Effects of exercise training on neuromuscular junction morphology and pre- to post-synaptic coupling in young and aged rats

The objective of this study was to determine whether pre- to post-synaptic coupling of the neuromuscular junction (NMJ) could be maintained in the face of significant morphological remodeling brought about by exercise training, and whether aging altered this capacity. Eighteen young adult (8 mo) and eighteen aged (24 mo) Fischer 344 rats were randomly assigned to either endurance trained (treadmill running) or untrained control conditions resulting in four groups (N=9/group). After the 10-week intervention rats were euthanized and hindlimb muscles were surgically removed, quickly frozen at approximate resting length and stored at -85°C. The plantaris and EDL muscles were selected for study as they have different functions (ankle extensor and ankle flexor, respectively) but both are similarly and overwhelmingly comprised of fast-twitch myofibers. NMJs were stained with immunofluorescent procedures and images were collected with confocal microscopy. Each variable of interest was analyzed with a 2-way ANOVA with main effects of age and endurance training; in all cases significance was set at P⩽0.05. Results showed that no main effects of aging were detected in NMJs of either the plantaris or the EDL. Similarly, endurance training failed to alter any synaptic parameters of EDL muscles. The same exercise stimulus in the plantaris however, resulted in significant pre- and post-synaptic remodeling, but without altering pre- to post-synaptic coupling of the NMJs. Myofiber profiles of the same plantaris and EDL muscles were also analyzed. Unlike NMJs, myofibers displayed significant age-related atrophy in both the plantaris and EDL muscles. Overall, these results confirm that despite significant training-induced reconfiguration of NMJs, pre- to post-synaptic coupling remains intact underscoring the importance of maintaining proper apposition of neurotransmitter release and binding sites so that effective nerve to muscle communication is assured.

Aging influences adaptations of the neuromuscular junction to endurance training

This investigation sought to determine if aging affected adaptations of the neuromuscular junction (NMJ) to exercise training. Twenty young adult (8 months) and 20 aged (24 months) rats were assigned to either a program of treadmill exercise, or sedentary conditions. Following the 10-week experimental period, rats were euthanized, and soleus and plantaris muscles were removed and frozen. Longitudinal sections of the muscles were fluorescently stained to visualize pre-synaptic nerve terminals and post-synaptic endplates on both slow- and fast-twitch fibers. Images were collected with confocal microscopy and quantified. Muscle cross-sections were histochemically stained to assess muscle fiber profiles (size and fiber type). Our analysis of NMJs revealed a high degree of specificity and sensitivity to aging, exercise training, and their interaction. In the soleus, slow-twitch NMJs demonstrated significant (P ≤ 0.05) training-induced adaptations in young adult, but not aged rats. In the fast-twitch NMJs of the soleus, aging, but not training, was associated with remodeling. In the plantaris, aging, but not training, remodeled the predominant fast-twitch NMJs, but only pre-synaptically. In contrast, the slow-twitch NMJs of the plantaris displayed morphologic adaptations to both aging and exercise in pre- and post-synaptic components. Muscle fiber profiles indicated that changes in NMJ size were unrelated to adaptations of their fibers. Our data show that aging interferes with the ability of NMJs to adapt to exercise training. Results also reveal complexity in the coordination of synaptic responses among different muscles, and different fiber types within muscles, in their adaptation to aging and exercise training.

Moderate aging does not modulate morphological responsiveness of the neuromuscular system to chronic overload in Fischer 344 rats

The purpose of this investigation was to determine the effect of aging on neuromuscular adaptations to chronic overload. Eight young adult (8 months old) and eight aged (22 months old) Fischer 344 rats underwent unilateral synergist ablation to overload the plantaris and soleus muscles of that hindlimb and to provide control muscles from the contralateral hindlimb. Cytofluorescent staining and confocal microscopy were used to quantify pre- and post-synaptic features of neuromuscular junctions (NMJs). Histochemical staining and light microscopy were used to assess adaptations of myofibers to chronic overload. Results demonstrate that NMJs of young adult and aged muscles did not undergo morphological remodeling as a result of 4 weeks of chronic overload. In contrast, myofibers of young and aged rats displayed significant (P<0.05), but similar hypertrophy ( approximately 18%) following that 4 week intervention. In both age groups, however, this hypertrophy was detected in the plantaris, but not the soleus. These data indicate that moderate aging (the equivalent of 65 years in human lifetime) does not modify the sensitivity of the neuromuscular system to chronic overload.

