Physiological responses to prolonged exercise in ultramarathon athletes.

The physiological responses of 10 ultramarathon athletes to prolonged exercise at the highest intensity level they could sustain for 4 h have been examined. Energy expenditure for the 4 h of exercise was 14,146 +/- 1,789 kJ, of which 63% was provided by the oxidation of fat. Plasma free fatty acids rose, but the changes in blood lactate concentration (delta 0.2 mmol/l) and exchange ratio (delta 0.05) were small, and the postexercise glycogen content (130 +/- 42 mumol/g) of the vastus lateralis muscles was estimated to be 37-53% of normal resting values. During exercise O2 intake (VO2) increased with time from the 50th to 240th min, the rise becoming significant (P less than 0.01) after 110 min of work. The change in VO2 was equivalent to a rise in relative intensity (%VO2max) of +9.1% and a change of speed of 1.49 km/h. A rise in cardiac frequency compensated for a fall in stroke volume (SV), so that cardiac output was maintained, and the increases in rectal temperature (Tre) (delta 0.63 degree C) and sweat loss (3.49 +/- 0.50 kg, equivalent to 5.5% of body wt) and the decreased mean skin temperature (Tsk) (-1.22 degree C) were within tolerable limits during exercise. Following exercise there was a loss (-25%) of ability to generate voluntary force of the quadriceps femoris, though electrically evoked mechanical properties of the muscle remained unchanged. The results suggest that neither thermal nor cardiovascular factors are limiting to prolonged (4 h) exercise, although the ability to utilize fat as a fuel may be important in ultradistance athletes.(ABSTRACT TRUNCATED AT 250 WORDS)