Can a Mare's Hormones Influence Performance?

 Can a Mare's Hormones Influence Performance?

By Loicia Johnson, Equine Emmett Therapist

The physiological changes that can occur through exercise are vast in both horse and human. In regards to the development of muscular strength, after challenging exercise, the formation of micro-tears in working muscles are registered by the body as muscle damage. This results in the body acting to repair the tissue, thus influencing increases in muscle strength, ability and function (Schoenfeld, 2010). As riders working towards competitive goals, we train consistently, meaning our mares tend to perform exercise throughout their natural reproductive cycle... but could this be detrimental?

The typical Oestrus cycle for a mare is around 22 days, as shown in the diagram below (Aurich, 2011). In a normal cycle, the mare will experience higher levels of oestrogen for 5-7 days of their cycle, followed by higher levels of progesterone from day 8-14 and then a gradual decline as oestrus arrives (Aurich, 2011). This in mind, could the fluctuation in hormone levels affect the muscular recovery of the performance mare? And how can the equine industry take this into consideration to preserve the performance mare’s health?

The Effect of Progesterone on Muscle Recovery and Growth

There has been little research performed on the mare’s oestrus cycle and how this could influence muscle metabolism. However, there are studies in other animals for which findings could be similar and therefore extrapolated to predict outcomes for the horse. Research has discovered, that as a result of progesterone treatment, a 30% reduction in muscle leukocytes (white blood cells) infiltration can be seen post-exercise (Iqbal et al., 2008). This research may be relevant in the preservation of the performance mare, as there is a suggestion that leukocytes (white blood cells) could have a function in the recovery of muscles after exercise (Enns et al, 2008).

A notion has been formed that leukocytes, in the form of macrophages (Ly-6Chi), could stimulate tissue recovery and anti-inflammatory molecule release. A study performed by Dunmant and Frenette (2010), found that in vitro, the presence of macrophages influenced myosin heavy chain content, which led to an increase of 10% after only 1 day. Therefore, through promoting progesterone this could lead to reductions in white blood cells within muscles and inhibit growth and repair after exercise.

The Effect of Oestrogen May Yield Beneficial Results

The research into elevated oestrogen levels further supports the changes in muscle metabolism throughout the oestrus cycle. A study by Thomas et al (2010) found a significant increase in myoblast (muscle cells) proliferation (formation of new cells) within muscle tissue. This was observed by the experimental group receiving oestrogen supplementation following exercise-induced muscle damage.

To support this study, research by Sung et al (2014) investigated the influence of female's menstrual cycle on resistance training. The authors found that during the follicular phase (high oestrogen), women experienced an increase in muscle fibre diameter after training (larger muscle growth) compared to the luteal phase (low oestrogen), which demonstrated no increase at all. Additionally, this study presented a large increase in the maximum isometric force (muscle strength) from muscles in the follicular phase (high oestrogen) of 2.9 ± 0.4 Newtons (N) pre-training to 3.8 ± 1.1(N) post-training. In comparison to the luteal phase (low oestrogen) of 3.4 ± 0.8(N) pre-training to 3.4 ± 0.7(N) post-training, which demonstrated a decrease in muscle strength. These results further support the notion that high oestrogen levels promote increased muscle strength and diameter, whereas low oestrogen inhibits growth (Haines et al, 2018).

Not only could oestrogen influence the development of muscular strength and growth but this may also prevent muscle damage (Williams et al, 2015). Thomas et al (2010), Sung et al (2014) and Enns et al (2008) found that skeletal muscles demonstrated an increase in muscle proliferation (formation of new cells) after exercise compared to pre-exercise values. These rats had been receiving oestrogen treatment and elicited up to 50% increases in positive muscle fibres when compared to the baseline at 72 hours post exercise.

The theory behind the effect that oestrogen may have on muscle has been studied and has been reported to increase the synthesis of nitric oxide (NO) in animals and humans. As a result of this, if muscle damage occurs, NO can regulate the release of Hepatic Growth Factors, which are said to regulate myoblast formation and proliferation (Thomas et al, 2010). Therefore, with the presence of oestrogen in the performance mare, during progressive training, muscle recovery may be more efficient in the oestrus phase of her cycle. In order to adjust a mare's training schedule to benefit her hormone cycle, it would be good practice to track her cycle on a regular basis based on signs of oestrus and behaviour.