Increased and decreased activity elicits specific morphological adaptations of the neuromuscular junction

The objective of this investigation was to compare the effects of decreased vs. increased activity on the neuromuscular system. Twenty-four young adult (7 weeks old) Sprague-Dawley rats were randomly assigned to three treatment groups (N=8/group). Increased activity was achieved by treadmill running for up to 1 h/day. Decreased activity was induced by muscle unloading via the hindlimb suspension model. Control animals engaged in normal weight bearing and ambulatory activity. At the end of the 10 week intervention period, animals were killed and soleus muscles were removed, quickly frozen, and examined using cytofluorescent (neuromuscular junctions) and histochemical (myofibers) procedures. Pre-synaptic morphology was quantified by measuring nerve terminal branching, and post-synaptic assessment was conducted by staining acetylcholine receptors at the endplate. Myofiber profiles of solei were compiled by determining fiber size (cross-sectional area) and fiber type composition. Results show that exercise training significantly (P<or=0.05) enhanced nerve terminal branching without modifying endplate size. In contrast, muscle unloading failed to influence nerve terminal branching, but significantly reduced endplate dimensions. Likely, the diminution of endplate size among unloaded myofibers was caused by the dramatic loss of size (approximately 60%) observed among those fibers. Both increased and decreased activity evoked reductions in the ratio of endplate area to length of nerve terminal branches, thus altering the pre- to post-synaptic relationship of neuromuscular junctions. In conclusion, the present data demonstrate that both increased and decreased activity significantly remodels the neuromuscular junction, but the locus (pre- vs. post-synaptic) and nature (expansion vs. reduction) of that remodeling are specific to the form of the alteration in activity.

Recovery of neuromuscular junction morphology following 16 days of spaceflight

It has previously been established that spaceflight elicits alterations in the morphology of the neuromuscular system that includes expansion of the neuromuscular junction (NMJ) and myofiber atrophy. The purpose of this study was to determine the capacity of the neuromuscular system to recover from spaceflight-induced modifications upon return to normal gravity. Soleus muscles were obtained from rats participating in the 16-day Neurolab space shuttle mission at 1 day and 14 days after returning to Earth: solei were also taken at the same time points from ground-based control rats. Cytofluorescent techniques, coupled with confocal microscopy, were used to assess NMJ morphology. Histochemistry, in conjunction with phase contrast microscopy, was employed to examine myofiber size and type. Results indicate that 1 day after landing both pre- and postsynaptic stained areas of the NMJ were significantly (P < or = 0.05) larger in the spaceflight group than in controls. Moreover, significant myofiber atrophy was demonstrated in animals subjected to 0 gravity. By 14 days following return to the Earth, however, NMJ stained areas and muscle fiber size were no longer different from control values at that same interval. These results suggest that the neuromuscular system possesses a robust capacity to recover from spaceflight-induced perturbations upon return to normal gravitational influences.

A comparison of the effects of unloading in young adult and aged skeletal muscle

PURPOSE

The objective of this investigation was to determine whether morphological adaptations to unloading are different in young adult and aged skeletal muscle.

METHODS

Sixteen young adult (8-month) Fischer 344 rats were randomly assigned to either a control or hindlimb suspension (HS) group. Sixteen aged (22-month) rats were similarly assigned to either control or HS conditions. After 4 wk, animals were euthanized and soleus and EDL muscles were histochemically analyzed.

RESULTS

In controls, neither the soleus nor EDL displayed age-related differences in fiber size or composition. Unloading elicited fiber atrophy of the soleus in both age groups but to a greater extent (P < 0.05) in aged rats. Only in aged solei were HS-induced fiber type conversions (Type I --> II) detected. In the EDL, unloading caused atrophy only among the aged.

CONCLUSION

These data suggest that aged muscle experiences greater detriment as a result of unloading. This may have important consequences in the aged because they are more likely to be restricted to bed rest or limb immobilization due to falls and other afflictions.

Muscle recruitment patterns regulate physiological responses during exercise of the same intensity

On different days, 10 men performed 30-min sessions of cycling at 50-55% of their peak oxygen uptake (VO(2)); one at 40 rpm and another at 80 rpm. Rectal temperature, heart rate (HR), mean arterial pressure (MAP), plasma lactate, glucose, insulin, and cortisol were measured before exercise, during the 15th and 30th min of exercise, and at 5 and 10 min postexercise. Rating of perceived exertion (RPE) was assessed 15 and 30 min into exercise. Electromyography established cadence-specific different intensities of quadriceps activation during cycling. At minute 30 of exercise and 5 min postexercise, HR was significantly (P < 0.05) greater at 40 rpm than at 80 rpm. MAP remained elevated longer after the 40-rpm than after the 80-rpm bout. Similarly, exercise-induced increases in plasma lactate persisted longer after the 40-rpm bout. Cortisol levels were elevated only at 40 rpm. RPE was higher during the slower cadence. These data indicated that the more pronounced muscle activation pattern associated with pedaling at 40 rpm resulted in greater physiological and psychophysiological stress than that observed at 80 rpm even though VO(2) was the same.

