Incremental treadmill exercise until onset of fatigue and its relationship to metabolic response and locomotion pattern.

The aim was to study metabolic response and locomotion pattern in Standardbred trotters during incremental treadmill exercise performed by increasing speed by 1 m/s in 1 min steps (start 7 m/s) until the onset of fatigue. The test protocol included determination of oxygen uptake, heart rate (HR), stride length (SL) and stride frequency (SF). Venous blood samples were collected at rest, at the end of each exercise step and after 30 min of recovery. Muscle biopsies were taken at rest and post exercise and muscle temperature was measured after exercise. As horses fatigued at different speed steps (9-11 m/s), variation was seen in running time (180-300 s), oxygen uptake (109-170 ml/kg bwt min), HR (200-225 beats/min), SL (4.4-5.7 m) and SF (116-130 strides/min) at the last speed step. Increased mean plasma lactate concentration (20.5 mmol/l) was seen at onset of fatigue and increased mean uric acid concentration after 30 min of recovery (112.8 mumol/l). After exercise, a decrease was seen in muscle ATP (7.1 mmol/kg d.w.), creatine phosphate (43.9 mmol/kg d.w.) and glycogen (160 mmol/kg d.w.), and an increase was seen in ADP (0.3 mmol/kg d.w.), AMP (0.18 mmol/kg d.w.), IMP (5.8 mmol/kg d.w.) and lactate (100.8 mmol/kg d.w.). At onset of fatigue, muscle temperature varied from 39.9-41.4 degrees C. Running time correlated with SL (r = 0.86), with an increase in IMP (r = 0.79) and AMP (r = 0.70) post exercise and with plasma uric acid concentration (r = 0.74) at 30 min of recovery. SF correlated negatively with the increase in ADP after exercise (r = 0.85). The results of this study indicate that running time during incremental treadmill exercise until the onset of fatigue is related to locomotion pattern and to a marked degree of anaerobic metabolism, especially adenine nucleotide degradation.