Prolactin responses to infant cues in men and women: effects of parental experience and recent infant contact

We used a longitudinal design to test whether parental experience differentially affects the development of prolactin responses to infant cues in men and women. Couples provided two blood samples at three tests, one test just before their babies were born, and two tests during the early postnatal period (n=21). Nine couples repeated the tests near the birth of their second babies. In the 30 min between the two samples, couples listened to recorded infant cries at the prenatal test and held their baby (fathers) or a doll (mothers) at the postnatal tests. Blood samples were analyzed for prolactin concentrations. Prolactin values were then related to sex and parity differences as well as to questionnaire data concerning emotional responses to infant cries and previous infant contact. We found that (1) prior to the birth of both the first and second babies, women's prolactin concentrations increased after exposure to infant stimuli, whereas men's prolactin concentrations decreased; postnatal sex differences varied with parity; (2) women's prolactin reactivity did not change significantly with parental experience; (3) the same men's prolactin concentrations decreased after holding their first newborns but increased after holding their second newborns; this change was not gradual or permanent; (4) men reporting concern after hearing recorded infant cries showed a different postnatal pattern of prolactin change after holding their babies than men not reporting concern; and (5) men who had little contact with their babies just prior to testing had a more positive prolactin response than men who had recently held their babies for longer periods. Although parental experience appears to affect men's prolactin responses, differences in reactivity were also related to patterns of recent infant contact and individual differences in responses to infant cues.     

The effects of social isolation on steroid hormone levels are modulated by previous social status and context in a cichlid fish

Social isolation is a major stressor which impacts the physiology, behaviour and health of individuals in gregarious species. However, depending on conditional and contextual factors, such as social status and group composition, social isolation may be perceived differently by different individuals or even by the same individuals at different times. Here we tested the effects of social status (territorial vs. non-territorial) and previous group composition (i.e. type of social group: mixed sex group with two territorial males, TT vs. mixed sex group with one territorial and one non-territorial male, TnT) on the hormonal response (androgens and cortisol) to social isolation in a cichlid fish (Oreochromis mossambicus). The different steroid hormones measured responded differentially to social isolation, and their response was modulated by social factors. Social isolation elicited a decrease of 11-keto formation only in territorial males, whereas non-territorial males present a non-significant trend for increasing KT levels. Testosterone did not respond to social isolation. Cortisol only increased in isolated individuals from TnT groups irrespective of social status (i.e. both in territorials and non-territorials). These results suggest that it is the perception of social isolation and not the objective structure of the situation that triggers the hormonal response to isolation.     

Inhibiting influence of testosterone on stress responsiveness during adolescence

The maturation of the hypothalamo-pituitary-adrenal (HPA) axis is a key-component of the changes that occur during adolescence. In guinea pigs, HPA responsiveness during late adolescence depends strongly on the quantity and quality of social interactions: Males that lived in a large mixed-sex colony over the course of adolescence exhibit a lower stress response than males that were kept in pairs (one male/one female). Since colony-housed males have higher testosterone (T) levels than pair-housed males, and inhibiting effects of T on HPA function are well known, we tested the hypothesis that the decrease in stress responsiveness found in colony-housed males is due to their high T concentrations. We manipulated T levels in two experiments: 1) gonadectomy/sham-gonadectomy of colony-housed males (which usually have high T levels), 2) application of T undecanoate/vehicle to pair-housed males (which usually have low T levels). As expected, gonadectomized males showed a significantly increased stress response in comparison with sham-gonadectomized males, and T-injected males had a significantly lower stress response than vehicle-injected males. Both experiments thus confirm an inhibiting effect of T on HPA responsiveness during adolescence, which can mediate the influence of social interactions. The reduction in stress responsiveness is hypothesized to have a biologically adaptive value: A sudden increase in glucocorticoid concentrations can enhance aggressive behavior. Thus, pair-housed males might be adapted to aggressively defend their female (‘resource defense strategy’), whereas colony-housed males display little aggressive behavior and are capable of integrating themselves into a colony (‘queuing strategy’).     

