Implementation of ultrasound-based technique for the assessment of muscle glycogen depletion after exercise in horses: preliminary results

Muscle glycogen is a form of carbohydrate storage, and a major source of energy during exercise. Glycogen concentration in muscle tissue is an important determinant for endurance capacity during high intensity exercise [1], so that its measurement provides a meaningful marker of athletes’ conditioning and diet/training appropriateness [2]. The aim of present study is to verify the feasibility on equines of a recently developed non-invasive approach for muscle glycogen assessment in man using ultrasound (US) imaging [3]. Intra muscular glycogen depletion following exercise was assessed after preliminary sessions for the definition of target muscles, probe position/orientation and exercise protocol. The measurements were performed by means of US system Esaote ClassC using a linear probe in a long axis orientation, set at 12.5 MHz frequency, on left middle gluteus (GL) and semitendinosus (ST) muscles. Images were obtained on 3 horses at rest and after exercise on a high speed treadmill (Sӓto I, SATO, Sweden). The horses performed 5 minutes warm-up and 15 minutes steady-state exercise (average speed: 6.5 m/s ±1.8 SD; heart rate: 171 bpm ± 21 SD). US images were obtained both before and within 20 minutes after exercise. Image intensity in the grey scale was considered as the outcome measure inversely related to tissue water content (an indirect measure for muscle glycogen concentration). The variation coefficient of consecutive measurements with probe re-positioning on the same muscle was 5.07% ± 2.98%. Preliminary data show that overall muscle image intensity significantly increased after exercise (+ 30% p = 0.017, paired t-test), being significant in ST (mean +22% ±11%, p=0.039) and not in GL muscle (mean +55% ± 60%, p=0.298). The use of US technique appears to be sensitive in detecting changes in tissue water content related to glycogen depletion following exercise. It is therefore a technique of potential impact for the evaluation of equine athletes, even though present results on a small sample need to be further implemented and validated through glycogen direct measurement by biopsy technique.