Effects of resistance training on neuromuscular junction morphology

The aim of this study was to determine the impact of resistance exercise on neuromuscular junction (NMJ) architecture. Eighteen Sprague-Dawley rats either participated in a 7-week resistance training program or served as untrained controls. Following the experimental period, the NMJs of soleus muscles were visualized with immunofluorescent techniques, and muscle fibers were stained histochemically. Results indicate that resistance training significantly (P < 0.05) increased endplate perimeter length (15%) and area (16%), and significantly enhanced the dispersion of acetylcholine receptors within the endplate region. Pre- and post-synaptic modifications to resistance exercise were well-coupled. No significant alterations in muscle fiber size or fiber type were detected. The data presented here indicate that the stimulus of resistance training was sufficiently potent to remodel NMJ structure, and that this effect cannot be attributed to muscle fiber hypertrophy or fiber type conversion.

Neuromuscular disturbance outlasts other symptoms of exercise-induced muscle damage

This study examined the biochemical, immunological, functional, and neuromuscular responses associated with exercise-induced muscle damage in the quadriceps of untrained men. Muscle damage and soreness was elicited with maximal concentric/eccentric muscle actions at 0.53 rads s(-1). Significant (P<0.05) soreness was evident 1, 2, and 3 days following muscle insult, while plasma creatine kinase, a marker of muscle damage, was elevated 3 and 5 days post-insult. Plasma interleukin-Ibeta was significantly increased within 5 min, and remained elevated 1, 2, 5, and 7 days post-insult. Maximal isometric quadriceps function was impaired (P<0. 05) for 5 days following muscle challenge. Maximal isokinetic performance at 1.09 rads s(-1) was diminished (P<0.05) for 2 days post-insult; no significant decrements at 3.14 rads s(-1) were noted. Average electrical activation (iEMG) of the quadriceps was unaltered, but iEMG activity of the rectus femoris - where soreness was focused - was significantly increased. Neuromuscular efficiency (torque/iEMG) was compromised throughout the 10-day post-insult period investigated. While other symptoms of exercise-induced muscle damage dissipate within 7 days, neuromuscular perturbation persists for at least 10 days.

Exercise stimulates neovascularization in occluded muscle without affecting bFGF content

PURPOSE

This study was conducted to determine whether exercise-induced improvements in capillarity in muscle with peripheral arterial insufficiency (PAI) was accompanied by endothelial cell mitosis, and whether that response could be explained by changes in the expression of basic fibroblast growth factor (bFGF), a known mitogen.

METHODS

After bilateral ligation of femoral arteries, Sprague-Dawley rats either remained sedentary or participated in a treadmill running protocol. Running time to exhaustion at each session was recorded. On days 1, 2, 3, 5, and 7 of the experimental period, trained-ligated and sedentary-ligated animals were euthanized, and segments of muscle from the gastrocnemius were obtained. Capillarity was determined with histochemistry, and endothelial cell mitotic activity (cell proliferation) was assayed via nuclear uptake of 5'-bromo-2'-deoxyuridine (BrdU), an analog of thymidine. Content of endogenous bFGF was assessed with immunoblotting techniques.

RESULTS

Exercise training resulted in augmented function of PAI affected muscle as evidenced by a nearly threefold increase in running time until exhaustion. Trained-ligated muscle demonstrated significantly (P < 0.05) greater capillarity than sedentary-ligated muscle. Vascular remodeling elicited by exercise included the formation of new capillaries (angiogenesis) as indicated by enhanced endothelial cell proliferation at days 3, 5, and 7 of the study. However, exercise training did not alter the content of bFGF in occluded muscle.

CONCLUSION

In muscle with PAI, exercise training improved functional capacity and capillarity. Angiogenesis was confirmed by the increased mitotic activity of endothelial cells. However, the content of bFGF, a potent angiogenic factor, was not altered. Thus, exercise-induced angiogenesis in PAI affected muscle is not dependent upon increased expression of endogenous bFGF.

Biorhythmic influences on functional capacity of human muscle and physiological responses

Previously, this laboratory has demonstrated that exhaustive aerobic exercise performance is not subject to significant chronobiological variation between 0800 and 2000 h, but certain physiological responses to maximal aerobic effort do fluctuate significantly within that time frame.

