Control of Aerobic Training with Individualized Physical Loads

A method was used for organizing training for aerobic working capacity. Its salient feature was individualization of the main components of the training process: dosing of the physical load at a level of 90–95% of the anaerobic threshold and optimizing the duration of rest on the basis of revealing the phase of superrecovery of the aerobic working capacity. The study, which enrolled 64 healthy males and females aged 16–20 years, provided evidence for the high effectiveness of the method. After 12 weeks of training, the anaerobic threshold, the systolic work index, and the results of Cooper’s 12-min test increased, respectively, by 9.7, 24, and 16% in the males and by 9.2, 32, and 25% in the females. The method of organizing the training process is proposed not only for the development of aerobic working capacity in healthy individuals but also for primary and secondary prophylaxis of cardiovascular diseases such as arterial hypertension and atherosclerosis.__________Translated from Fiziologiya Cheloveka, Vol. 31, No. 4, 2005, pp. 131–133.Original Russian Text Copyright © 2005 by El’chaninova, Varshavskii, Ladanov, Kalachev, Filippova.     

Morphological and biochemical changes in old rat muscles: effect of increased use

Male Wistar rats were strength and swim trained during a substantial period of old age to determine the influence of aging and activity on the histochemical and metabolic characteristics of a predominantly slow (soleus) and a predominantly fast (plantaris) skeletal muscle. Strength training counteracted the age-related atrophy of the fibers and the age-induced changes in fiber-type distribution of both muscles. Swim training, on the other hand, was without any effect on these parameters. The activity of both mitochondrial and cytoplasmic enzymes became lower with aging in the soleus muscle, whereas only the activity of the cytoplasmic enzymes became lower in the plantaris. Strength training reduced the aerobic capacity of both muscles, whereas swim training had the opposite effect. Aging induced a lower glycogen concentration of the lateral gastrocnemius muscle. This was avoided by swim training. The phosphocreatine and adenosine 5'-triphosphate concentrations were unchanged with aging but became higher with strength training. The activity pattern, therefore, seems to have a considerable influence on the age-related modification of the histochemical and metabolic characteristics of skeletal muscles of the rat. The effect, however, is related to the recruitment pattern of the fiber populations and the form of activity.     

Optimization of function of breath by means of training with additional resistive resistance

Research on studying influence of the rate of muscular training in conditions of additional resistive resistance to breath on a level of physical working capacity, aerobic productivity and a functional condition of system of breath was performed. It is shown, that regular muscular loadings in a combination to resistive resistance to breath in a mode of an interval exposition provide an increase of profitability and efficiency of functioning of breathing system as expressed in increase of expiratory force respiratory muscles, rationalization of parity of volumetric-time parameters of breath pattern that promotes more effective gas exchange and decrease in power cost of respiratory movements and as a result leads to increase in aerobic productivity and growth of the general and special working capacity. At continuous exposition of resistive resistance against a background of muscular training, the increase of inspiratory force of respiratory muscles, increase of functional capacity of system of breath were revealed at physical activity as well as a significant gain of the general physical working capacity.     

Non-bicarbonate intracellular pH buffering of reptilian muscle

Summary Non-bicarbonate intracellular pH buffering values of skeletal and cardiac muscles were measured for 16 species of Australian reptiles from four orders (snakes, skelctal 19–36 slykes, cardiac 9–17 slykes; lizards, skeletal 25–54 slykes, cardiac 17–19 slykes; turtles, skeletal 25–43 slykes, cardiac 11–24 slykes; crocodile, skeletal 43 slykes). Although a positive correlation between pH buffering capacity and dependence on anaerobic muscle work was found, even the highest reptilian pH buffering values were low relative to equivalent white anaerobic muscles of fish, birds, and mammals. The low non-bicarbonate intracellular pH buffering capacity of reptilian muscle arises through lower contributions from proteins (10–14 slykes), non-protein histidine (7–18 slykes) and phosphate (5–15 slykes). It is concluded that while other vertebrates depend on these intracellular buffers for regulating muscle pH during anaerobic muscle work, reptiles rely less on buffering and instead may tolerate greater pH fluctuations.Abbreviations intracellular pH buffering capacity - EDTA ethylenediaminetetra-acetic acid - HPLC high performance liquid chromatography - I.D. internal diameter - LDH lactate dehydrogenase     

Effects of application resistive and flex resistance to breath in training sportsmen

The research on studying influence of a rate of muscular training in conditions of additional aerodynamic and flex resistance to breath on a level of physical working capacity, aerobic productivity and a functional condition of system of breath is carried out. It is shown, that regular muscular training on a background of use of the dosed out breath with increased aerodynamic and flex resistance to respiratory streams and respiratory movements, first of all, provides essential growth of functionalities of the respiratory muscles expressed in authentic increase of power and speed-power parameters, shortening of time of impellent reaction of respiratory muscles. Improvement of a functional condition of respiratory system, and, in particular, respiratory muscles, causes significant growth of physical working capacity and physical readiness of young sportsmen.     