Rapid endocrine responses of young men to social interactions with young women

It is well-established that males of many nonhuman vertebrate species exhibit hormonal reactions to stimuli from potential mates. The present studies were designed to test replication of preliminary findings suggesting that human males may exhibit such reactions as well. In Experiment 1, young men (n=115) provided saliva samples before and after either conversing with a woman confederate or sitting alone for 15 min. Changes from baseline in salivary testosterone concentrations were significantly greater among the men exposed to women, but only among subjects tested in the afternoon. In Experiment 2, male subjects (n=99) interacted with either a male or a female confederate with saliva samples collected before and after these interactions and all experimental sessions conducted in the afternoon. Men who interacted with women exhibited significant elevations of testosterone relative to both their own baseline concentrations and to change scores among the men who interacted with other men. In addition, women confederates' ratings of men's extraversion and degree of self-disclosure were positively correlated with changes in testosterone. In both experiments, furthermore, changes in cortisol concentrations from baseline were significantly greater among men who spoke with women relative to men in the control conditions. The results provide evidence that social interactions with potential mates can in fact trigger specific patterns of endocrine responses in human males.     

Hormonal correlates of human paternal interactions: a hospital-based investigation in urban Jamaica

To expand our understanding of the neuroendocrine mechanisms underlying human fatherhood, including its cross-cultural expression, we investigated the hormonal correlates of fatherhood in the greater Kingston, Jamaica area. We recruited 43 men, aged 18-38, to participate: 15 single men; 16 "coresidential" fathers (men who live with their adult female partner and youngest child); and 12 "visiting" fathers (men who live apart from their adult female partner and youngest child). The research protocol entailed biological sampling before and after a 20-min behavioral session during which single men sat alone and fathers interacted with their partner and youngest child. Hormone measures relied upon minimally invasive techniques (salivary testosterone and cortisol, finger prick blood spot prolactin, urinary oxytocin and vasopressin). Results revealed significant group differences in average male testosterone levels (p=0.006), with post hoc contrasts indicating that visiting fathers had significantly (p<0.05) lower testosterone levels than single men. Prolactin profiles also differed significantly across groups (p=0.010) whereby post hoc contrasts showed that prolactin levels of single men declined significantly compared with the flat levels of visiting fathers (p<0.05). No group differences in cortisol, oxytocin or vasopressin levels were observed. However, among fathers, vasopressin levels were significantly and negatively (r=-.431, p=0.022) correlated with the age of a man's youngest child. These results thus implicate lower testosterone levels as well as prolactin and vasopressin in human fatherhood. These findings also highlight the importance of sociocultural context in human fatherhood while exhibiting parallels with existing data on the non-human vertebrate hormonal bases of paternal care.     

Comparative analysis of male androgen responsiveness to social environment in birds: the effects of mating system and paternal incubation

Male androgen responses to social challenges have been predicted to vary with mating system, male-male aggressiveness, and the degree of paternal investment in birds ("challenge hypothesis," Am. Nat. 136 (1990), 829). This study focused on the interspecific predictions of the challenge hypothesis. Comparative methods were used to control for effects of the phylogenetic relatedness among the sampled species. Male androgen data of 84 bird species were collected from literature records on seasonal androgen patterns. From these, the androgen responsiveness (AR) was calculated as described in the original challenge hypothesis (i.e., maximum physiological level/breeding baseline). Scatterplots of AR versus mating strategy, male-male aggressiveness, and the degree of paternal care confirmed the expected interspecific patterns. When phylogenetic analyses were performed among all of the sampled species, the effects of paternal investment disappeared while the AR remained covarying to a high degree with mating system and male-male aggressiveness. Although these mechanisms may be different at the intraspecific level, this suggests that interspecific differences of AR in male birds may have evolved in response to changes of mating strategies, rather than in response to altered paternal duties. However, control for phylogeny among the subsample of 32 passerine species revealed that if any paternal investment contributed to the observed variance in AR, then the change from "no male incubation" to "male shares incubation duties" represented the most effective, whereas the male's contribution to feeding offspring did not explain the observed variation of AR.     

The hormonal and behavioral response to group formation, seasonal changes, and restraint stress in the highly social Malayan Flying Fox (Pteropus vampyrus) and the less social Little Golden-mantled Flying Fox (Pteropus pumilus) (Chiroptera: Pteropodidae)

