Structure and function of the ankle dorsiflexor muscles in young and moderately active men and women.

The aim was to investigate determinants of ankle dorsiflexor muscle (DF) strength and size in moderately active young men and women (n = 30; age 20-31 yr). Concentric (Con) and eccentric (Ecc) strength were measured isokinetically. Magnetic resonance imaging was used to determine the muscle cross-sectional area (CSA). Multiple biopsies were obtained from the tibialis anterior muscle to determine total numbers, areas (Area I and II) and proportions (Prop I and II) of type I and II fibers, respectively, and relative contents of myosin heavy chain (MHC) isoforms MHC1, MHC2a, and MHC2x. Women had lower Con and Ecc strength (24 and 27%; P < 0.01), smaller CSA (19%; P < 0.001), lower Ecc DF specific strength (strength/CSA) (10%; P < 0.01), and smaller Area I and Area II (21 and 31%; P < 0.01) than men. Prop I, MHC1, estimated total number of fibers, and Con DF specific strength were similar for both sexes. Con DF strength was up to 72% determined by CSA and Prop I, and Ecc DF strength was up to 81% determined by CSA, Prop I, and sex; variables other than CSA explained at most 9%. Body weight and fiber areas explained >50% of the variation in CSA. In conclusion, CSA was the predominant determinant of DF strength, CSA was to a great extent determined by the body weight and the sizes of muscle fibers, and sex differences in Ecc specific strength require further study.     

Motor unit firing rates and contractile properties in tibialis anterior of young and old men.

The effects of aging on motoneuron firing rates and muscle contractile properties were studied in tibialis anterior muscle by comparing results from six young (20.8 +/- 0.8 yr) and six old men (82.0 +/- 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal voluntary contraction (MVC) with twitch interpolation and stimulated twitch contractions. The old men had 26% lower MVC torque (P < 0.01) than did the young men, but percent activation was not different (99.1 and 99.3%, respectively). Twitch contraction durations were 23% longer (P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, motor unit firing rates were recorded. Results from approximately 950 motor unit trains in each subject group indicated that at all relative torque levels mean firing rates were 30-35% lower (P < 0.01) in the old subjects. Comparisons between young and old subjects' mean firing rates at each of 10%, 50%, and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations (r = -0.67 to -0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neuromuscular age-related remodeling principle.     

Skeletal muscle oxidative capacity in young and older women and men.

It has been suggested that a decline in skeletal muscle oxidative capacity is a general consequence of aging in humans. However, previous studies have not always controlled for the effects of varying levels of physical activity on muscle oxidative capacity. To test the hypothesis that, when matched for comparable habitual physical activity levels, there would be no age-related decline in the oxidative capacity of a locomotor muscle, the postexercise recovery time of phosphocreatine was compared in the tibialis anterior muscle of young [n = 19; 33.8 +/- 4.8 (SD) yr] and older [n = 18; 75.5 +/- 4.5 yr] healthy women and men of similar, relatively low, activity levels. The intramuscular metabolic measurements were accomplished by using phosphorus magnetic resonance spectroscopy. The results indicate that there was no age effect on the postexercise recovery time of phosphocreatine recovery, thus supporting the stated hypothesis. These data suggest that there is no requisite decline in skeletal muscle oxidative capacity with aging in humans, at least through the seventh decade.     

Correlation between active and passive isometric force and intramuscular pressure in the isolated rabbit tibialis anterior muscle