A Broad-Based Estimation of the Physical Work Capacity of Young Men

The physical work capacity of young men (17–21 years old) was estimated during this broad-based study. The subjects were divided into two groups depending on the level of their motor activity: those who were in general sport fitness programs (166 subjects) and those who had achieved high recognition in sports (149 subjects). Two stages in the development of physical work capacity in young men were detected. The first stage covered the 17- to 18-year-old men and was characterized by a significant increase in physical development and aerobic capacity, the improved hemodynamic responses to physical exercise, and a high tension in the cardiac regulatory mechanisms. The second stage (18–21 years) was a period of relative stabilization of physical development and body functions, during which the organism came to a new functional level typical of adulthood. A comparative analysis of the physical work capacity in subjects with different levels of motor activity showed that the indices of physical development, physical fitness, and aerobic capacity in 17-year-old athletes exceeded the same indices in the untrained age-matched group. Physical work capacity was significantly higher in athletes than in the untrained subjects during the whole juvenile period. In athletes the age-related physical fitness and aerobic capacity increased to a greater extent but physical development increased to a lesser extent than the same in the untrained subjects.     

Fiber types and metabolic characteristics in muscles of wild boars, normal and halothane sensitive Swedish landrace pigs

Muscle fiber characteristics, glycogen content and enzyme activities were studied in muscles of six Swedish landrace pigs, six wild boars and in three halothane sensitive landrace pigs. The wild boars have a higher proportion of type I and IIA fibers compared with Swedish landrace pigs. Fiber composition is similar in landrace pigs and halothane sensitive landrace pigs. The wild boars revealed the highest citrate synthase (CS) and 3-OH-acyl-CoA dehydrogenase (HAD) activities and the lowest glycogen content compared with the other two groups. Lower CS and HAD activities were observed in the halothane sensitive pigs compared with the other pigs. The data show that wild boars have a higher and halothane sensitive landrace pigs a lower aerobic capacity in skeletal muscles compared with Swedish landrace pigs.     

The effect of chronic and acute dietary restriction on the growth and protein turnover of fast and slow types of rat skeletal muscle

Changes in the growth and protein turnover of the anterior tibialis and soleus muscles were studied in response to acute and chronic dietary restriction (50% of ad libitum intake) between 3 and 149 weeks post partum. The effect of long-term dietary restriction from weaning to senescence was to retard the growth and normal developmental of the two types of skeletal muscle. This was evident from measurements of various parameters of growth, i.e. total protein, RNA and DNA and protein/DNA-P, which were reduced by approximately 50% when compared with age-matched controls. These decreases, however, were not accompanied by a decline in the fractional rate of synthesis (%/day) or ribosomal activity (mg protein/day per mg RNAP). The slowing down of the age-related decline in muscle growth has been attributed to a reduction in RNA capacity (RNA/protein), with similar responses in the fast- and slow-twitch skeletal muscles. The initial effects of piecemeal feeding of this restricted diet on the two types of muscle were also monitored. Short term starvation effects, i.e. 24 hr after feeding a reduced ration, were measured on the protein content and RNA/protein of both the anterior tibialis and soleus muscles; both parameters were unchanged within 24 hr. In contrast, a rapid and significant decline in the ribosomal synthetic activity (mg/d per mg RNAP), and a corresponding fall in the fractional rate of synthesis, occurred within 24 hr of feeding.     

