Relationship between performance at different exercise intensities and skeletal muscle characteristics

The hypothesis investigated whether exercise performance over a broad range of intensities is determined by specific skeletal muscle characteristics. Seven subjects performed 8-10 exhaustive cycle trials at different workloads, ranging from 150 to 700 W (150 min to 20 s). No relationships between the performance times at high and low workloads were observed. A relationship (P < 0.05) was noticed between the percentage of fast-twitch x fibers and the exercise time at 579 ± 21 W (～30 s; r(2) = 0.88). Capillary-to-fiber-ratio (r(2): 0.58-0.85) was related (P < 0.05) to exercise time at work intensities ranging from 395 to 270 W (2.5-21 min). Capillary density was correlated (r(2) = 0.68; P < 0.05) with the net rate of plasma K(+) accumulation during an ～3-min bout and was estimated to explain 50-80% (P < 0.05) of the total variance observed in exercise performances lasting ～30 s to 3 min. The Na(+)-K(+) pump β(1)-subunit expression was found to account for 13-34% (P < 0.05) during exhaustive exercise of ～1-4 min. In conclusion, exercise performance at different intensities is related to specific physiological variables. A large distribution of fast-twitch x fibers may play a role during very intense efforts, i.e., ～30 s. Muscle capillaries and the Na(+)-K(+) pump β(1)-subunit seem to be important determinants for performance during exhaustive high-intensity exercises lasting between 30 s and 4 min.     

Lesions in the rat soleus muscle following eccentrically biased exercise

Morphological changes in skeletal muscle related to lengthening (eccentric) contractions have been noted by several laboratories. However, a systematic examination of skeletal muscle following repetitive eccentric contractions is lacking. This study was undertaken to study lesions and determine their relative densities in rat soleus muscle 30 min following level or downhill treadmill exercise. Following fixation in situ by vascular perfusion, toluidine-blue-stained 1-micron sections of the muscle samples selected at 73-micron intervals were scanned with a light microscope. Three types of lesions were noted: focal disruptions in the A-band, localized dissolution of Z-lines, and clotting of muscle fibers. Soleus muscle from the caged controls and the tibialis anterior muscle from downhill-exercised rats were essentially free of lesions. Eighty-nine percent of the soleus m. lesions in the downhill runner group and 97% of those in the level runner group were A-band disruptions. A-band lesion density was significantly higher in the soleus muscle of the downhill runners compared to level runners with the highest incidence in the distal half. A-band lesion density was lower in soleus muscles from level runners; however, the highest intramuscular incidence was in the proximal rather than the distal end. The results indicate that a disruption of the A-band is a principal change within some skeletal muscle fibers 30 min following repetitive eccentric contractions.     

Changes in fiber composition of soleus muscle during rat hindlimb suspension

Chronic reduction of gravitational load in the rear limbs of rats to simulate the influence of near-zero gravity in skeletal muscles has been shown previously to elicit atrophy in the soleus muscle. Use of this model by the present investigation indicates that soleus atrophy was characterized by a decline in the number of fibers in groups that contained the slow isoenzyme of myosin and which were classified as type I from intensity of staining to myofibrillar actomyosin adenosinetriphosphatase (ATPase) and to NADH tetrazolium reductase. Furthermore total fiber number was not changed, whereas fibers containing the intermediate isoenzyme and those classified as type IIa increased. There results could be explained by either a change in the composition within existing fibers or a simultaneous loss of slow fibers and de novo synthesis of intermediate and fast fibers. Evidence for transformation included an absence of embryonic or neonatal myosin in muscles from suspended rats and the constant fiber number that was unchanged by 4 wk of suspension. Furthermore although fiber areas of both groups of type I and IIa fibers declined during suspension, variability of the fiber areas within each group did not increase.     

Habitual exercise enhances neuromuscular transmission efficacy of rat soleus muscle in situ.

Rat motor nerve terminals and the endplates they interact with exhibit changes to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neuromuscular transmission in rat soleus muscle were investigated using a novel in situ approach. Consistent with previous reports, miniature endplate potential frequency was enhanced by habitual exercise. Other passive properties, such as resting membrane potential, miniature endplate potential amplitude, and "giant" miniature endplate potential characteristics were unaltered by the training program. Full-size endplate potentials were obtained by blocking soleus muscle action potentials with mu-conotoxin GIIIb. Quantal content values were 91.5 and 119.9 for control and active groups, respectively (P < 0.01). We also measured the rate and extent of endplate potential amplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10--20%) in a specific population of cells from active rats (P < 0.05). The results establish the degree of activity-dependent plasticity as it pertains to neuromuscular transmission in a mammalian slow-twitch muscle.     

