No evidence of cardiac fatigue in tissue velocity curves at rest after 6 days of ultra-endurance exercise

Purpose: The aim of this study was to investigate if extreme workload would induce signs of cardiac fatigue similar to that in skeletal muscle, e.g. decreased velocity of contraction. Methods: The subjects were 12 men and 3 women who participated in the Adventure Racing World Championship, a 5-7 days non-stop competition open for mixed gender teams of four. All subjects were healthy, well-trained ultra-endurance athletes with experince from several years of training and competition at international elite level. Measurements of the heart's contraction velocities were conducted using tissue Doppler imaging (VIVID7) in a resting situation at baseline, immediately after the race, and after 24 hours of recovery. Results: Characteristics for the subjects were at baseline (mean ± SD, for men and women): age 30±3 and 27±4; interventricular septal thickness 10.5±0.7 and 8.0±0.0 mm; left ventricular end-diastolic diameter 54.4±3.4 and 45.0±3.0 mm; posterior wall thickness 10.4±0.9 and 8.0±1.0 mm; early to late diastolic filling velocity (E/A) 2.3±0.6 and 2.2±0.2. Exercise duration was approx. 150 hours, and the calculated average work intensity was 40% of respective VO2peak, including time for rest, change of equipment, and food intake. Values of contraction velocities are presented in the table. Conclusions: All athletes had normally sized hearts. Based on contraction velocities we found no evidence of cardiac fatigue after ultra-endurance exercise. A difference compared to studies that found cardiac fatigue in other sports (e.g. marathon, triathlon) is that even though our population exercised for an extreme duration the average intensity was low. This might point towards that exercise intensity, not duration, is the primary source for cardiac fatigue.
Physiology of Adventure Racing