Involvement of IL-1 in the Maintenance of Masseter Muscle Activity and Glucose Homeostasis

Physical exercise reportedly stimulates IL-1 production within working skeletal muscles, but its physiological significance remains unknown due to the existence of two distinct IL-1 isoforms, IL-1α and IL-1β. The regulatory complexities of these two isoforms, in terms of which cells in muscles produce them and their distinct/redundant biological actions, have yet to be elucidated. Taking advantage of our masticatory behavior (Restrained/Gnawing) model, we herein show that IL-1α/1β-double-knockout (IL-1-KO) mice exhibit compromised masseter muscle (MM) activity which is at least partially attributable to abnormalities of glucose handling (rapid glycogen depletion along with impaired glucose uptake) and dysfunction of IL-6 upregulation in working MMs. In wild-type mice, masticatory behavior clearly increased IL-1β mRNA expression but no incremental protein abundance was detectable in whole MM homogenates, whereas immunohistochemical staining analysis revealed that both IL-1α- and IL-1β-immunopositive cells were recruited around blood vessels in the perimysium of MMs after masticatory behavior. In addition to the aforementioned phenotype of IL-1-KO mice, we found the IL-6 mRNA and protein levels in MMs after masticatory behavior to be significantly lower in IL-1-KO than in WT. Thus, our findings confirm that the locally-increased IL-1 elicited by masticatory behavior, although present small in amounts, contributes to supporting MM activity by maintaining normal glucose homeostasis in these muscles. Our data also underscore the importance of IL-1-mediated local interplay between autocrine myokines including IL-6 and paracrine cytokines in active skeletal muscles. This interplay is directly involved in MM performance and fatigability, perhaps mediated through maintaining muscular glucose homeostasis.     

HIGH-INTENSITY EXERCISE TRAINING INDUCES MORPHOLOGICAL AND BIOCHEMICAL CHANGES IN SKELETAL MUSCLES

In the present study we investigated the effect of two different exercise protocols on fibre composition and metabolism of two specific muscles of mice: the quadriceps and the gastrocnemius. Mice were run daily on a motorized treadmill, at a velocity corresponding to 60% or 90% of the maximal running velocity. Blood lactate and body weight were measured during exercise training. We found that at the end of training the body weight significantly increased in high-intensity exercise mice compared to the control group (P=0.0268), whereas it decreased in low-intensity exercise mice compared to controls (P=0.30). In contrast, the food intake was greater in both trained mice compared to controls (P < 0.0001 and P < 0.0001 for low-intensity and high-intensity exercise mice, respectively). These effects were accompanied by a progressive reduction in blood lactate levels at the end of training in both the exercised mice compared with controls (P=0.03 and P < 0.0001 for low-intensity and high-intensity exercise mice, respectively); in particular, blood lactate levels after high-intensity exercise were significantly lower than those measured in low-intensity exercise mice (P=0.0044). Immunoblotting analysis demonstrated that high-intensity exercise training produced a significant increase in the expression of mitochondrial enzymes contained within gastrocnemius and quadriceps muscles. These changes were associated with an increase in the amount of slow fibres in both these muscles of high-intensity exercise mice, as revealed by the counts of slow fibres stained with specific antibodies (P < 0.0001 for the gastrocnemius; P=0.0002 for the quadriceps). Our results demonstrate that high-intensity exercise, in addition to metabolic changes consisting of a decrease in blood lactate and body weight, induces an increase in the mitochondrial enzymes and slow fibres in different skeletal muscles of mice, which indicates an exercise-induced increase in the aerobic metabolism.     

Changes in Bioenergetic Functions and Performance Capacity of Athletes after Administration of a Polyphenolic Antihypoxic Agent

A study was made of the effects of a polyphenolic antihypoxic agent on respiration in isolated mitochondria and homogenates of skeletal muscles and on the bioenergetic functions and performance capacity of runners. Polarographic analysis showed that preincubation of mitochondria isolated from skeletal muscles with the antihypoxic agent inhibited glutamate oxidation more than twofold in state 4. In homogenates of skeletal muscles, the antihypoxic agent in the presence or absence of an uncoupler strongly inhibited or slightly stimulated glutamate oxidation in the initial mitochondrial state, respectively. Thus, the antihypoxic agent inhibited mitochondrial ATPase but did not change the ATP synthase activity or exert a direct effect on the respiratory chain. The antihypoxic agent administered to runners at a concentration of about 30 mg/kg body mass 1 h prior to exercise at maximum power with a maximum duration of 1–10 min allowed higher acidification but did not affect the major bioenergetic functions such as O₂ consumption, excess CO₂ production, O₂ debt, and O₂ demand. The agent substantially increased the performance capacity of runners during exercise with a maximum duration of 3–10 min. Under competition conditions, administration of the agent to runners improved the results by 2–6% in 200- to 10000-m races.     

