Muscle energetics in exercising horses Dominique-Marie Votion 1,2, * , †, Rachel Navet 3, †, Ve ´ronique Anne Lacombe 4 , Francis Sluse 3 , Birgitta Esse ´n- Gustavsson 5 , Kenneth William Hinchcliff 6 , Jose ´ -Luis L. Rivero 7 , Didier Serteyn 1 and Stephanie Valberg 8 1 Equine Teaching Hospital, University of Liege, Lie `ge, Belgium 2 Equine European Centre of Mont-Le-Soie, University of Liege, Lie `ge, Belgium 3 Laboratory of Bioenergetics, University of Liege, Lie `ge, Belgium 4 College of Pharmacy, The Ohio State University, Columbus, OH, USA 5 Department of Clinical Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden 6 Faculty of Veterinary Science, University of Melbourne, Werribee, Victoria, Australia 7 Muscle Biology Laboratory, Department of Comparative Anatomy and Pathological Anatomy, Faculty of Veterinary Science, University of Cordoba, Cordoba, Spain 8 Department of Veterinary Population Medicine, University of Minnesota, St Paul, MN, USA * Corresponding author: dominique.votion@ulg.ac.be Submitted 28 February 2007: Accepted 17 September 2007 Review Abstract An optimally functional musculoskeletal system is crucial for athletic performance and even minor perturbations can limit athletic ability. The introduction of the muscle biopsy technique in the 1970s created a window of opportunity to examine the form and function of equine skeletal muscle. Muscle histochemical and biochemical analyses have allowed characterization of the properties of equine muscle fibres and their influence on, and adaptation to, physical exertion. Analyses of exercise responses during standardized treadmill exercise and field studies have illustrated the role of cellular energetics in determining athletic suitability for specific disciplines, mechanisms of fatigue, adaptations to training and the affect of diet on metabolic responses. This article provides a review of the tools available to study muscle energetics in the horse, discusses the muscular metabolic pathways and summarizes the energetics of exercise. Keywords: muscle biopsy; exercise; glycogen; glucose; lipid; muscle fibre Introduction The tremendous range of athletic capacity of equine athletes can be attributed to both years of genetic selec- tion for prowess for a particular form of exercise as well as the remarkable plasticity of muscle, which readily adapts to physical training. The introduction of the muscle biopsy technique and the expanding array of histochemical, biochemical and molecular applications developed over the last three decades have improved our understanding of muscle structure, function, adaptation to training and limitations of performance. In this review, modern methods to assess the relationship between the physical characteristics of skeletal muscle and the biochemical response and adaptation to exercise will be reviewed; the complexity of muscle energetics in equine athletes will also be briefly summarized. Supplemental detailed reviews of muscle biochemistry and physiology can be found elsewhere 1–3 . Techniques to measure metabolic and contractile properties of muscle The first part of this paper discusses the techniques avail- able for assessment of substrate metabolism, especially in relation to the contractile properties of the muscle fibres. These techniques allow a quantitative and †Both authors have equally contributed to the review. Equine and Comparative Exercise Physiology 4(3/4); 105–118 DOI: 10.1017/S1478061507853667 qCambridge University Press 2008