Paraspinal muscle responses during sudden upper limb loading.

The paraspinal muscle responses for unexpected and expected upper limb loading were investigated by surface EMG of 20 healthy volunteers. The simultaneous trunk and hand accelerations with paraspinal, biceps brachii and soleus muscles EMG were measured in four subjects. A short-latency response of approximately 50 ms was observed in paraspinal muscles. The latency was approximately 3 ms shorter (P = 0.017) during "expected" trials on average and the latency shortened during the first three expected trials (P = 0.02). Anticipation also decreased the magnitude of the response (P < 0.05). Trunk movement initiated approximately 35 ms and approximately 50 ms after the impact of the load at T6 and T12 levels, respectively. In conclusion, visual expectation shortens the latency and decreases the magnitude of the paraspinal muscle response to sudden upper limb loading. Also, the trial repetition has an effect on reflex latency if visual information is available. These results indicate that anticipation modulates the reflex control of paraspinal muscles, which may be significant in understanding spinal function.