Distinct α2 Na,K-ATPase membrane pools are differently involved in early skeletal muscle remodeling during disuse

The Na,K-ATPase is essential for the contractile function of skeletal muscle, which expresses the α1 and α2 subunit isoforms of Na,K-ATPase. The α2 isozyme is predominant in adult skeletal muscles and makes a greater contribution in working compared with noncontracting muscles. Hindlimb suspension (HS) is a widely used model of muscle disuse that leads to progressive atrophy of postural skeletal muscles. This study examines the consequences of acute (6–12 h) HS on the functioning of the Na,K-ATPase α1 and α2 isozymes in rat soleus (disused) and diaphragm (contracting) muscles. Acute disuse dynamically and isoform-specifically regulates the electrogenic activity, protein, and mRNA content of Na,K-ATPase α2 isozyme in rat soleus muscle. Earlier disuse-induced remodeling events also include phospholemman phosphorylation as well as its increased abundance and association with α2 Na,K-ATPase. The loss of α2 Na,K-ATPase activity results in reduced electrogenic pump transport and depolarized resting membrane potential. The decreased α2 Na,K-ATPase activity is caused by a decrease in enzyme activity rather than by altered protein and mRNA content, localization in the sarcolemma, or functional interaction with the nicotinic acetylcholine receptors. The loss of extrajunctional α2 Na,K-ATPase activity depends strongly on muscle use, and even the increased protein and mRNA content as well as enhanced α2 Na,K-ATPase abundance at this membrane region after 12 h of HS cannot counteract this sustained inhibition. In contrast, additional factors may regulate the subset of junctional α2 Na,K-ATPase pool that is able to recover during HS. Notably, acute, low-intensity muscle workload restores functioning of both α2 Na,K-ATPase pools. These results demonstrate that the α2 Na,K-ATPase in rat skeletal muscle is dynamically and acutely regulated by muscle use and provide the first evidence that the junctional and extrajunctional pools of the α2 Na,K-ATPase are regulated differently.     

High aerobic capacities in the skeletal muscles of pinnipeds: adaptations to diving hypoxia.

The objective was to assess the aerobic capacity of skeletal muscles in pinnipeds. Samples of swimming and nonswimming muscles were collected from Steller sea lions (Eumetopias jubatus, n = 27), Northern fur seals (Callorhinus ursinus, n = 5), and harbor seals (Phoca vitulina, n = 37) by using a needle biopsy technique. Samples were either immediately fixed in 2% glutaraldehyde or frozen in liquid nitrogen. The volume density of mitochondria, myoglobin concentration, citrate synthase activity, and beta-hydroxyacyl-CoA dehydrogenase was determined for all samples. The swimming muscles of seals had an average total mitochondrial volume density per volume of fiber of 9.7%. The swimming muscles of sea lions and fur seals had average mitochondrial volume densities of 6.2 and 8.8%, respectively. These values were 1.7- to 2.0-fold greater than in the nonswimming muscles. Myoglobin concentration, citrate synthase activity, and beta-hydroxyacyl-CoA dehydrogenase were 1.1- to 2. 3-fold greater in the swimming vs. nonswimming muscles. The swimming muscles of pinnipeds appear to be adapted for aerobic lipid metabolism under the hypoxic conditions that occur during diving.     

INVERSE RELATIONSHIP BETWEEN FUNCTIONAL MATURITY AND EXPONENTIAL GROWTH RATE OF AVIAN SKELETAL MUSCLE: A CONSTRAINT ON EVOLUTIONARY RESPONSE

In this study, we investigate whether a tissue-level constraint can explain the general inverse relationship between growth rate and precocity of development in birds. On the whole, altricial (dependent) chicks grow three to four times faster than the less dependent, more able chicks of precocial species of similar adult mass. We suggest that an antagonism between growth and acquisition of mature function in skeletal muscle constrains postnatal growth and development in most species of birds. Altricial species, represented by European starlings in this study, hatch with skeletal muscle having low capacity for generating force but grow rapidly. Conversely, precocial species (northern bobwhite quail and Japanese quail), hatch with relatively mature skeletal muscle, especially in their legs, but grow more slowly. As development proceeds in all species, exponential growth rates decrease as muscles acquire adult levels of function. Among four variables associated with muscle function, exponential growth rate (EGR) was negatively correlated with pyruvate kinase activity (glycolysis), potassium concentration (electrical potential), and dry weight fraction (contractile proteins) in both pectoral and leg muscles but not with citrate synthase activity (aerobic metabolism) in either set of muscles. For pectoral muscle, these variables accounted for 87% of the total variation in EGR in all three species combined despite a twofold difference in growth rates between the starling and quail. EGRs of leg muscle (51% of variation accounted for) were less than predicted by the pectoral-muscle equation in quail during the early part of the postnatal period and in starlings during the late postnatal period. This result would not contradict a growth rate/maturity constraint hypothesis if EGRs were down-regulated for allometric or other considerations.     