Tracking a Mare’s Natural Cycle

Trackener is a piece of sensory equipment that can be placed comfortably on the horse in a bib or girth sleeve. This device tracks not only heart rate but also skin temperature, movement and behavioural changes. Therefore the analytics, which is processed for you, could yield intuitive information as to when your mare is experiencing higher levels of oestrogen or progesterone through behavioural changes and activity. Activity has long been scrutinised in female mammals after the first publication surfaced 70 years ago providing large increases in physical activity during oestrus using pedometry. This was later observed in cows fitted with pedometers who showed a 218% increase in locomotory activity (walking/pacing) during oestrous compared to diestrus (high progesterone) (Nebel et al, 2000). As Trackener enables users to track the distance covered and analyse movement patterns around the clock, feedback of any changes could aid the tracking of oestrus in a performance mare, thus benefiting her welfare and helping optimise training and recovery.


In conclusion, exercising a mare during high progesterone periods could cause muscle damage and inhibit growth. Whereas, high oestrogen levels a couple days before ovulation may have the potential to increase the efficiency of muscle growth and development, as well as acting to protect against muscle damage from challenging exercises. These findings suggest a benefit to tracking the mare's oestrus cycle which may help yield improved training schedules and performance.

About the author

Loicia Johnson is an Equine Emmett Therapist from Kent and recent winner of the Hadlow Equine Award for dedication to Horse Welfare. Having completed her degree in Equine Sports Therapy and Rehabilitation earlier this year, Loicia’s love for horses and determination to help them recover from injury led her to begin her career in equine therapy where for the last two years she has been practising Emmett therapy, kinesiology taping and equine hydrotherapy. To find out more about what Loicia does, send her a message.


Aurich, C. (2011). ‘Reproductive Cycles of Horses’. Animal Reproduction Science, 124(3–4), 220–228pp. 

Dumont, N., and Frenette, J. (2010). ‘Macrophages Protect against Muscle Atrophy and Promote Muscle Recovery in Vivo and in Vitro’. The American Journal of Pathology, 176(5), 2228–2235pp.

Enns, L., Iqbal, S., and Tiidus, M. (2008). ‘Oestrogen Receptors Mediate Oestrogen-Induced Increases in Post-Exercise Rat Skeletal Muscle Satellite Cells’. Acta Physiologica, 194(1), 81–93pp. 

Haines, M., McKinley-Barnard, S., Andre, T., Gann, J., and Hwang, P. (2018). ‘Skeletal Muscle Estrogen Receptor Activation in Response to Eccentric Exercise Up-Regulates Myogenic-Related Gene Expression Independent of Differing Serum Estradiol Levels Occurring during the Human Menstrual Cycle’. Journal of Sports Science and Medicine, 17, 31–39pp. 

Iqbal, S., Thomas, A., Bunyan, K., and Tiidus, M. (2008). ‘Progesterone and Estrogen Influence Postexercise Leukocyte Infiltration in Overiectomized Female Rats’. Applied Physiology, Nutrition, and Metabolism, 33(6), 1207–1212pp. 

Nebel, L., Dransfield, M., Jobst, S., and Bame, H. (2000). ‘Automated Electronic Systems for The Detection of Oestrus and Timing of AI in Cattle. Animal Reproduction Science, 60–61(1), 713–723pp.

Sung, E., Han, A., Hinrichs, T., Vorgerd, M., Manchado, C., and Platen, P. (2014). ‘Effects of Follicular Versus Luteal Phase-Based Strength Training in Young Women’. SpringerPlus, 3(1), 668pp.

Schoenfeld, J. (2010). ‘The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training’. Journal of Strength and Conditioning Research, 24(10), 2857–2872pp. 

Thomas, A., Bunyan, K., and Tiidus, M. (2010). ‘Oestrogen Receptor-Alpha Activation Augments Post-Exercise Myoblast Proliferation’. Acta Physiologica, 198(1), 81–89pp. 

Williams, T., Walz, E., Lane, A., Pebole, M., and Hackney, A. (2015). ‘The Effect of Estrogen On Muscle Damage Biomarkers Following Prolonged Aerobic Exercise in Eumenorrheic Women’. Biology of Sport, 32(3), 193–198pp.