The role of oestrogen in exercise-induced muscle damage
Oestrogen is believed to be a potent antioxidant, with potential membrane stabilising and gene regulatory effects. Oestrogen has been shown to be an effective cardioprotectant, with for example a lower incidence of atherosclerosis in pre menopausal females compared to age-matched males and in post menopausal females on hormone replacement therapy compared to age-matched males. What has yet to be determined is the extent to which oestrogen can protect skeletal muscle. It has been shown that certain markers of exercise-induced muscle damage (EIMD) are lower in females in both animal and human studies, but as yet no conclusive evidence from human studies has shown that oestrogen provides a protective mechanism against ERvID or whether susceptibility to EIKID varies across the normal menstrual cycle, where oestrogen fluctuations are high. Furthermore, if oestrogen provides a protective mechanism against EIlVID, it is unknown at which phase during the muscle damage and repair cycle this occurs. It is also debatable if the potential inhibitory effects that oestrogen has on the inflammatory response, with regard to repair and regeneration of the skeletal muscle are positive or negative. The thesis is comprised of a critical review of the nature of EIMD and the potential effects that oestrogen has on the muscle damage and repair cycle. This is followed by three empirical studies which were designed to explore this question. These are outlined below: Study 1 Study one was in two parts. The first part of this study aimed at determining if the phase of the menstrual cycle, could in anyway affect eumennorheic (normally 2 menstruating) females in their susceptibility to exercise-induced muscle damage. An eccentric exercise procedure (elbow flexor muscle group) was performed on a randomly assigned arm during either the menses or ovulatory phase of the menstrual cycle. The contra-lateral limb underwent the same procedure during the alternate phase (random assignment determined in which phase the participant was first damaged). Simple markers of EDM were assessed at baseline and every 24 h up to three days post exercise, during both phases. No significant differences were seen in any markers of BIMD across phases of the menstrual cycle. The second part of this study investigated whether prolonged ingestion of exogenous oestrogen, in the form of the combined oral contraceptive pill attenuated any of the symptoms associated with EIMD. The only symptom to show a significant interaction between groups was perceived soreness, with the pill users reporting significantly (P<0.01) less soreness than the eumennorheic females in the days following the exercise protocol. This suggested that oestrogen may modulate the pain associated with EIMD. Study 2 The second study focussed on gender differences in exercise-induced muscle damage, with particular focus on the secondary symptoms and events which occur following ERVID. Male and female participants performed a bout of eccentrically biased exercise. Markers of both EIMD and inflammation were taken prior to the eccentric exercise and across a 7-day follow up period. Gender differences in the response to EIMD were seen in creatine kinase activity and mid-thigh circumference, with males showing a larger response on both variables. In addition to this, males reported significantly less soreness than females following the exercise protocol. 3 Interestingly, with the exception of neutrophil elastase release, there were no differences in other markers of inflammation between men and women. Total elastase concentration, a marker of neutrophil activation, did not differ between genders. However, elastase release per neutrophil was significantly lower in females, which may be indicative of gender differences in the inflammatory response associated with EDvID. Study 3 With the recognition that oestrogen could potentially reduce or inhibit the inflammatory response the third and final study investigated whether female skeletal muscle was more susceptible to exercise-induced muscle damage after a second bout of eccentric exercise, due to poor regeneration and repair following the initial bout. Males and females performed a bout of eccentrically biased exercise. Markers of EEVM included creatine kinase, soreness, isometric strength and isokinetic strength assessment. The procedure was then repeated two weeks later to determine if gender differences existed in terms of the repeated bout effect associated with EIlvID. Only one variable showed a gender x time x bout interaction (P<0.05), that was the fatigue index. It was shown that following the initial bout of damage, males and females responded very differently, with female muscle being less fatiguable in the 48 h following damage compared to the males, but with both groups responding very similarly in the repeated bout. This may be due to differences in gender and their response to EIlVID, or due to differences in fibre type between genders.