The purpose of this study was to quantify the relationship between intramuscular pressure (IMP) and muscle force during isometric muscle contraction of the rabbit tibialis anterior (TA) absent the effect of either bone or fascia. To quantify this relationship, length-tension experiments were performed on the isolated TA of the New Zealand White rabbit (mass=2.5+/-0.5kg, n=12). The knee was fixed in a custom jig, the distal tendon of the TA was attached to a servomotor, and a 360 microm fiber optic pressure transducer was inserted into the TA. The peroneal nerve was stimulated to define optimal length (L(0)). The length-tension curve was created using 40Hz isometric contractions with 2-min rest intervals between each contraction. Measurements began at L(0)-50%L(f) and progressed to L(0)+50%L(f), changing the length-tension in 5% L(f) increments after each contraction. Qualitatively, the length-tension curve for isometric contractions was mimicked by the length-pressure curve for both active and passive conditions. Linear regression was performed individually for each animal for the ascending and descending limb of the length-tension curve and for active and passive conditions. Pressure-force coefficients of determination ranged from 0.138-0.963 for the active ascending limb and 0.343-0.947 for the active descending limb. Passive pressure coefficients of determination ranged from 0.045-0.842 for the ascending limb and 0.672-0.982 for the descending limb. These data indicate that IMP measurement provide a fairly accurate index of relative muscle force, especially at muscle lengths longer than optimal.     

Fiber type composition of the vastus lateralis muscle of young men and women.

This study presents data collected over the past 10 years on the muscle fiber type composition of the vastus lateralis muscle of young men and women. Biopsies were taken from the vastus lateralis muscle of 55 women (21.2+/-2.2 yr) and 95 men (21.5+/-2.4 yr) who had volunteered to participate in various research projects. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were classified using mATPase histochemistry, and cross-sectional area was measured for the major fiber types (I, IIA, and IIB). Myosin heavy chain (MHC) content was determined electrophoretically on all of the samples from the men and on 26 samples from the women. With the exception of fiber Type IC, no significant differences were found between men and women for muscle fiber type distribution. The vastus lateralis muscle of both the men and women contained approximately 41% I, 1% IC, 1% IIC, 31% IIA, 6% IIAB, and 20% IIB. However, the cross-sectional area of all three major fiber types was larger for the men compared to the women. In addition, the Type IIA fibers were the largest for the men, whereas the Type I fibers tended to be the largest for the women. Therefore, gender differences were found with regard to the area occupied by each specific fiber type: IIA>I>IIB for the men and I>IIA>IIB for the women. These data establish normative values for the mATPase-based fiber type distribution and sizes in untrained young men and women.     

Skeletal muscle contractile and noncontractile components in young and older women and men.

To examine the influences of age, gender, and habitual physical activity level on human skeletal muscle composition, we developed a relatively simple magnetic resonance imaging method for the quantitation of leg anterior compartment contractile and noncontractile content. We studied 23 young (11 women and 12 men, 26-44 yr old) and 21 older (10 women and 11 men, 65-83 yr old) healthy adults. Analysis was by two-factor (age, gender) ANOVA. Physical activity, quantitated by three-dimensional accelerometer worn about the waist for 1 wk, was not different between groups. Men had larger contractile and noncontractile cross-sectional areas (cm(2)) than women, with no gender effect on percent noncontractile area. Young subjects had larger contractile areas and smaller absolute (cm(2)) and relative (percent total) noncontractile areas than older subjects. There was a significant linear relationship between physical activity and percent noncontractile area in older (r = -0.68, P = 0.002) but not young subjects. These data demonstrate a more than twofold increase in the noncontractile content of locomotor muscles in older adults and provide novel support for physical activity as a modulator of this age-related change in muscle composition.     

Specific strength and voluntary muscle activation in young and elderly women and men.

The extents to which decreased muscle size or activation are responsible for the decrease in strength commonly observed with aging remain unclear. Our purpose was to compare muscle isometric strength [maximum voluntary contraction (MVC)], cross-sectional area (CSA), specific strength (MVC/CSA), and voluntary activation in the ankle dorsiflexor muscles of 24 young (32 +/- 1 yr) and 24 elderly (72 +/- 1 yr) healthy men and women of similar physical activity level. Three measures of voluntary muscle activation were used: the central activation ratio [MVC/(MVC + superimposed force)], the maximal rate of voluntary isometric force development, and foot tap speed. Men had higher MVC and CSA than did women. Young men had higher MVC compared with elderly men [262 +/- 19 (SE) vs. 197 +/- 22 N, respectively], whereas MVC was similar in young and elderly women (136 +/- 15 vs. 149 +/- 16 N, respectively). CSA was greater in young compared with elderly subjects. There was no age-related impairment of specific strength, central activation ratio, or the rate of voluntary force development. Foot tap speed was reduced in elderly (34 +/- 1 taps/10 s) compared with young subjects (47 +/- 1 taps/10 s). These results suggest that isometric specific strength and the ability to fully and rapidly activate the dorsiflexor muscles during a single isometric contraction were unimpaired by aging. However, there was an age-related deficit in the ability to perform rapid repetitive dynamic contractions.     

