Race horses train on various surfaces, including dirt, turf, wood chips, and all-weather synthetic materials. A method commonly used to enhance cardiovascular fitness in horses is to train them by having them run uphill. Several investigators have measured or modeled the relationship between uphill running and energy expenditure, HR, and o2.1–5 However, few reports6,7 exist regarding the effects of downhill locomotion on horses.
Downhill locomotion requires that muscles perform eccentric contractions (ie, do negative work), during which the muscles generate tension even as they are lengthening. Because efficiency is defined as metabolic power input divided by mechanical power output, eccentric contractions enable skeletal muscle to contract with a larger magnitude efficiency (approx −1.20 on slopes exceeding a 20% decline)8,9 than for concentric contractions (approx 0.25 on slopes exceeding a 20% incline).10,11 Eccentric muscle contractions affect human skeletal muscle differently than concentric contractions, increasing muscle size and strength more than concentric exercise training and with reduced o2.4,12–20 Some human athletes train by running downhill to increase their running speed.21 The additional potential energy available during downhill running may translate into faster fibers that can accelerate the speed of horizontal running.21 However, during horse racing, Thoroughbreds reportedly run slower than on horizontal surfaces, not only when going uphill but also going downhill.7 In addition, larger animals have a greater difference in energy cost than do smaller animals when running uphill, downhill, and horizontally.22,23
A better understanding of the skeletal muscle and cardiopulmonary responses to downhill running may suggest a beneficial role for downhill running in Thoroughbred training. To our knowledge, only 1 study24 has been conducted to evaluate the effects on energetics and O2 transport in the same group of horses during locomotion uphill and downhill at equivalent gradients. We hypothesized that locomotion of Thoroughbreds on a treadmill at a decline would result in a lower metabolic energy cost and reduced cardiopulmonary function, compared with locomotion at equivalent speed on a treadmill at a horizontal plane. Furthermore, we hypothesized that the reduction in those energy costs going downhill would equal the increase in energy costs when going uphill against gravity. Restated, we hypothesized that the difference in work done in raising a Thoroughbred's Mb against gravity going uphill and saved when going downhill would be equal and result in metabolic power changes of equal magnitude.
Supported by the Equine Research Institute, Japan Racing Association, Tochigi, Japan.
Presented as an abstract and poster at the 9th International Conference on Equine Exercise Physiology, Chester, England, June 2014.
Arterial oxygen concentration
Cost of transport
Mixed-venous oxygen concentration
Rate of metabolic oxygen consumption
Rate of metabolic carbon dioxide production
Säto I, Säto AB, Knivsta, Sweden.
Surflow, Terumo Corp, Tokyo, Japan.
MO95H-8.5, Baxter, Tokyo, Japan.
Criticath, Ohmeda, Madison, Wis.
Statham P23d, Viggo-Spectramed, Tokyo, Japan.
S810, Polar Electro Oy, Kempele, Finland.
OSM3 hemoximeter, Radiometer-Copenhagen, Brønshøj, Denmark.
KH120A, Kubota, Tokyo, Japan.
Biosen C-line glucose lactate analyser, EKF-diagnostic GmbH, Barleben, Germany.
LF-150B, G. N. Sensor, Chiba, Japan.
TF-105, G. N. Sensor, Chiba, Japan.
METS-900, VISE Medical, Chiba, Japan.
Nafion drying tube, Perma Pure LLC, Lakewood, NJ.
Model DPM3, Kofloc, Tokyo, Japan.
DI-720-USB, DATAQ Instruments, Akron, Ohio.
Windaq Pro+, DATAQ Instruments, Akron, Ohio.
SigmaPlot 12.5, Systat, Chicago, Ill.
Hoyt DF, California State Polytechnic University, Pomona, Calif: Personal communication, 2016.
Hiraga A, Japan Racing Association Hidaka Research and Training Center, Hokkaido, Japan: Personal communication, 2016.
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