Muscle characteristics in young Norwegian-Swedish Coldblooded Trotters and associations with breeding index, body size and early training

Reasons for performing study: The trotting speed of Norwegian‐Swedish Coldblooded Trotters (NSCTs) is, although inferior to Standardbred speeds, gradually increasing. This development is mainly attributed to breeding progresses. Characteristics of the middle gluteal muscle have recently been described in a small number of these horses but the actual spread in muscle parameters within the population is unknown. Objectives: To describe the characteristics of the middle gluteal muscle and their naturally occurring variability in young NSCT horses and investigate possible association with body size, individual breeding index and early training. Methods: Biopsies of the middle gluteal muscle of 144 NSCT horses born in 2005 were collected at age 17–24 months. The horses were randomly selected by stratified sampling with gender and paternal progeny as stratification factors. The muscle samples were analysed for fibre type composition, fibre type areas, capillary supply and the activity of the key metabolic enzymes citrate synthase, 3‐OH‐acyl‐CoA‐dehydrogenase and lactate dehydrogenase. Correlation analysis including muscle parameters and body size, individual breeding index and early training was performed. Results: Large interindividual differences were identified in muscle characteristics. Male horses had higher percentages of type IIA fibres and lower percentages of type IIX fibres than age‐matched females. Fibre type areas followed the order type IIX>IIAX>IIA>I, while the capillary supply per fibre type area increased in the opposite order. No gender‐related differences were found in fibre type areas or capillary supply. Height was correlated to the relative area of type I fibres. Early training was associated with increased oxidative capacity and increased percentage of type IIA fibres. The lowest breeding index was found in untrained horses. Conclusions: Muscle characteristics differ among young NSCT horses, but the innate oxidative capacity seems to be lower than in Standardbreds of comparable age. Early training may influence on these parameters.