Effects of beta-hydroxy beta-methylbutyrate (HMB) supplementation on gluteus medius muscle fiber composition and muscle performance in adult Thoroughbred horses exercising to fatigue on a high-speed treadmill
Abstract
Consumption of β-hydroxy β-methylbutyrate (HBM), a leucine metabolite, alters muscle
composition and metabolism leading to strength and agility improvements in human athletes. To
determine if HMB affects athletic performance and muscle function in horses, Thoroughbred
geldings were fed a control (CON; n=5) or HMB (n=6) supplement (30 mg/kg/day) for 6 weeks
prior to completing a standardized exercise test (SET). Gluteus medius (GM) muscle samples were
obtained before the SET for fiber-typing and venous blood was collected before and immediately
upon completion of the SET for lactate measurements. Heart rate (HR), biceps femoris (BF) and
semitendinosus (ST) surface electromyograms, and fore- and hindlimb metacarpophalangeal joint
angles were captured for the duration of the SET. Results demonstrate that HMB supplementation
increased (P < 0.05) the percentage of type IIA muscle fibers in the GM with a corresponding
decrease (P < 0.05) in type IIX fibers. The percentage of type I fibers was unaffected by diet.
Supplementation with HMB did not result in any significant effects on performance, muscle
function or biomechanical properties by comparison to CON. Increasing treadmill speed resulted
in an increase (P < 0.05) in stride length and maximal extension angle of the fore fetlock, and a
shortening (P < 0.05) of the stance phase of the gait cycle. Integrated EMG (iEMG) increased (P
< 0.05) with increasing treadmill speeds for both the BF and ST, with the BF exhibiting greater
iEMG values than the ST. In summary, HMB increased the percentage of type IIA fibers which
did not translate into immediate, improved athletic performance
General Audience Abstract
Muscles depend on their fibers, innervation, energy supply, and blood flow to contract.
Failure to meet one or more of these requirements precludes muscle tissue from performing work,
situation termed fatigue. Identification of fatigue indicators is of interest to the horse industry for
a number of reasons, including horse and human safety, prevention of unnecessary expenses, and
general public opinion of the sport disciplines. Diet supplementation with legal, performanceenhancing compounds is of interest to riders and horse owners alike. Molecules such as betahydroxy beta-methylbutyrate (HMB) improve muscle function, protein synthesis, and muscle
tissue repair. Assessment of the athletic capacity and performance of horses by evaluating fatigue
indicators favors responsible training regimes. Techniques to achieve this goal include muscle
sampling, biochemical, electromyographic, and biomechanical analysis.
We hypothesized that dietary supplementation of HMB would have positive effects on the
athletic performance of horses. This study evaluated the effects of 45-day HMB supplementation
on muscle fiber composition, muscle performance, and rates of fatigue in adult Thoroughbred
horses by use of a high-speed treadmill. Muscle biopsies, blood lactate, high-speed video captures, and electromyography were analyzed. These analyses revealed that HMB supplementation
increased the number of fatigue-resistant fibers in muscles but caused no substantial, immediate
improvements on the athletic performance of horses.
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