Electromyogram power spectrum during dynamic contractions at different intensities of exercise

During dynamic contractions performed on a cycle ergometer, we studied the influence of motor unit (MU) recruitment on the electromyographic (EMG) spectral content by exerting mechanical power of different intensities, which was chosen to remain below the maximal aerobic power (VO2max). The spectral parameters: EMG total power (PEMG), mean (MPF) and median (MED) power frequencies, which are the most representative of the EMG spectral content, were calculated according to the EMG activity of the vastus medialis muscle (VM) and soleus muscle (SOL) of the right leg. For VM and SOL, PEMG increased linearly with exerted power demonstrating an enhancement of MU recruitment. Moreover these relationships were less scattered when exerted power was expressed as a percentage of VO2max. Changes in MPF and MED with varying exercise intensities were different from one subject to another. For a set of subjects, MPF and MED were found to be independent of exerted power. Although VM and SOL muscles are different in fibre type composition, similar results were obtained for both EMG activities. We have concluded that for dynamic contractions performed at different intensities below VO2max, the recruitment of the MU has a poor effect on the EMG spectral content whatever the predominant type of fibre.     

Changes in rat soleus muscle phenotype consecutive to a growth in hypergravity followed by normogravity

It has been demonstrated that a long-term stay in hypergravity (HG: 2G) modified the phenotype and the contractile properties of rat soleus muscle. The ability of this muscle to contract was drastically reduced, which is a sign of anticipated aging. Consequently, our aim was to determine whether rats conceived, born, and reared in hypergravity showed adaptative capacities in normogravity (NG: 1G). This study was performed on rats divided into two series: the first was reared in HG until 100 days and was submitted to normogravity until 115 to 220 postnatal days (HG-NG rats); the second was made up of age paired groups reared in normogravity (NG rats). The contractile, morphological, and phenotypical properties of soleus muscle were studied. Our results showed that the NG rats were characterized by coexpressions of slow and fast myosin, respectively, 76.5 and 23.5% at 115 days. During their postnatal maturation, the fast isoform was gradually replaced by slow myosin. At 220 days, the relative proportions were respectively 91.05% and 8.95%. From 115 to 220 days, the HG-NG rats expressed 100% of slow myosin isoform and they presented a slower contractile behavior compared with their age-matched groups; at 115 days, the whole muscle contraction time was increased by 35%, and by 15%, at 220 days. Our study underlined the importance of gravity in the muscular development and suggested the existence of critical periods in muscle phenotype installation.     

Growth of different types of muscle fibers of the soleus muscle during postnatal ontogeny in the rat

In order to study the development of the m. soleus muscle fibers during postnatal ontogenesis in the rat, methods for revealing ATPase activity of myosin at preincubation in acidic and alcaline medium and lactate dehydrogenase and succinate dehydrogenase activity have been used. The m. soleus undergoes three stages of development. The first stage--from birth of the animal up to the 7th day. During this time the muscle is homogenous. The second stage is characterized by appearance of certain histochemical differences in the muscle fibers. The muscle becomes mixed. During the whole period (in males from the 7th up to the 175th, and in females from the 7th up to the 60th-70th day) transferring of glycolytic fibers into oxidative-glycolytic ones with their successive transformation into oxidative fibers is observed. During the third stage (in males older than 175, and in females older than 60-70 days) the m. soleus converts from the mixed into the homogenous one consisting of oxidative fibers.     

Breathing pattern during and after exercise of different intensities

We recently observed rapid shallow breathing during recovery from maximal exercise in some normal subjects. We wondered whether this phenomenon is randomly related to level of exercise or is limited to recovery from very high levels of exercise. We monitored ventilation, tidal volume, and respiratory frequency in seven normal subjects during and after exercise. Each subject exercised on several occasions on separate days. At least two of the tests were maximal (i.e., subject terminated). In the other tests exercise was terminated by the experimenter at different fractions of the highest level attained by the subject. There was no systematic difference between breathing pattern during exercise and recovery in tests where final O2 consumption (VO2) was 45-92% of the subjects' highest VO2. By contrast 13 of 19 studies in which final VO2 was 92-100% of highest VO2 were followed by relative rapid shallow breathing. We conclude that rapid shallow breathing during recovery from exercise is a phenomenon that is limited to very high exercise levels. On consideration of the various mechanisms that may be entertained to explain this phenomenon, we believe that development of pulmonary congestion-interstitial edema at very high levels of exercise is the most consistent with our findings.     

Lactate dehydrogenase expression at the onset of altered loading in rat soleus muscle.

Both functional overload and hindlimb disuse induce significant energy-dependent remodeling of skeletal muscle. Lactate dehydrogenase (LDH), an important enzyme involved in anaerobic glycolysis, catalyzes the interconversion of lactate and pyruvate critical for meeting rapid high-energy demands. The purpose of this study was to determine rat soleus LDH-A and -B isoform expression, mRNA abundance, and enzymatic activity at the onset of increased or decreased loading in the rat soleus muscle. The soleus muscles from male Sprague-Dawley rats were functionally overloaded for up to 3 days by a modified synergist ablation or subjected to disuse by hindlimb suspension for 3 days. LDH mRNA concentration was determined by Northern blotting, LDH protein isoenzyme composition was determined by zymogram analysis, and LDH enzymatic activity was determined spectrophotometrically. LDH-A mRNA abundance increased by 372%, and LDH-B mRNA abundance decreased by 43 and 31% after 24 h and 3 days of functional overload, respectively, compared with that in control rats. LDH protein expression demonstrated a shift by decreasing LDH-B isoforms and increasing LDH-A isoforms. LDH-B activity decreased 80% after 3 days of functional overload. Additionally, LDH-A activity increased by 234% following 3 days of hindlimb suspension. However, neither LDH-A or LDH-B mRNA abundance was affected following 3 days of hindlimb suspension. In summary, the onset of altered loading induced a differential expression of LDH-A and -B in the rat soleus muscle, favoring rapid energy production. Long-term altered loading is associated with myofiber conversion; however, the rapid changes in LDH at the onset of altered loading may be involved in other physiological processes.     