Transcranial magnetic stimulation during resistance training of the tibialis anterior muscle.

During the first few weeks of resistance training, maximal voluntary contraction (MVC) force increases at a faster rate than can be accounted for by increases in protein synthesis. This early increase in MVC force has been attributed to neural mechanisms but the sources have not been identified. The purpose of this study was to measure changes in cortical excitability with transcranial magnetic stimulation during 4 weeks of resistance training of the tibialis anterior muscle. Ten individuals performed 6 sets of 10 MVCs 3 times per week for 4 weeks and ten participated as a control group. There were no changes in any parameters tested in the control group over the 4 weeks. In the training group, TA muscle strength increased significantly by 10% at week 2 and by 18% at week 4. As hypothesized, cortical excitability during resistance training also increased. The amplitude of the TA surface EMG motor evoked potential elicited by TMS during a low-level contraction increased by 32% after training with no change in the M-wave. These data indicate that there may be an increase in cortical excitability during the first few weeks of resistance training of the TA muscle.     

Fractional cholesterol esterification rate and muscle cholesterol of healthy young men

A group of fourteen healthy young male volunteers was examined to define more exactly the relations between lecithin cholesterol acyltransferase activity (LCAT), fractional cholesterol esterification rate (FER), total cholesterol (TC) and its free and esterified fractions (FC, CE) in skeletal muscles under physiological conditions. The mean values (+/- S.D.) of LCAT activity (95.4 +/- 16.3 mumol .1(-1) per hour), and FER (7.45 +/- 1.54% per hour) corresponded to published data on normolipidaemic healthy men of normal body weight. The mean value of TC in muscles was 332 +/- 83 micrograms per 100 mg of non-collagen protein, of which 14 +/- 7.4 per cent was formed by cholesterol esters. There was positive correlation between TC in muscles and age. Significant positive correlations between FER and the content of esterified cholesterol in muscles, and between FER and the proportion of esterified to total muscle cholesterol were found. These results suggest a close interrelation of cholesterol ester metabolism in the plasma and in slow pool tissues.     

Clinical characteristics of OGTT-derived hepatic- and muscle insulin resistance in healthy young men

[Purpose]

Insulin inhibits glucose release in the liver but increases glucose absorption in muscles. When insulin cannot properly control glucose, it negatively affects glucose metabolism and, furthermore, contributes to the onset of metabolic syndrome and chronic disease. Therefore, this study''s goal is to understand the clinical characteristics of hepatic insulin resistance and muscle insulin sensitivity in healthy young men.

[Methods]

Twenty-eight healthy young men (age 23.3 ± 0.5; mean ± SE) participated in this study. Liver function and blood lipids were measured by blood sampling from brachial vein after participants fasted the previous day. Hepatic insulin resistance and muscle insulin sensitivity were evaluated using two-hour OGTT along with surrogate index related to insulin sensitivity. The VO_2max was evaluated using cycle ergometer. Systemic insulin sensitivity was evaluated using two-hour euglycemic hyperinsulinemic clamp method.

[Results]

Hepatic insulin resistance showed a significant correlation with body fat (r = 0.609, p < 0.05). Also, hepatic insulin resistance showed a significant correlation with GOT (r = 0.467), GPT (r = 0.434), and γ-GTP (r = 0.375), reflecting liver functions, as well as showing a significant correlation with hs-CRP (r = 0.492, p < 0.05). On the other hand, muscle insulin sensitivity had no correlation with neither body fat nor liver function index (p > 0.05), and among surrogate indexes, it showed a significant correlation with Avignon (r = -0.493) and Matsuda index (r = -0.577). Glucose infusion rate, using the clamp method, showed a significant correlation with muscle insulin sensitivity (r = 0.448, p < 0.05). The VO_2max had a significant correlation with hepatic insulin resistance (r = -0.435, p < 0.05) and muscle insulin sensitivity (r = 0.474, p < 0.05), respectively.