Aerobic capacity and force-velocity characteristics in cross-country skiers at the end of preparatory period and at the beginning of period of competitions

The goal of this study is to compare the dynamics of aerobic and force–velocity characteristics of the shoulder girdle muscles and leg muscles during high-volume aerobic training in junior cross-country skiers in the time interval from the end of the preparatory period to the beginning of the competition period. Eleven junior cross-country skiers were repeatedly tested from December to February. In this period, the volume of aerobic training was 22–23 h per week. During the experimental period, the knee extensors torque in the range of angular velocities of 30–300 degrees/s was observed to decrease, with no changes in the volume of the quadriceps muscle of thigh and knee flexor muscles. The maximal oxygen consumption rate (VO₂ max) was observed to decrease by 6% (P < 0.05) in the treadmill test, while the oxygen consumption at anaerobic threshold (VO₂ at La = 4 mmol/L) was unchanged. On the contrary, the functional capacity of the shoulder girdle muscles was enhanced. The force-velocity characteristics of the shoulder girdle muscles that were estimated in the maximal anaerobic power test at a double poling ergometer increased by 16% (P < 0.01), and the volume of the triceps muscle of arm increased by 4.6 (P < 0.01). The aerobic capacity of the shoulder girdle muscles that were estimated by the VO₂ at La = 4 mmol/L increased by 30% (P < 0.05). The potential for increasing the performance of junior cross-country skiers seems to be associated with the increased functional capacity of the shoulder girdle muscles.     

The Order of Exercise during Concurrent Training for Rehabilitation Does Not Alter Acute Genetic Expression,Mitochondrial Enzyme Activity or Improvements in Muscle Function

Concurrent exercise combines different modes of exercise (e.g., aerobic and resistance) into one training protocol, providing stimuli meant to increase muscle strength, aerobic capacity and mass. As disuse is associated with decrements in strength, aerobic capacity and muscle size concurrent training is an attractive modality for rehabilitation. However, interference between the signaling pathways may result in preferential improvements for one of the exercise modes. We recruited 18 young adults (10 ♂, 8 ♀) to determine if order of exercise mode during concurrent training would differentially affect gene expression, protein content and measures of strength and aerobic capacity after 2 weeks of knee-brace induced disuse. Concurrent exercise sessions were performed 3x/week for 6 weeks at gradually increasing intensities either with endurance exercise preceding (END>RES) or following (RES>END) resistance exercise. Biopsies were collected from the vastus lateralis before, 3 h after the first exercise bout and 48 h after the end of training. Concurrent exercise altered the expression of genes involved in mitochondrial biogenesis (PGC-1α, PRC, PPARγ), hypertrophy (PGC-1α4, REDD2, Rheb) and atrophy (MuRF-1, Runx1), increased electron transport chain complex protein content, citrate synthase and mitochondrial cytochrome c oxidase enzyme activity, muscle mass, maximum isometric strength and VO_2peak. However, the order in which exercise was completed (END>RES or RES>END) only affected the protein content of mitochondrial complex II subunit. In conclusion, concurrent exercise training is an effective modality for the rehabilitation of the loss of skeletal muscle mass, maximum strength, and peak aerobic capacity resulting from disuse, regardless of the order in which the modes of exercise are performed.     