PURPOSE

The purpose of the present investigation was to determine whether muscle performance, and selected physiological responses to resistance exercise, was significantly influenced by time of day effects.

METHODS

Ten healthy, but untrained, men (21.1+/-0.6 yr, mean +/- SE) volunteered to participate in the study. In a balanced and randomized study design, each subject performed resistance exercise protocols on an isokinetic dynamometer with maximal effort at 0800 h, 1200 h, 1600 h, and 2000 h. Selected physiological and hormonal data were recorded before and immediately following the exercise stimulus.

RESULTS

The data demonstrated significant chronobiological oscillation in peak torque, average power, maximal work in a single repetition, and total work per set. Interestingly, this oscillation was manifested only at the fastest velocities of limb movement utilized. Pre- and postexercise rectal temperature exhibited significant time of day fluctuation, as did postexercise blood pressure. Plasma levels of testosterone and cortisol also displayed significant biorhythmicity under both pre- and postexercise conditions. However, exercise-induced responses (pre- to postexercise differences) of those steroid hormones did not exhibit significant biorythmic variation. Although plasma concentrations of both testosterone and cortisol were highest at 0800 h, testosterone to cortisol ratios were greatest at 2000 h.

CONCLUSIONS

In summary, these data suggest that maximal muscle performance does vary within the segment of the day when exercise typically occurs (0800-2000 h) but that this variation is specific to speed of movement.

Chronobiological effects on exercise performance and selected physiological responses

Previous studies investigating the impact of circadian rhythms on physiological variables during exercise have yielded conflicting results. The purpose of the present investigation was to examine maximal aerobic exercise performance, as well as the physiological and psychophysiological responses to exercise, at four different intervals (0800 hours, 1200 hours, 1600 hours, and 2000 hours) within the segment of the 24-h day in which strenuous physical activity is typically performed. Ten physically fit, but untrained, male university students served as subjects. The results revealed that exercise performance was unaffected by chronobiological effects. Similarly, oxygen uptake, minute ventilation and heart rate showed no time of day influences under pre-, submaximal, and maximal exercise conditions. Ratings of perceived exertion were unaffected by time of day effects during submaximal and maximal exercise. In contrast, rectal temperature exhibited a significant chronobiological rhythm under all three conditions. Under pre- and submaximal exercise conditions, significant time of day effects were noted for respiratory exchange ratio, while a significant rhythmicity of blood pressure was evident during maximal exercise. However, none of these physiological variables exhibited significant differential responses (percent change from pre-exercise values) to the exercise stimulus at any of the four time points selected for study. Conversely, resting plasma lactate levels and lactate responses to maximal exercise were found to be significantly sensitive to chronobiological influences. Absolute post-exercise plasma norepinephrine values, and norepinephrine responses to exercise (percent change from pre-exercise values), also fluctuated significantly among the time points studied. In summary, these data suggest that aerobic exercise performance does not vary during the time frame within which exercise is normally conducted, despite the fact that some important physiological responses to exercise do fluctuate within that time period.

Exercise training of moderate intensity does not abate the effects of denervation on muscle morphology

Denervation elicits profound alterations in the morphometry of skeletal muscle. These alterations include changes in fiber type composition as well as reductions in fiber size. There is evidence that the increased mechanical load placed upon muscle via rhythmic stretching attenuates denervation induced alterations in muscle morphology. The purpose of the present study was to determine whether the mechanical stimuli associated with exercise training, i.e. rhythmic stretching and mechanical loading, would effectively moderate the changes in muscle morphometry observed following denervation. Unilateral denervation of the soleus muscle of eight male Sprague-Dawley rats was performed under aseptic conditions. The animals were then randomly assigned to two groups: sedentary controls and exercise trained. The exercise training protocol featured treadmill running five days per week for six weeks. At the conclusion of the experimental period, animals from both groups were sacrificed and soleus muscles were histochemically analyzed for muscle morphometry. Results demonstrated that denervation caused marked alterations in fiber type profile and in fiber cross-sectional areas. Interestingly, the degree of denervation induced atrophy appeared to be fiber type specific. However, the data presented here indicate that in denervated soleus muscles there were no significant differences in fiber type composition or fiber size between the sedentary and exercise trained groups. Hence, it appears that the mechanical stimuli provided by treadmill running of moderate intensity and duration are not sufficiently potent to ameliorate muscle morphometric responses to denervation.