[Conclusion]

For young men in their 20''s, the OGTT-based hepatic insulin sensitivity was an indicator of hepatic function and body fat but muscle insulin sensitivity was related to peripheral insulin sensitivity. Also, for young men, higher VO_2max indicated lower hepatic insulin resistance and higher muscle insulin sensitivity.     

Testosterone-induced increase in muscle size in healthy young men is associated with muscle fiber hypertrophy

Administration of replacement doses of testosterone to healthy hypogonadal men and supraphysiological doses to eugonadal men increases muscle size. To determine whether testosterone-induced increase in muscle size is due to muscle fiber hypertrophy, 61 healthy men, 18-35 yr of age, received monthly injections of a long-acting gonadotropin-releasing hormone (GnRH) agonist to suppress endogenous testosterone secretion and weekly injections of 25, 50, 125, 300, or 600 mg testosterone enanthate (TE) for 20 wk. Thigh muscle volume was measured by magnetic resonance imaging (MRI) scan, and muscle biopsies were obtained from vastus lateralis muscle in 39 men before and after 20 wk of combined treatment with GnRH agonist and testosterone. Administration of GnRH agonist plus TE resulted in mean nadir testosterone concentrations of 234, 289, 695, 1,344, and 2,435 ng/dl at the 25-, 50-, 125-, 300-, and 600-mg doses, respectively. Graded doses of testosterone administration were associated with testosterone dose and concentration-dependent increase in muscle volume measured by MRI (changes in vastus lateralis volume, -4, +7, +15, +32, and +48 ml at 25-, 50-, 125-, 300-, and 600-mg doses, respectively). Changes in cross-sectional areas of both type I and II fibers were dependent on testosterone dose and significantly correlated with total (r = 0.35, and 0.44, P < 0.0001 for type I and II fibers, respectively) and free (r = 0.34 and 0.35, P < 0.005) testosterone concentrations during treatment. The men receiving 300 and 600 mg of TE weekly experienced significant increases from baseline in areas of type I (baseline vs. 20 wk, 3,176 +/- 186 vs. 4,201 +/- 252 microm(2), P < 0.05 at 300-mg dose, and 3,347 +/- 253 vs. 4,984 +/- 374 microm(2), P = 0.006 at 600-mg dose) muscle fibers; the men in the 600-mg group also had significant increments in cross-sectional area of type II (4,060 +/- 401 vs. 5,526 +/- 544 microm(2), P = 0.03) fibers. The relative proportions of type I and type II fibers did not change significantly after treatment in any group. The myonuclear number per fiber increased significantly in men receiving the 300- and 600-mg doses of TE and was significantly correlated with testosterone concentration and muscle fiber cross-sectional area. In conclusion, the increases in muscle volume in healthy eugonadal men treated with graded doses of testosterone are associated with concentration-dependent increases in cross-sectional areas of both type I and type II muscle fibers and myonuclear number. We conclude that the testosterone induced increase in muscle volume is due to muscle fiber hypertrophy.     