This study examined behavioral and physiological responses (changes in inter-animal spacing, total glucocorticoids, testosterone, and body mass) to the formation of breeding and same-sex groups in two bat species, the socially gregarious Malayan Flying Fox (Pteropus vampyrus) and the less social Little Golden-mantled Flying Fox (Pteropus pumilus). We hypothesized that social instability, especially in the breeding groups and especially in P. vampyrus, would result in elevated glucocorticoids and that social facilitation of breeding and/or male-male competition would result in persistently higher levels of testosterone in breeding males. Seasonal rhythms in all measures were also predicted, and the glucocorticoid stress response was expected to vary by sex, season, and group type. Nearly all animals responded to group formation with elevated glucocorticoids, but, for breeding animals (especially aggressive male P. vampyrus), these responses persisted over time. In both species, breeding group formation resulted in elevated testosterone in males. Glucocorticoids, testosterone, testes volume, and body mass generally peaked in the breeding season in males (late summer and early autumn), but the seasonal glucocorticoid peak in females occurred in late winter and early spring. All animals responded to restraint stress with elevations in glucocorticoids that largely did not differ by sex, time of year, reproductive condition, group type, or, in lactating females, the presence of her pup. Changes in both behavior and physiology were more evident in P. vampyrus than in P. pumilus, and we believe that their underlying social differences influenced their responses to group formation and to the changing seasonal environment.     

Behavioural and hormonal effects of social isolation and neophobia in a gregarious bird species, the European starling (Sturnus vulgaris)

Separating gregarious individuals from their group members often results in behavioural and physiological changes, like increased levels of corticosterone. Testosterone and corticosterone, in particular, have been implicated in the response of mammals to novelty. Data in birds are, however, rare. The presence or absence of group members may also influence an individual's response to novel stimuli. We assessed the behaviour and hormonal response of European starlings (Sturnus vulgaris) to a novel object in two different situations and seasons: each starling was tested when separated and when in contact with its group members in May/June (breeding season) and again in September/October (non-breeding season). Starlings are gregarious throughout the year, but as foraging flocks are small during the breeding season and large during the non-breeding season, we assumed that non-breeding starlings would be more affected by social isolation. Overall, starlings had higher levels of corticosterone, lost more body mass, and were more active when they were separated from their group. Isolated individuals, however, did not show a greater neophobic response than individuals in the presence of their group members in either season. Circulating levels of testosterone and corticosterone were higher after a test with novel object than after a test with only the familiar feeding dish in both sexes and seasons. However, control tests for handling effects confirmed only the increase in testosterone. Our study shows that social isolation is stressful for unrelated and unpaired members of a wild flocking bird species and demonstrates that novelty can lead to a rise in testosterone in birds.     

Impact of social environment on female chimpanzee reproductive cycles

The socio-sexual environment of a female is known to affect ovarian function. Increased male contact can enhance menstrual cycle regularity. Conversely, social deprivation constitutes a form of stress that often alters cyclicity and the secretion of reproductive hormones. The present study was carried out on captive female chimpanzees to examine possible interactions among housing conditions, menstrual cycle length, morphological changes in secondary sexual character expression and endocrine release patterns related to follicular and luteal function. Animals were housed over a period of 2 years either with a male conspecific or singly. Blood samples were collected over three cycles, and anogenital swelling changes registered to define menstrual cycle phases. Fecal sampling techniques were used to monitor cortisol as a measure of stress-load. Male presence seemed to affect female cyclicity. Females housed with a male had shorter and more regular cycles than singly housed females. Prolactin, gonadotropins and estradiol levels were generally higher in paired females during specific cycle phases. Group variation was not always significant. No differences were found in progesterone. Sexually cohabited females tended to have lower fecal cortisol metabolites immediately before and after maximum tumescence. We suggest that the close behavioral, physical and olfactory contact with a male conspecific can act as a sort of zeitgeber to modulate ovarian function by stabilizing the female cycle and, perhaps, enhancing folliculogenesis and ovulation.     

Hypothalamic and hormonal control of the photoperiodically induced vernal functions in the White-crowned Sparrow,Zonotrichia leucophrys gambelii

To assess the role of the hypothalamo-hypophysial complex in the control of photoperiodically induced vernal premigratory responses in the White-crowned Sparrow, the effects of hypothalamic lesions and systemic administration of several hormones on these responses were investigated. Lesions that destroyed the posterior median eminence (PME) or the entire median eminence (ME) inhibited photoperiodically induced testicular growth, premigratory fattening and Zugunruhe. Lesions in the basal infundibular nucleus (IN) that resulted in complete inhibition of testicular growth abolished Zugunruhe, but allowed varying degrees of fattening. The systemic administration of prolactin, testosterone propionate (TP) or the combination thereof in the PME-lesioned birds induced fattening similar to that observed in photostimulated controls but did not induce Zugunruhe. It is concluded that testosterone and prolactin are the most important hormones involved in the control of vernal premigratory fattening. The role of these hormones in the induction of vernal Zugunruhe is not positively proven.     

