscle activity and muscle fatigue characteristics were quite similar between AC and CLBP subjects after repetitive golf swings. Despite this, it was clear that repetitive golf swings were aggravating some part of the musculoskeletal system in CLBP subjects, which resulted in increased pain in the low back area.