Kristine Urschel, PhD, assistant professor of equine science in the University of Kentucky's Department of Animal and Food Sciences, lectured on "Muscle Power: Regulation of Muscle Mass in Growing, Athletic, and Aging Horses" at the UK Veterinary Diagnostic Laboratory Aug. 25. Her lecture covered the factors regulating muscle protein accretion in horses, and applied relevant research in humans, pigs, and rats since equine research in this area is limited.

"Exercise and dietary protein together provide a big jump in protein synthesis," Urschel said. "Exercise, amino acids, protein, and carbohydrates result in the greatest jump in protein synthesis."

Urschel said this correlation is directly relevant to horse owners because protein synthesis affects muscle growth. Muscle mass generally equates to greater power output; more muscle allows both greater generation of force and faster muscle contraction. For the equine athlete, this increase might translate into more explosive movement for cutting horses, greater jumping ability for hunters, jumpers, and eventers, increased suspension and flexion for dressage horses, and enhanced speed for racehorses.

Fueled by exercise and protein, muscle mass is the balance between the processes that synthesize and break down protein. Nutrition, age, hormones, and physical activity can all affect synthesis and breakdown.

"Muscle works as a storage unit for amino acids that the body uses for other metabolic functions. Amino acids, exercise, and insulin activate pathways that lead to muscle synthesis," Urschel explained.

Muscle power changes over a horse's lifespan, with three phases (adolescent, adult, and aging) that can be manipulated. The greatest muscle mass increase comes with both increased exercise and increased dietary protein, according to Urschel.

"Adolescent horses are putting muscle down," Urschel said. "With an increase in exercise, young horses between the ages of 1 to 3 will exhibit a subtle increase in muscle mass over time that naturally follows development."

Muscle mass also is a determinant of mobility and quality of life. Horses, like humans, are prone to injury and accident as they lose muscle mass, and the aging process has a negative impact on muscle's responsiveness. In general, with age, a lower rate of protein synthesis and increased protein breakdown result in a net loss of muscle.

But, Urschel pointed out, studies in older men show that exercise makes muscle more sensitive to feeding, which might prove to be an important parallel in the horse world. Extrapolating from research on rats, Urschel also theorized that low-grade or chronic inflammation in aged horses might interfere with muscle-building pathways.

Owners and trainers in the Standardbred and Thoroughbred industries often want to know if a higher fat-free muscle mass can help a horse to run faster, Urschel said. But the comprehensive research needed to answer these questions about the effects of diet and exercise on muscle lack funding so far.

Urschel's presentation is available for viewing on TheHorse.com.

Karin Pekarchik is an editorial officer in UK's Agricultural Communications Services.