The effects of short-term resistance training on endocrine function in men and women

This investigation examined hormonal adaptations to acute resistance exercise and determined whether training adaptations are observed within an 8-week period in untrained men and women. The protocol consisted of a 1-week pre-conditioning orientation phase followed by 8 weeks of heavy resistance training. Three lower-limb exercises for the quadriceps femoris muscle group (squat, leg press, knee extension) were performed twice a week (Monday and Friday) with every other Wednesday used for maximal dynamic 1 RM strength testing. Blood samples were obtained pre-exercise (Pre-Ex), immediately post-exercise (IP), and 5 min post-exercise (5-P) during the first week of training (T-1), after 6 weeks (T-2) and 8 weeks (T-3) of training to determine blood concentrations of whole-blood lactate (LAC), serum total testosterone (TT), sex-hormone binding globulin (SHBG), cortisol (CORT) and growth hormone (GH). Serum TT concentrations were significantly (P ≤ 0.05) higher for men at all time points measured. Men did not demonstrate an increase due to exercise until T-2. An increase in pre-exercise concentrations of TT were observed both for men and women at T-2 and T-3. No differences were observed for CORT between men and women; increases in CORT above pre-exercise values were observed for men at all training phases and at T-2 and T-3 for women. A reduction in CORT concentrations at rest was observed both in men and women at T-3. Women demonstrated higher pre-exercise GH values than men at all training phases; no changes with training were observed for GH concentrations. Exercise-induced increases in GH above pre-exercise values were observed at all phases of training. Women demonstrated higher serum concentrations of SHBG at all time points. No exercise-induced increases were observed in men over the training period but women increased SHBG with exercise at T-3. SHBG concentrations in women were also significantly higher at T-2 and T-3 when compared to T-1 values. Increases in LAC concentrations due to exercise were observed both for men and women for all training phases but no significant differences were observed with training. These data illustrate that untrained individuals may exhibit early-phase endocrine adaptations during a resistance training program. These hormonal adaptations may influence and help to mediate other adaptations in the nervous system and muscle fibers, which have been shown to be very responsive in the early phase of strength adaptations with resistance training. Accepted: 11 December 1997     

Short-term changes in fathers' hormones during father-child play: impacts of paternal attitudes and experience

Hormonal differences between fathers and non-fathers may reflect an effect of paternal care on hormones. However, few studies have evaluated the hormonal responses of fathers after interacting with their offspring. Here we report results of a 30-minute in-home experiment in which Filipino fathers played with their toddlers and consider whether paternal experience and men's perceptions of themselves as fathers affect hormonal changes. Fathers provided saliva and dried blood spot samples at baseline (B) and 30 (P30) and 60 (P60, saliva only) minutes after the interaction. We tested whether testosterone (T), cortisol (CORT), and prolactin (PRL) shifted after the intervention. In the total sample, T did not vary over the study period, while CORT declined from B to P30 and P60, and PRL also declined from B to P30. Fathers who spent more time in daily caregiving and men who thought their spouses evaluated them positively as parental caregivers experienced a larger decline in PRL (B to P30) compared to other fathers. First-time fathers also had larger declines in PRL compared to experienced fathers. Experienced fathers also showed a greater decline in CORT (B to P60) compared to first-time fathers. These results suggest that males' paternal experience and age of offspring affect hormonal responses to father–child play and that there is a psychobiological connection between men's perceptions of themselves as fathers and their hormonal responsivity to childcare.     

Women's intercollegiate athletic competition: Cortisol,testosterone, and the dual-hormone hypothesis as it relates to status among teammates

Recent research suggests that testosterone and cortisol jointly regulate dominance motivation and, perhaps, the status relationships that are affected by it. For this article, the results of six different studies of women's intercollegiate athletic competition were combined to give a sample size of almost ninety women for whom we had before- and after-competition values for salivary cortisol and testosterone for at least one and sometimes two competitions. For many of these women, we had surveys that allowed us to assess their status with teammates. In no matter what sport (soccer, softball, volleyball, and tennis) levels of salivary cortisol and testosterone increased when women participated in athletic competition. Salivary levels of C and T appear to rise in parallel during competition and increases in levels of one hormone are significantly related to increases in the other. Salivary levels of these hormones typically decreased for teammates who did not play but watched the competition from the sidelines. For women who played in two competitions, individual differences in the positive effect of competition on cortisol and testosterone were conserved from one competition to the next, affirming the personal consistency of endocrine responses to competition. Status with teammates was positively related to before-competition levels of testosterone, but only for women with relatively low before-competition levels of cortisol. This result provides novel support for the “dual-hormone hypothesis” as it relates to predicting social status in women's athletic teams — natural social groups of individuals who know each other and whose social hierarchy has evolved over the course of practice and play for at least one and, in some cases, several years of intercollegiate athletic competition.     