Twitch potentiation and caffeine contractures in isolated rat soleus muscle

1. Electrically-evoked twitch and tetanic tension were measured in isolated rat soleus muscle after exposure to caffeine. 2. Between 0.01 and 2.5 mM caffeine twitch tension was potentiated, reaching a peak of 150% of Resting Tension at 0.5 mM. 3. Biphasic Tension development with relaxation was observed at 2.5 mM caffeine with maximal contractures (110% tetanic tension) occurring at 20 mM. 4. Creatine phosphate and ATP stores were maintained throughout the period of tension development and relaxation. 5. In contrast with amphibian muscle, the isolated soleus is very sensitive to low doses of caffeine and produces biphasic caffeine contractures which relax in the presence of caffeine.     

Changes in the rat skeletal muscle proteome induced by moderate-intensity endurance exercise

The adaptation of skeletal muscle to endurance exercise has not previously been investigated using proteomic techniques. Such work could improve our understanding and generate novel information regarding the effects of exercise. Plantaris muscles were investigated from rats exercised on treadmills at 70-75% peak oxygen uptake (V O(2)peak) for 30 min, 4 days per week for 5 weeks or sedentary controls. Analysis of 2-D gels matched 187 spots across control and exercised muscles and 80 proteins corresponding to 40 gene products were identified by MALDI-ToF MS. Exercise increased the animals' V O(2)peak by 14% and altered the expression of 15 spots consistent with a shift from glycolysis toward greater fatty-acid oxidation. The majority of differentially expressed gene products were present as multi-spot series of similar M(r) but different pI. Mitochondrial aconitase focused to 5 spots, 2 spots (pI 7.6 and 7.7) decreased (57%) whereas the pI 8.0 spot increased (51%) and was found to contain protein carbonyls. This adaptation may be related to exercise-induced oxidative stress and translocation of aconitase to mitochondrial DNA. In conclusion, proteomic techniques simultaneously demonstrated well-established effects, and identified novel changes not previously associated with the adaptation of muscle to exercise.     

Effect of exercise training at different intensities on fat metabolism of obese men.

The present study investigated the effect of exercise training at different intensities on fat oxidation in obese men. Twenty-four healthy male obese subjects were randomly divided in either a low- [40% maximal oxygen consumption (VO(2 max))] or high-intensity exercise training program (70% VO(2 max)) for 12 wk, or a non-exercising control group. Before and after the intervention, measurements of fat metabolism at rest and during exercise were performed by using indirect calorimetry, [U-(13)C]palmitate, and [1,2-(13)C]acetate. Furthermore, body composition and maximal aerobic capacity were measured. Total fat oxidation did not change at rest in any group. During exercise, after low-intensity exercise training, fat oxidation was increased by 40% (P < 0.05) because of an increased non-plasma fatty acid oxidation (P < 0.05). High-intensity exercise training did not affect total fat oxidation during exercise. Changes in fat oxidation were not significantly different among groups. It was concluded that low-intensity exercise training in obese subjects seemed to increase fat oxidation during exercise but not at rest. No effect of high-intensity exercise training on fat oxidation could be shown.     

Substrate metabolism during different exercise intensities in endurance-trained women.

We have studied eight endurance-trained women at rest and during exercise at 25, 65, and 85% of maximal oxygen uptake. The rate of appearance (R(a)) of free fatty acids (FFA) was determined by infusion of [(2)H(2)]palmitate, and fat oxidation rates were determined by indirect calorimetry. Glucose kinetics were assessed with [6,6-(2)H(2)]glucose. Glucose R(a) increased in relation to exercise intensity. In contrast, whereas FFA R(a) was significantly increased to the same extent in low- and moderate-intensity exercise, during high-intensity exercise, FFA R(a) was reduced compared with the other exercise values. Carbohydrate oxidation increased progressively with exercise intensity, whereas the highest rate of fat oxidation was during exercise at 65% of maximal oxygen uptake. After correction for differences in lean body mass, there were no differences between these results and previously reported data in endurance-trained men studied under the same conditions, except for slight differences in glucose metabolism during low-intensity exercise (Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, and Wolfe RR. Am J Physiol Endocrinol Metab 265: E380-E391, 1993). We conclude that the patterns of changes in substrate kinetics during moderate- and high-intensity exercise are similar in trained men and women.