Intramuscular pressure of human tibialis anterior muscle detects age-related changes in muscle performance

Intramuscular pressure (IMP) reflects forces produced by a muscle. Age is one of the determinants of skeletal muscle performance. The present study aimed to test whether IMP mirrors known age-related muscular changes. We simultaneously measured the tibialis anterior (TA) IMP, compound muscle action potential (CMAP), and ankle torque in thirteen older adults (60–80 years old) in vivo by applying different stimulation intensities and frequencies. We found significant positive correlations between the stimulation intensity and IMP and CMAP. Increasing stimulation frequency caused ankle torque and IMP to increase. The electromechanical delay (EMD) (36 ms) was longer than the onset of IMP (IMPD) (29 ms). Compared to the previously published data collected from young adults (21–40 years old) in identical conditions, the TA CMAP and IMP of older adults at maximum intensity of stimulation were 23.8% and 39.6% lower, respectively. For different stimulation frequencies, CMAP, IMP, as well as ankle torque of older adults were 20.5%, 24.2%, and 13.2% lower, respectively. Surprisingly, the EMD did not exhibit any difference between young and older adults and the IMPD was consistent with the EMD. Data supporting the hypotheses suggest that IMP measurement is an indicator of muscle performance in older adults.     

Protein metabolism in rat tibialis anterior muscle after stimulated chronic eccentric exercise.

In another study (J. Appl. Physiol. 69: 1709-1717, 1990) we reported that gastrocnemius (GAST) muscle enlargement failed to occur after 10 wk of 192 contractions performed every 3rd or 4th day. This result was surprising because increased protein synthesis rates were determined after an initial acute exercise bout with the same paradigms. In the same set of animals, tibialis anterior (TA) muscles were enlarged 16-30% compared with sedentary control muscles after the same chronic training regimen. This indicated that the regulation of protein expression may be different between the GAST and TA muscles. The present experiment attempted to define and explain these differences by comparing changes in various indexes of protein metabolism in TA with the same parameters determined in the accompanying study for the GAST. As in the GAST, results showed that TA protein synthesis rates are increased by acute exercise and principally regulated by translational and possibly posttranslational mechanisms. The differential response in muscle mass between the GAST and TA muscles after training may be due, in part, to greater relative resistances imposed on the TA than on the GAST that result in a more-prolonged effect on protein synthesis rates, with lower numbers of stimulated contractions required to stimulate increases in protein synthesis. Data also revealed that although as little as 1 min of total contractile duration (24 repetitions) increased TA protein synthesis rate by 30%, 8 min of total contractile duration (192 repetitions) further increased TA protein synthesis rates to only 45% above control.     

Genioglossus muscle activity at rest and in response to brief hypoxia in healthy men and women.

Obstructive sleep apnea (OSA) is more common in men than in women for reasons that are not clearly understood. An underlying difference between men and women in the respiratory-related neural control of upper airway dilator muscles has been suggested as a possible reason for the gender difference. We have compared three aspects of upper airway dilator muscle function in healthy men and women: 1) resting inspiratory genioglossus electromyogram (EMGgg) activity, 2) the respiratory EMGgg "afterdischarge" after a brief hypoxic stimulus, and 3) the relationship between the EMGgg and pharyngeal airway pressure. Inspired minute ventilation (VI), epiglottic pressure (P(epi)), and EMGgg and diaphragm EMG (EMGdi) activity were measured in 24 subjects (12 men, 12 women in the luteal menstrual phase) and were compared between genders while lying supine awake. Every 7-8 min over 2 h, subjects were exposed to 45-s periods of isocapnic hypoxia (9% O(2) in N(2)) that were abruptly terminated with one breath of 100% O(2). The relationship between P(epi) and EMGgg activity was also compared between genders. The results of 117 trials with satisfactory end-tidal PCO(2) control and no sighs or swallows are reported. There was no gender difference in the resting level of peak inspiratory EMGgg [3.7 +/- 0.8 (women) vs. 3.2 +/- 0.6% maximal activity (men)]. Repeated-measures ANOVA showed no gender or gender-by-time interaction effect between men and women in VI or EMGgg or EMGdi activity during or after the hypoxic stimulus. The relationship between P(epi) and EMGgg was not different between men (slope -0.63 +/- 0.20) and women (slope -0.69 +/- 0.33). These results do not support the hypothesis that the higher prevalence of OSA in men is related to an underlying gender difference in respiratory neural control of upper airway dilator muscles.     

Ventilatory decline after hypoxia and hypercapnia is not different between healthy young men and women.

