Scientists believe it may be possible to measure glycogen concentrations in the muscles of horses without the need for invasive biopsies.
The answer, they believe, lies in the use of high-frequency ultrasound.
The ability to easily measure muscle glycogen levels may provide a meaningful marker of how well-prepared a horse is for an event, and could be a possible predictor of performance.
Glycogen in skeletal muscle is a major source of energy during exercise and an important determinant of endurance capacity.
During vigorous exercise — as is the case with most competitive horse activities — the blood-derived glucose can guarantee less than 10% of the energy used.
It is the presence of adequate deposits of glycogen — a multibranched polysaccharide of glucose — in the muscle cells that allows the horse to sustain prolonged exercise.
It has been shown that skeletal muscle glycogen depletion is linked with a reduction of exercise capacity and a significant lengthening of recovery times.
“Since muscle glycogen content is modifiable by adjustments of appropriate dietary regimen in relation with training protocols, the measurement of muscular reserves is a key point in the management of the athlete horse,” Sarah Tabozzi and her colleagues in Italy noted in the journal BMC Veterinary Research.
The study team said the gold standard of glycogen concentration measurement is the histochemical and biochemical analysis of biopsy-derived muscle tissue — an invasive and potentially injuring procedure.
The researchers noted that high-frequency ultrasound technology had been used recently in human sports medicine to estimate muscle glycogen content.
They set out to evaluate the feasibility of its use in assessing muscle glycogen in horses.
The study used eight privately owned horses, comprising three Thoroughbreds, three Standardbreds and two Hackney ponies. All were actively in training, either as racehorses, showjumpers or in combined driving.
High-frequency ultrasound images were taken of each horse’s gluteus medius and semitendinosus muscles (both major muscles that help power the hind end) before and after a session on an exercise treadmill tailored to the individual level of physical conditioning of each horse.
The authors noted that, when comparing the baseline images to those taken after exercise, there was a significant increase in the greyscale intensity in the semitendinosus muscles but not in the gluteus medius.
They found that the volume of the exercise was significantly correlated with exercise-dependent change in image intensity, consistent with a reduction of glycogen muscle stores from aerobic activity.
“We postulate that these changes qualitatively correspond to the reduction of muscle glycogen stores induced by the energy demand of exercise,” they concluded.
The use of ultrasound technology shows promise as a low-cost, non-invasive and simple alternative to muscle biopsy analysis for glycogen assessment in horses, they said.
“However, due to the limited number of horses and tested muscles and absence of validation against muscle biopsy in equines, some caution should be used in interpreting present results.”
Further investigation seems warranted in the future to validate the technique in horses, they added.
The authors, who monitored heart rate and assessed oxygen consumption of the horses during exercise, found that the overall balance of glycogen depletion was dominated by the total volume of exercise more than the absolute intensity of exercise.
“During sessions of near to maximal exercise, due to the low sustainability, the percentage of total glycogen used is relatively lower, compared to a more protracted exercise of lower intensity, when a substantial depletion of total deposits ensues, albeit at a slower rate.
The full study team comprised Tabozzi, with the Italian Red Cross National Committee; Giovanni Stancari, Enrica Zucca, Michela Tajoli, Luca Stucchi and Francesco Ferrucci, all with the University of Milan; and Claudio Lafortuna, with Italy’s National Research Council.
Tabozzi, S.A., Stancari, G., Zucca, E. et al. Variation of skeletal muscle ultrasound imaging intensity in horses after treadmill exercise: a proof of concept for glycogen content estimation. BMC Vet Res 17, 121 (2021). https://doi.org/10.1186/s12917-021-02818-9