BORGHUIS, A. J., K. A. P. M. LEMMINK, and A. L. HOF. Core Muscle Response Times and Postural Reactions in Soccer Players and Nonplayers. Med. Sci. Sports Exerc., Vol. 43, No. 1, pp. 108-114, 2011. Decreased core stability has been suggested to be associated with a higher occurrence of lower extremity injuries and low back pain. In a physical contact sport like soccer, direction-specific muscle reflex responses are crucial in maintaining core stability. Delayed core muscle response times repeatedly have been reported in patients with low back pain, but no study has compared core muscle reflex latencies and postural control between soccer players and less active nonplayers. Purpose: The goal of this study was to investigate whether soccer players will exhibit shorter average core muscle reflex latencies along with less postural sway in response to a sudden trunk perturbation compared with nonplayers. A second goal was to see whether postural control measures are a valid, more practical alternative for the use of surface EMG in measuring reflexive core neuromuscular control. Methods: Sudden trunk loading in the frontal and sagittal plane was used in 10 high-level amateur soccer players and 11 less active nonplayers to study core muscle reflex latencies, using surface EMG of six major trunk muscles. Simultaneously, kinematic response data of a balance seat were obtained using gyroscopes measuring seat angular velocity. Results: Soccer players demonstrated shorter reflex latencies compared with nonplayers for the rectus abdominis, erector spinae, and externus obliquus muscles in response to sagittal plane perturbations. These shorter reflex latencies went along with greater seat movement in response to sudden trunk loading, with moderate correlations between the two measures. Conclusions: The results showing shorter reflex latencies and greater balancing movements for soccer players add to the debate whether more postural sway is an appropriate indicator of having less neuromuscular control.