Muscular Response to Sudden Load

STUDY DESIGN: Subjects were exposed to fatiguing and restorative interventions to assess their response to sudden loads. OBJECTIVES: To investigate the erector spinae and rectus abdominis response characteristics to "sudden load" and the effect of fatigue and rehabilitation. SUMMARY OF BACKGROUND DATA: Unexpected loads which people often experience, can lead to high forces in the spine and may be a cause of low back injury. METHODS: Muscle responses to sudden load were mediated by fatigue, walking, expectation, method of load application, exposure to vibration, and cognitive-behavioral rehabilitation in patients with chronic low back pain. A novel technique, perfected in this work, called wavelet analysis, was used to analyze these data. RESULTS: Reaction time was affected by fatigue and expectation. Vibration exposure significantly increased the muscle response time. Walking was able to ameliorate that effect. Back muscles responded differently, depending on whether loads were applied to the back through the hands or through the trunk. Electromyographic reaction time and magnitude decreased in patients after a 2-week rehabilitation program. CONCLUSIONS: Sudden loads can exacerbate fatigue effects. Walking after driving reduces the risk to the back caused by handling unpredictable loads. Vibration exposure guidelines should be more conservative. Patients have longer response times than healthy subjects, but patients can improve their response to sudden loads via rehabilitation. Patients exhibit a flexion-extension oscillation at 5 Hz in response to a sudden load, suggesting that the 5-Hz, seated, natural frequency observed during whole-body vibration may result from neurophysiologic control limits.