1.2. Endurance

An endurance test is carried out on a treadmill or bicycle ergometer at a workload (VO₂) of 70-80% of the subject's maximum until exhaustion supervenes, usually with the heart rate within about five beats of the maximum. Because of the difficulty in performing this test, only a few laboratories have attempted measurement of endurance times in normal individuals and, to our knowledge, none except our own, in malnourished subjects.

From a number of sources, it is known that the maximum relative workload that can be sustained for an 8-hour workday usually does not exceed about 35-40% VO₂ max. Thus, MICHAEL et al. (1961) found in laboratory treadmill work that 8 hours could be tolerated without undue fatigue when the relative load did not exceed 35% VO₂ max. Subjects rested for 10 minutes each hour and for one half-hour between the 4th and 5th hours of work. In the building industry, ÅSTRAND (1967) reported that about 40% VO₂ max was the upper limit that could be tolerated for an 8-hour workday, and we have estimated that sugar-cane cutters worked at an average of about 35% of their VO₂ max during an 8-hour day (SPURR, BARCO-NIETO and MAKSUD, 1975). These studies were performed in physically fit subjects. Sedentary individuals can be expected to have lower upper limits for 8 hours of work (ÅSTRAND and RODAHL, 1970, p. 292).

We have measured maximum endurance times at 80% VO₂ max (T₈₀) in the groups of malnourished subjects described above (BARAC-NIETO et al., 1978; 1980). We did not find any significant differences between the three groups (M, I and S) of malnourished men; T₈₀ averaged 97 ± 12 min (mean ± SE) in all subjects (BARAC-NIETO et al., 1978). However, it might be assumed that the VO₂ max of Group S subjects would be about 2.4 L/min had they not been malnourished, and that about 35% (0.84 L/min) could be sustained for an 8-hour workday. The value of 0.84 L/min is 80% of the VO₂ max (1.05 L/min) for these subjects who had maximum endurance times at this relative workload of a little over 1.5 hours, a loss of about 6.5 hours of daily working time or about an 80% reduction in productive potential (BARAC-NIETO et al., 1978). Using a similar method of estimation, BARAC-NIETO (1984) has calculated a 16% reduction in work output of the M subjects, a 35% decrease in I and a 78% reduction in S men.

In the case of Group S during dietary repletion, an interesting change in T₈₀ was observed. Endurance times were significantly reduced from 113 minutes at the first measurement of the basal period to 42 minutes at the end of the dietary repletion (BARAC-NIETO et al., 1980). The explanation for this surprising reduction is still not clear. HANSON-SMITH et al. (1977) reported decreased work-endurance times in rats on high-protein diets compared to animals ingesting an isocaloric carbohydrate diet, and BERGSTRÖM et al. (1967) and GOLLNICK et al. (1972) have shown that diets in which the energy value of carbohydrate has been replaced with fat and/or protein lead to reduced stores of muscle glycogen. Furthermore, BERGSTRÖM et al. (1967) demonstrated that the maximum endurance time in humans is directly related to the initial glycogen content of skeletal muscle.

During the dietary repletion period of the Group S subjects, carbohydrate intake was reduced from 64 to 50% of calories. In a normal individual this amount of carbohydrate should be sufficient to maintain muscle glycogen stores, but definitive studies seem not to have been done (DURNIN, 1982). The rebuilt muscle tissue of Group S subjects may not store glycogen normally and, together with the lack of regular exercise in the protracted sedentary existence in the metabolic ward, may lead to reduced muscle glycogen and shorter endurance times. Nutritive supply to muscles, and the metabolic and endocrine responses which regulate it during both short term and prolonged exercise, have not been investigated in malnourished individuals. Even though there is little reason at the moment to suspect abnormal muscle function in acute exercise testing to maximum levels, the responses to prolonged exercise may be worth investigating.