# Bilateral Deficits during Maximal Grip Force Production in Late Postmenopausal Women
## 4 Discussion  
This study examined bilateral deficit patterns between late postmenopausal and young premenopausal women by estimating the maximal hand-grip force production. Late postmenopausal women showed significantly less hand-grip forces produced in both unilateral (i.e., dominant and nondominant hand) and bilateral tests, and further revealed greater bilateral deficit patterns than young premenopausal women. For late postmenopausal women, decreased maximal hand-grip forces generated by the dominant hand and both hands were significantly related to greater bilateral deficit patterns and increased values in central pulse pressure. Despite inconsistent findings on the presence of a bilateral deficit pattern in the aging population [28,42], we found that a greater bilateral deficit in the upper extremities appeared in late postmenopausal women. These results expanded previous findings that mainly reported the bilateral deficit phenomenon in the lower extremities [24, 25, 26, 27, 28]. Reduced maximal hand-grip forces from each hand during bilateral contraction as compared to those during unilateral contraction may be related to higher interhemispheric inhibition between hemispheres in late menopausal women. Some previous studies asserted that bilateral deficit may be related to suppressive effects of interhemispheric inhibitions between hemispheres during bilateral movement execution [43,44]. In unimanual contraction, increased interhemispheric inhibition from the dominant hemisphere may influence the nondominant hemisphere to suppress the mirror movements of contralateral extremities [45,46]. In a bilateral contraction, both hemispheres may be affected by interhemispheric inhibitions, and these suppressions potentially interfere with motor outputs from each limb [47, 48, 49]. Interestingly, previous studies reported that greater levels of interhemispheric inhibition in premenopausal women were related to decreased estradiol level during the ovarian cycle, whereas these changes in interhemispheric inhibition level were not observed in males between pre- and post-tests with an interval of 14 days [50,51]. These findings raised a possibility that greater interhemispheric inhibition levels in late postmenopausal women induced by estrogen deficiency may be related to their bilateral deficit patterns during maximal hand-grip force production. Moreover, our correlation findings indicated that greater reduction of maximal hand-grip strength in the dominant hand was significantly related to increased bilateral deficit patterns in late postmenopausal women. Previous studies reported that maximal hand-grip strength of the dominant hand in postmenopausal women significantly decreased as compared to those in either premenopausal women or age-matched men because of potential interactive effects of aging and estrogen deficiency [16,52]. Impaired muscle strength is frequently observed in older adults because of decreased muscle mass and quality (i.e., muscle strength per muscle mass) as referred to age-related sarcopenia [53]. Moreover, the occurrence rate of sarcopenia highly increases around 50s in women who may experience menopause [54,55]. Several studies posited that estrogen may show an anabolic effect on muscles by stimulating insulin-like growth factor-1 (IGF-1) receptors [56], and decreased levels of estrogen may be related to greater oxidative stress that potentially engenders muscle atrophy [57, 58, 59]. Moreover, postmenopausal women may have deficits in activation of estrogen receptors highly observed in type II muscle fibers [60,61] influenced by less estrogen and IGF-1 levels, and the deactivation of estrogen receptors presumably impairs muscle strength [58,62]. Consequently, estrogen deficiency in late postmenopausal