Transgenerational effects of social stress on social behavior,corticosterone, oxytocin,and prolactin in rats

Social stressors such as depressed maternal care and family conflict are robust challenges which can have long-term physiological and behavioral effects on offspring and future generations. The current study investigates the transgenerational effects of an ethologically relevant chronic social stress on the behavior and endocrinology of juvenile and adult rats. Exposure to chronic social stress during lactation impairs maternal care in F0 lactating dams and the maternal care of the F1 offspring of those stressed F0 dams. The overall hypothesis was that the male and female F2 offspring of stressed F1 dams would display decreased social behavior as both juveniles and adults and that these behavioral effects would be accompanied by changes in plasma corticosterone, prolactin, and oxytocin. Both the female and male F2 offspring of dams exposed to chronic social stress displayed decreased social behavior as juveniles and adults, and these behavioral effects were accompanied by decreases in basal concentrations of corticosterone in both sexes, as well as elevated juvenile oxytocin and decreased adult prolactin in the female offspring. The data support the conclusion that social stress has transgenerational effects on the social behavior of the female and male offspring which are mediated by changes in the hypothalamic–pituitary–adrenal axis and hypothalamic–pituitary–gonadal axis. Social stress models are valuable resources in the study of the transgenerational effects of stress on the behavioral endocrinology of disorders such as depression, anxiety, autism, and other disorders involving disrupted social behavior.     

Identification of QTLs for behavioral reactivity to social separation and humans in sheep using the OvineSNP50 BeadChip

Background

Current trends in sheep farming practices rely on animals with a greater level of behavioral autonomy than before, a phenotype that actively contributes to the sustainability of animal production. Social reactivity and reactivity to humans are relevant behavioral traits in sheep, known for their strong gregariousness and weak tolerance to handling, which have previously been reported with moderate to high heritabilities. To identify loci underlying such behaviors, we performed a genome study in Romane lambs.

Results

The experiment was carried out on 934 male and female lambs allocated into 9 half-sib families (average of 103 lambs per family) and reared outside. After weaning, all the lambs were individually exposed to 4 standardized behavioral tests combining social isolation, exposure to humans or handling, confinement and novelty (i.e. arena test, corridor test, isolation box test, shearing test). A broad range of behaviors including vocalizations, locomotion, vigilance and flight distance, as well as the cortisol response to handling, were collected. All lambs were genotyped using the Illumina OvineSNP50 BeadChip. QTL detection was performed by linkage, association and joint linkage and association analyses using the QTLmap software. Five main QTL regions were identified on sheep chromosomes (Ovis Aries Region, OAR) 12, 16, 19, 21 and 23 among many other QTLs with small to moderate effects. The QTLs on OAR12, 16 and 21 showed significant associations with social reactivity. The QTLs on OAR19 and 23 were found to be associated with reactivity to humans. No overlapping QTLs were identified for the different traits measured in the behavioral tests, supporting the hypothesis that different genetic factors influence social reactivity and tolerance to humans.

Conclusion

The results of this study using ovine SNP data suggest that in domestic sheep the behavioral responses to social separation and exposure to humans are under polygenic influence. The most relevant QTLs reported in the present study contain interesting candidate genes previously described to be associated with various emotional and social behaviors in mammals.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-778) contains supplementary material, which is available to authorized users.     

The effects of cortisol and testosterone on renal function in male Antechinus stuartii (Marsupialia)

Seasonal changes in the physiology of Antechinus stuartii result in complete male mortality after mating. The most important endocrine changes in males are large rises in plasma testosterone and cortisol concentrations. Glomerular filtration rate (GFR) in males declines coincident with high plasma testosterone and cortisol. In the present study GFRs were measured in males captured in May (when endogenous plasma testosterone and cortisol levels are low) and given depot injections of either saline, testosterone-only, cortisol-only or testosterone plus cortisol at doses designed to mimic plasma levels during the mating period. GFR decreased significantly with testosterone injection, independent of cortisol treatment. Urinary concentrations of sodium and chloride, and osmolality decreased significantly with cortisol treatment, although the addition of testosterone reversed the effect. Total urinary excretion of electrolytes was similar between groups. Plasma potassium levels significantly increased in testosterone plus cortisol treated males. Plasma sodium levels significantly increased and plasma chloride significantly decreased in all groups treated with cortisol. Water consumption significantly increased in all cortisol-treated males and food consumption significantly increased in all testosterone-treated males. The seasonal renal functional changes observed in A. stuartii were mimicked by testosterone administration. Accepted: 23 January 1998     

