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Investigations using animal models show that estrogen is related to enzyme release, specifically creatine kinase, from exercised skeletal muscle. In humans, women have lower resting blood creatine kinase levels than men and have an attenuated blood creatine kinase response after prolonged endurance exercise. These results have led to the common belief that women may be protected from exercise-induced muscle damage due to circulating estrogen. Studies using laboratory models to examine gender differences in exercise-induced muscle damage, however, have not consistently documented that women have an attenuated response compared with men. Furthermore, research on exercise responses in women with different circulating levels of estrogen has not found estrogen to be related to indicators of muscle damage. Recent studies, in fact, have reported that women may experience more muscle damage, based on indirect measures, than men. Although some data exist that women may have a faster recovery from exercise-induced muscle damage, these results are tentative at this time.