RESUMEN
BACKGROUND: Swimming has been used empirically for rehabilitation and conditioning of horses. However, due to challenges imposed by recording physiological parameters in water, the intensity of free swimming effort is unknown. OBJECTIVES: Measure the physiological workload associated with untethered swimming in horses. Five fit Arabian endurance horses were assessed while swimming in a 100 m-long indoor pool. Horses were equipped with a modified ergospirometry facemask to measure oxygen consumption (VÌO2) and ventilatory parameters (inspired/expired volumes, VI, VE; peak inspiratory/expiratory flows, PkVI, PkVE; respiratory frequency, Rf; minute ventilation, VE; inspiratory/expiratory durations and ratios, tI, tE, tI/ttot, tE/ttot); and an underwater electrocardiogram that recorded heart rate (HR). Postexercise venous blood lactate and ammonia concentrations were measured. Data are reported as median (interquartile ranges). RESULTS: Horses showed bradypnea (12 breaths/min (10-16)) for the first 30 s of swimming. VÌO2 during swimming was 43.2 ml/(kg.min) (36.0-56.6). Ventilatory parameters were: VI = 16.7 L (15.3-21.8), VE = 14.7 L (12.4-18.9), PkVI = 47.8 L/s (45.8-56.5), PkVE = 55.8 L/s (38.3-72.5), Rf = 31.4 breaths/min (20.0-33.8), VE = 522.9 L/min (414.7-580.0), tI = 0.5 s (0.5-0.6), tE = 1.2 s (1.1-1.6), tI/ttot = 0.3 (0.2-0.4), tE/ttot = 0.7 (0.6-0.8). Expiratory flow tracings showed marked oscillations that coincided with a vibrating expiratory sound. HR was 178.0 bpm (148.5-182.0), lactate = 1.5 mmol/L (1.0-1.9) and ammonia = 41.0 µmol/L (36.5-43.5). CONCLUSIONS: Free (untethered) swimming represents a submaximal, primarily aerobic exercise in horses. The breathing pattern during swimming is unique, with a relatively longer apneic period at the beginning of the exercise and an inspiratory time less than half that of expiration.