Trunk Muscle Loading in Non-Sagittally Symmetric Postures as a Result of Sudden Unexpected Loading Conditions

The present study investigated the effect of warning time and magnitude of an external loading on the trunk muscular response to sudden loading conditions while in a non-sagittally symmetric posture. Eleven subjects were asked to catch falling weights of three magnitudes (3, 6, and 9 kg) with four levels of warning time (0, 100, 200, and 400 ms) in an asymmetric posture. For each of the eight muscles sampled with surface electrodes the integrated electromyographic (EMG) signal was interpreted in terms of its peak value, mean value, onset rate, and lead/lag time with reference to the weight drop. Results show monotonic relations between muscle force and levels of warning time, and muscle force and levels of weight. In addition, muscular forces in the left posterior trunk musculature ranged between two and five times greater than the right posterior trunk musculature in response to sudden loading conditions. This experiment demonstrates how sudden asymmetric loading, and specifically sudden loading without adequate warning time may be involved in the development of low back pain.