Catabolic enzyme activities in relation to premigratory fattening and muscle hypertrophy in the gray catbird (Dumetella carolinensis)

Summary The flight muscles of the gray catbird (Dumetella carolinensis) were examined to determine if short term adjustments occur in the activity of key catabolic enzymes during preparation for long distance migration. The aerobic capacity of the pectoralis muscle as indicated by citrate synthase activity (CS) is among the highest reported for skeletal muscle (200 moles [min·g fresh mass]^–1 at 25°C). The mass specific aerobic capacity as indicated by CS activity or cytochromec concentration does not change during premigratory fattening (Fig. 2) or in relation to the muscle hypertrophy that occurs concomitantly. The maintenance of mass specific aerobic capacity indicates that the total aerobic capacity increases in proportion to the increase in muscle size. The augmented potential for total aerobic power output is considered an adaptation to meet the increased power requirements of flight due to the increased body mass. Additionally, the capacity to oxidize fatty acids, as indicated by -hydroxyacyl-CoA dehydrogenase activity, approximately doubles during premigratory fattening (from 35 to 70 moles [min·g fresh mass]^–1 at 25°C; Fig. 1A). This adaptation should favor fatty acid oxidation, thereby sparing carbohydrate and prolonging endurance. The activity of phosphofructokinase, a key glycolytic enzyme, does not change before migration.Abbreviations CPT carnitine palmitoyl transferase - CS citrate synthase - HOAD -hydroxyacyl-CoA-dehydrogenase - PFK phosphofructokinase     

Field Tests for Evaluating the Aerobic Work Capacity of Firefighters

Working as a firefighter is physically strenuous, and a high level of physical fitness increases a firefighter’s ability to cope with the physical stress of their profession. Direct measurements of aerobic capacity, however, are often complicated, time consuming, and expensive. The first aim of the present study was to evaluate the correlations between direct (laboratory) and indirect (field) aerobic capacity tests with common and physically demanding firefighting tasks. The second aim was to give recommendations as to which field tests may be the most useful for evaluating firefighters’ aerobic work capacity. A total of 38 subjects (26 men and 12 women) were included. Two aerobic capacity tests, six field tests, and seven firefighting tasks were performed. Lactate threshold and onset of blood lactate accumulation were found to be correlated to the performance of one work task (r_s = −0.65 and −0.63, p<0.01, respectively). Absolute (mL·min⁻¹) and relative (mL·kg⁻¹·min⁻¹) maximal aerobic capacity was correlated to all but one of the work tasks (r_s = −0.79 to 0.55 and −0.74 to 0.47, p<0.01, respectively). Aerobic capacity is important for firefighters’ work performance, and we have concluded that the time to row 500 m, the time to run 3000 m relative to body weight (s·kg⁻¹), and the percent of maximal heart rate achieved during treadmill walking are the most valid field tests for evaluating a firefighter’s aerobic work capacity.     

Involvement of the adenylyl cyclase signaling system in the action of insulin and mollusk insulin-like peptide

Involvement of the adenylyl cyclase signaling system in the mechanism of action of the mammalian insulin and epidermal growth factor as well as of insulin-like peptide isolated from the bivalve mollusk Anodonta cygnea has been studied. It was shown for the first time that insulin and insulin-like peptide exert in vitro the GTP-dependent stimulating action on the adenylyl cyclase activity. Epidermal growth factor has an analogous effect. Effectiveness of the peptides decreased in the order insulin-like peptide > epidermal growth factor > insulin in the foot smooth muscles of A. cygnea and insulin > epidermal growth factor > insulin-like peptide in the skeletal muscles of rat.     

Testosterone stimulates myoglobin expression in different muscles of the mouse

The regulation of energy metabolism is one of the major functions of steroid hormones. This study was performed to explore whether testosterone can regulate the aerobic capacity of skeletal muscles via myoglobin expression. To study this, changes in testosterone level were quantified, and the level of myoglobin protein was analyzed using Western blot in mice subjected to 6 weeks of training (T) or testosterone administration (A). Both treatments significantly increased the plasma testosterone level when compared to the untrained (U) or control (C) group. Training induced a significant increase in the myoglobin content in gastrocnemius and plantaris muscles (287 and 83%, respectively). Testosterone administration increased myoglobin concentration in plantaris (183%) but not in gastrocnemius. In extensor digitorum longus muscle the protein content decreased slightly after exercise, but increased 78% after testosterone administration. In soleus and rectus femoris muscles the myoglobin content was unchanged after both treatments. The data show that testosterone and training have differential effects on the concentration of myoglobin in some, but not all muscles. This may have an influence on the aerobic capacity in mouse skeletal muscles. The data demonstrated that both testosterone administration and training induced an increase in plasma testosterone level. However, the effects of the treatments on the myoglobin concentration differ.