Flexion relaxation of the hamstring muscles during lumbar-pelvic rhythm.

Objective: This study investigated the simultaneous activity of back muscles and hamstring muscles during sagittal forward body flexion and extension in healthy persons. The study was cross-sectional.

Design: A descriptive study of paraspinal and hamstring muscle activity in normal persons during lumbar-pelvic rhythm.

Setting: A university hospital.

Participants: Forty healthy volunteers (21 men, 19 women, ages 17 to 48 years), all without back pain or other pain syndromes.

Main Outcome Measures: Surface electromyography (EMG) was used to follow activities in the back and the hamstring muscles. With movement sensors, real lumbar flexion was separated from simultaneous pelvic motion by monitoring the components of motion with a two-inclinometer method continuously from the initial upright posture into full flexion. All signals were sampled during real-time monitoring for off-line analyses.

Results: Back muscle activity ceased (ie, flexion relaxation [FR] occurred) at lumbar flexion with a mean of 79 degrees. Hamstring activity lasted longer and EMG activity ceased in the hamstrings when nearly full lumbar flexion (97%) was reached. After this point total flexion and pelvic flexion continued further, so that the last part of lumbar flexion and the last part of pelvic flexion happened without back muscle activity or hamstring bracing, respectively.

Conclusions: FR of the back muscles during body flexion has been well established and its clinical significance in low back pain has been confirmed. In this study, it was shown for the first time that the hip extensors (ie, hamstring muscles) relax during forward flexion but with different timing. FR in hamstrings is not dependent on or coupled firmly with back muscle behavior in spinal disorders and the lumbar pelvic rhythm can be locally and only partially disturbed.