Behavioral and hormonal responses of men to brief interactions with women

This study tested for behavioral and hormonal reactions of young men to brief social encounters with potential mating partners. Male college students were randomly assigned to engage in a short conversation with either a young man (male condition) or a young woman (female condition). Participants provided saliva samples before and after the conversation, completed a battery of psychological measures after the interaction, and had their behavior rated by their conversation partners. Salivary testosterone (T) increased significantly over baseline levels in the female condition only, though differences between conditions were not significant. In addition, change in T was significantly correlated with the degree to which the female confederates thought the male participants were trying to impress them. These behavioral ratings, in turn, were correlated with the participants' ratings of the female confederates as potential romantic partners. Results were generally consistent with the hypothesis that human males may exhibit a behavioral and endocrine courtship response that is similar to that observed in males of many nonhuman vertebrate species.     

Environmental effects on hormonal regulation of testicular descent

Regulation of testicular descent is hormonally regulated, but the reasons for maldescent remain unknown in most cases. The main regulatory hormones are Leydig cell-derived testosterone and insulin-like factor 3 (INSL3). Luteinizing hormone (LH) stimulates the secretion of these hormones, but the secretory responses to LH are different: INSL3 secretion increases slowly and may reflect the LH dependent differentiated status of Leydig cells, whereas testosterone response to LH is immediate. Testosterone contributes to the involution of the suspensory ligament and to the inguinoscrotal phase of the descent, while INSL3 acts mainly in transabdominal descent by stimulating the growth of the gubernaculum. INSL3 acts through a G-protein coupled receptor LGR8. In the absence of either INSL3 or LGR8 mice remain cryptorchid. In humans only few INSL3 mutations have been described, whereas LGR8 mutations may cause some cases of undescended testis. Similarly, androgen insensitivity or androgen deficiency can cause cryptorchidism. Estrogens have been shown to down regulate INSL3 and thereby cause maldescent. Thus, a reduced androgen–estrogen ratio may disturb testicular descent. Environmental effects changing the ratio can thereby influence cryptorchidism rate. Estrogens and anti-androgens cause cryptorchidism in experimental animals. In our cohort study we found higher LH/testosterone ratios in 3-month-old cryptorchid boys than in normal control boys, suggesting that cryptorchid testes are not cabable of normal hormone secretion without increased gonadotropin drive. This may be either the cause or consequence of cryptorchidism. Some phthalates act as anti-androgens and cause cryptorchidism in rodents. In our human material we found an association of a high phthalate exposure with a high LH/testosterone ratio. We hypothesize that an exposure to a mixture of chemicals with anti-androgenic or estrogenic properties (either their own activity or their effect on androgen–estrogen ratio) may be involved in cryptorchidism.     

Acute metabolic,hormonal, and psychological responses to strength training with superimposed EMS at the beginning and the end of a 6 week training period

Objectives:The aim was to determine metabolic and hormonal responses to strength training with or without superimposed electromyostimulation (EMS) at the beginning and the end of a 6 week training period.Methods:20 strength trained subjects were randomly assigned to two groups. The first group (S) performed 4 sets of back squats with a constantly adjusted additional load of their individual 10 repetition maximum (10 RM) twice a week over 6 weeks. The second group (S+E) did the same training program with superimposed EMS on leg and trunk muscles. Physiological responses were determined before and after the first (TS 1) and the last training session (TS 12).Results:No significant differences of hormonal responses could be observed between groups and TSs. However, small to large effects on metabolism occurred between groups and TSs. Delayed onset muscle soreness (DOMS) was significantly higher 48h after TS 1 for S+E.Conclusions:Despite a higher DOMS after S+E, there is no acute effect of superimposed EMS on hormonal response to exhaustive resistance exercise. We suggest that, because of the high resistance during 10 RM bouts, most of the muscle fibers are already activated and superimposed EMS only activates few additional muscle fibers.