Equine Science Update
Reports from the world of  equine research.
Join our FREE Email Newsletter List
Researchers at Utrecht University in the Netherlands have succeeded in diagnosing equine overtraining syndrome by measuring nocturnal growth hormone secretion.
Dr Ellen de Graaf-Roelfsema and colleagues conducted the study in conjunction with researchers at Maastricht University, the Free University of Brussels and the University of Virginia (USA).
A successful training program gets the athlete to peak performance in time for the event. There’s a fine balance between doing enough suitable training and doing too much.
Pushing the athlete, human or equine, too hard may lead to “overtraining syndrome” and result in performance getting worse rather than better.

Overtraining is thought to be due to accumulated stress, from training or other factors, resulting in long term reduction in performance. It may take several months to regain the previous level of performance.

A less severe result of intensified exercise is “overreaching”. This is often regarded as a normal consequence for elite athletes as it only takes a couple of weeks for them to recover their previous form.

Growth hormone (GH) is thought to be important in controlling the recovery from stressful events and helping the individual restore their normal homeostatic balance.

The Utrecht study involved twelve Standardbred geldings (average 20 months old) - trained in pairs. Both horses in the pair were managed identically apart from their exercise regime.

The study covered a 32-week training period, which was divided into four phases.

Phase: 1: habituation. (4 weeks duration) Horses followed a regime of light, but increasing, work to get them used to training on the treadmill.
Phase 2: normal training.  (18 weeks) Horses received a mixture of endurance training and high intensity interval training.
Phase 3: intensified training phase (6 weeks). During this phase one horse of each pair underwent an intensified exercise program. The other horse continued on the normal exercise regime (as in phase 2)
Phase 4: recovery / reduced training phase (4 weeks) Horses were exercised lightly.

The morning after the last day of each phase the horses were subjected to a standardized exercise test. Throughout the following night, from 22.00 - 06.00, samples were collected every five minutes for GH assay.

GH tends to be released in pulses. The pattern of those pulses is more important than the overall GH concentration level in the blood - so it’s not enough to simply measure the average levels in the blood.

Samples were collected at night because work (in humans) has shown that GH is released mainly during the recovery phase - during slow wave sleep.

They analyzed the samples using firstly deconvolution analysis and secondly a complex approach known as “approximate entropy”. This allowed them to identify changes in the pattern of GH secretion.

Twelve horses were originally enrolled in the study. One pair had to drop out, so the results were based on results from ten horses.

“The intensively trained group clearly showed a loss of performance, indicative for over-reaching or maybe even over training.” Dr Graaf-Roelfsema reports.

Intensively trained horses tired more quickly during the standard exercise test than did normally trained horses. On average, they took 19% less time to fatigue compared with the control horses.

GH secretion became increasingly irregular after the intensive training phase. Indeed it had still not fully returned to normal even after the reduced exercise phase. Insensitively trained horses showed more frequent, smaller concentration peaks of GH secretion.

“We have demonstrated that intensified training for 6 weeks alters nocturnal GH pulsatility in young Standardbred horses compared to a control group by a diminished coordinated GH secretion”.

"We were able to distinguish between overtrained and control horses by evaluation of their nocturnal GH secretion pattern." Dr Graaf-Roelfsema points out. "However, it is a very labor intensive method and not very practical to use for the individual patient. We are working on that right now."

Having developed this method for diagnosing overtraining syndrome, the researchers can now focus on determining effective prevention and treatment methods.

The findings also offer the prospect of improved or preventive treatment for comparable stress related syndromes in humans such as burnout.

For more details see:

Overtrained horses alter their resting pulsatile growth hormone secretion.
E de Graaf-Roelfsema, PP Veldhuis, HA Keizer, MME van Ginneken, KG van Dam, ML Johnson, A Barneveld, PPCA Menheere, E van Breda, ID Wijnberg, JH van der Kolk.
Am J Physiol Regul Integr Comp Physiol (2009) 297, R403- R411
Written by Mark Andrews. Published online 25.09.09.
Amended 28.09.09
© Copyright Equine Science Update  2009
Method developed to diagnose overtraining syndrome.
Sign up for our FREE e-mail newsletter.
Email: