Muscle fiber diameters and numbers of capillaries per fiber, per square millimeter, and around each fiber were determined in needle biopsies from the lateral part of the quadriceps muscle of 23 young men. Twelve subjects were untrained (UT) and eleven were endurance-trained (ET) athletes. Average values for maximal oxygen uptake were 51.3 (UT) and 72.0 ml/kg-min (ET). Mean fiber diameters were not significantly different in the two groups (48.8 and 49.1 micron). The capillaries per fiber ratios were 1.77+/-0.10 and 2.49+/-0.08 (mean+/-SE) in the UT and ET groups, respectively. The numbers of capillaries around each fiber were 4.43+/-0.19 (UT) and 5.87+/-0.18 (ET). The numbers of capillaries per mm2 were 585+/-40 (UT) and 821+/-28 (ET). Fiber diameters were 28% smaller in ultrathin than in fresh-frozen sections from the same biopsies. After correction for this difference, the numbers of capillaries per mm2 were 305 and 425 in the UT and ET, respectively. The capillaries per fiber ratio increased with increasing fiber diameter, but not sufficiently to maintain the number of capillaries per mm2. Fibers containing many mitochondria are surrounded by more capillaries than fibers with few mitochondria.

SUMMARYSeven young females were subjected to 24 weeks of intensive endurance training. Adaptive changes in myofibrillary ATP-ase activity, capillary supply and mitochondrial content were investigated with light-and electron microscopy in needle biopsies from the quadriceps femoris.1. The average value for the maximal oxygen uptake increased from 45-7 to 57-2 (ml. kg-1 min-') (25.2 %, P < 0-005).2. The average number of capillaries per muscle fibre increased from 1P39 to 1-79 (28.8 %, P < 0-005). Since no significant change in fibre area was found, this suggests that a considerable number of new capillaries have been formed during the training period.3. An increased capillary supply of all fibre types was found, being greatest for type I and smallest for type IIB.4. The relative amount of type I fibres before and after the training period was 57-9 and 56*5 % respectively (n.s.), for type IIA fibres 26-4 and 31 5 % (P < 0.005), for type JIB fibres 9-2 and 3.4 % (P < 0.005) and for type IIC fibres 0 4 and 2.2 % (P < 0.005). Thus, in the type II group, significant changes in subtypes take place during the endurance training. The data suggest that type IIAB may represent a transitional state between type IIA and IIB. 5. Correlation of capillary supply, myofibrillar ATP-ase activity and mitochondrial content (determined semiquantitatively) of individual muscle fibres indicates that the capillary supply to a given fibre is more closely related to its mitochondrial content than to the fibre type as determined on the basis of myofibrillar ATP-ase activity.

Six male and six female runners were chosen on the basis of age (20-30 years) and their performance over the marathon distance (mean time = 199.4, SEM 2.3 min for men and 201.8, SEM 1.8 min for women). The purpose was to find possible sex differences in maximal aerobic power (VO2max), anaerobic threshold, running economy, degree and utilization of VO2max (when running a marathon) and amount of training. The results showed that performance-matched male and female marathon runners had approximately the same VO2max (about 60 ml.kg-1.min-1). For both sexes the anaerobic threshold was reached at an exercise intensity of about 83% of VO2max, or 88%-90% of maximal heart rate. The females' running economy was poorer, i.e. their oxygen uptake during running at a standard submaximal speed was higher (P less than 0.05). The heart rate, respiratory exchange ratio and blood lactate concentration also confirmed that a given running speed resulted in higher physiological strain for the females. The percentage utilization of VO2max at the average marathon running speed was somewhat higher for the females, but the difference was not significant. For both sexes the oxygen uptake at average speed was 93%-94% of the oxygen uptake corresponding to the anaerobic threshold. Answers to a questionnaire showed that the females' training programme over the last 2 months prior to running the actual marathon comprised almost twice as many kilometers of running per week compared to the males (60 and 33 km, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The number of capillaries per fiber, per mm2, around each fiber type and relative to fiber area was determined in six untrained subjects (UT) and six elite cross-country skiers (ET). Average values for maximal oxygen uptake were 49.8 ml . kg-1 . min-1 (UT) and 77.9 ml . kg-1 . min-1 (ET). Type I fibers constituted 39.2% (UT) and 68.6% (ET), type II A fibers 39.6% (UT) and 19.2% (ET), while 12.8% (UT) and 6.6% (ET) of the fibers were type II B. The mean fiber area for the type II A fibers was significantly greater (p less than 0.01) than the areas for type I and II B in the untrained group. The average numbers of capillaries around each fiber type (CA) were 4.76-4.84-2.94 (UT) and 7.79-6.63-4.5 (ET) for type I, II A, and II B, respectively. There was a significant difference (p less than 0.01) in the CA values relative to fiber area for all fiber types in both groups, being highest for type I and lowest for type II B. The CA increased linearly with increasing size of the fibers for all fiber types in both groups. The mitochondrial content was determined semiquantitatively for each fiber type. The differences in capillary supply between the fiber types are accompanied by similar differences in mitochondrial content. The results indicate that endurance training increases the capillary supply of all fiber types in the human quadriceps muscle. The fact that light microscopical studies have given lower capillarization values than those obtained with the electron microscope is discussed.