The gradual decay in ventilation after removal of a respiratory stimulus has been proposed to protect against cyclic breathing disorders such as obstructive sleep apnea (OSA). The male predominance of OSA, and the increased incidence of OSA in women after menopause, indicates that the respiratory-stimulating effect of progesterone may provide protection against OSA by altering the rate of poststimulus ventilatory decline (PSVD). It was therefore hypothesized that PSVD is longer in premenopausal women than in men and is longer in the luteal menstrual phase compared with the follicular phase. PSVD was measured in 12 men and in 11 women at both their luteal and follicular phases, after cessation of isocapnic hypoxia and normoxic hypercapnia. PSVD was compared between genders and between women in the luteal and follicular phases by repeated-measures ANOVA. There were no significant differences in PSVD between any of the groups after either respiratory stimulus. This suggests that the higher occurrence of OSA in men does not reflect an underlying gender difference in PSVD and implies the increased prevalence of OSA in women after menopause is not representative of an effect of progesterone on PSVD.     

Kidneys extract BNP and NT-proBNP in healthy young men.

Renal metabolism of the cardiac marker NH2-terminal-pro-brain natriuretic peptide (NT-proBNP) has been suggested. Therefore, we determined the renal extraction ratios of NT-proBNP and its bioactive coproduct brain natriuretic peptide (BNP) at rest and during exercise. In addition, the cerebral ratios were evaluated. Ten young healthy men were investigated at baseline, during moderate cycle exercise (heart rate: 140, Borg scale: 14-15), and in the recovery with BNP and NT-proBNP measured from the brachial artery and the jugular and renal veins, and the renal and cerebral extraction ratios (Ext-Ren and Ext-Cer, respectively) were calculated. Cardiac output, stroke volume, heart rate, mean arterial pressures, and estimated glomerular filtration were determined. BNP and NT-proBNP were extracted by the kidneys but not by the brain. We observed no effect of exercise. The mean values (+/- SE) of Ext-Ren of NT-proBNP were similar (0.19 +/- 0.05, 0.21 +/- 0.06, and 0.12 +/- 0.03, respectively) during the three sessions (P > 0.05). Also the Ext-Ren of BNP were similar (0.18 +/- 0.07, 0.15 +/- 0.11, and 0.14 +/- 0.06, respectively; P > 0.05). There were no significant differences between Ext-Ren of BNP and NT-proBNP during the three sessions (P > 0.05). The Ext-Cer of both peptides varied insignificantly between -0.21 +/- 0.15 and 0.11 +/- 0.08. The renal extraction ratio of both BNP and NT-proBNP is approximately 0.15-0.20. There is no cerebral extraction, and short-term moderate exercise does not affect these values. Our findings suggest that the kidneys extract BNP and NT-proBNP to a similar extent in healthy young men.     

Proteomic analysis of rat soleus and tibialis anterior muscle following immobilization

A proteomic analysis was performed comparing normal slow twitch type fiber rat soleus muscle and normal fast twitch type fiber tibialis anterior muscle to immobilized soleus and tibialis anterior muscles at 0.5, 1, 2, 4, 6, 8 and 10 days post immobilization. Muscle mass measurements demonstrate mass changes throughout the period of immobilization. Proteomic analysis of normal and atrophied soleus muscle demonstrated statistically significant changes in the relative levels of 17 proteins. Proteomic analysis of normal and atrophied tibialis anterior muscle demonstrated statistically significant changes in the relative levels of 45 proteins. Protein identification using mass spectrometry was attempted for all differentially regulated proteins from both soleus and tibialis anterior muscles. Four differentially regulated soleus proteins and six differentially regulated tibialis anterior proteins were identified. The identified proteins can be grouped according to function as metabolic proteins, chaperone proteins, and contractile apparatus proteins. Together these data demonstrate that coordinated temporally regulated changes in the proteome occur during immobilization-induced atrophy in both slow twitch and fast twitch fiber type skeletal muscle.     

Capillary supply and mitochondrial content of different skeletal muscle fiber types in untrained and endurance-trained men. A histochemical and ultrastructural study.

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.