Basic fibroblast growth factor increases collateral blood flow in rats with femoral arterial ligation

The potential for exogenous infusion of basic fibroblast growth factor (bFGF) to increase collateral blood flow to dependent tissue was quantified in adult male rats with peripheral arterial insufficiency. Occlusion of the femoral artery at a proximal site did not infringe on resting blood flow to the distal hindlimb muscle, but did remove the blood flow reserve. Blood flow to the hindlimb muscles was measured with radiolabeled microspheres using an isolated hindlimb preparation perfused in the descending aorta (Krebs-Henseleit bicarbonate, 5% albumin medium containing red blood cells [40% hematocrit]) at 100 mm Hg. Calf muscle blood flow changed modestly (approximately equal to 50%) with infusion of only the carrier (heparin/saline), increased markedly over the first 2 weeks of bFGF infusion (1 microgram/d into the femoral artery), but did not change further with infusion for 4 weeks. Waiting 2 weeks after 1 week of bFGF infusion did not further increase the intermediate improvement in blood flow. The improved collateral blood flow and increased muscle capillary density likely contributed to the enhanced muscle performance observed during nerve stimulation in situ. X-ray films of arterial casts identified an expansion of upper thigh vessels that likely served as collaterals. In animals with peripheral arterial insufficiency, short-term exogenous infusion of bFGF is effective at inducing vascular expansion that is sufficient to improve the flow reserve of dependent distal tissue and enhance muscle function. This raises the expectation that a similar response in patients with peripheral arterial insufficiency would significantly improve morbidity, including the symptoms of intermittent claudication.

The effects of different treadmill running programs on the muscle morphology of adult rats

The biochemical adaptations of different muscle fiber types to endurance training of various intensities and durations have previously been investigated. The objective of this study was to determine the effects of two different endurance training programs on muscle fiber morphology. Twenty-four male Sprague-Dawley rats were randomly assigned to three groups: high intensity/low duration endurance trained (HILD), low intensity/high duration endurance trained (LIHD), or untrained (controls). Following the twelve week training period, muscle fibers of the gastrocnemius muscle of the rats were histochemically classified as type I, type IIa, or type IIb following myosin ATPase staining with pre-incubation at pH 4.6. Muscle fiber type distribution and cross-sectional areas were then determined. Neither HILD nor LIHD rats demonstrated significant differences in fiber type distribution compared to controls. When all fiber types were pooled together and analyzed, there were no differences between the three groups with respect to fiber size. However, when the three fiber types were analyzed individually, HILD animals demonstrated a significant reduction in the size of type IIa fibers while LIHD rats experienced a significant diminution in the size of type I and type IIb fibers. Thus, the morphological adaptations of the muscle fibers in the HILD and LIHD groups were fiber type specific.

Compatibility of high-intensity strength and endurance training on hormonal and skeletal muscle adaptations

Thirty-five healthy men were matched and randomly assigned to one of four training groups that performed high-intensity strength and endurance training (C; n = 9), upper body only high-intensity strength and endurance training (UC; n = 9), high-intensity endurance training (E; n = 8), or high-intensity strength training (ST; n = 9). The C and ST groups significantly increased one-repetition maximum strength for all exercises (P < 0.05). Only the C, UC, and E groups demonstrated significant increases in treadmill maximal oxygen consumption. The ST group showed significant increases in power output. Hormonal responses to treadmill exercise demonstrated a differential response to the different training programs, indicating that the underlying physiological milieu differed with the training program. Significant changes in muscle fiber areas were as follows: types I, IIa, and IIc increased in the ST group; types I and IIc decreased in the E group; type IIa increased in the C group; and there were no changes in the UC group. Significant shifts in percentage from type IIb to type IIa were observed in all training groups, with the greatest shift in the groups in which resistance trained the thigh musculature. This investigation indicates that the combination of strength and endurance training results in an attenuation of the performance improvements and physiological adaptations typical of single-mode training.

Dietary supplementation and improved anaerobic performance

In the present study, the effects of an increased daily dose of a dietary supplement (ATP-E, 0.2 g.kg-1.day-1) on Wingate test performance were examined in 12 men (21 +/- 1.6 years) prior to and following 14 days of supplement and placebo ingestion. A double-blind and counterbalanced design was used. Results revealed higher (p < .007) preexercise blood ATP (95.4 +/- 10.5 mumol.dl-1) for the entire group following 14 days of ATP-E ingestion compared to placebo measures (87.6 +/- 10.9 mumol.dl-1). Mean power (667 +/- 73 W) was higher (p < .008) after 14 days of ATP-E ingestion versus placebo (619 +/- 67 W). Peak plasma lactate was lower (p < .07) after 14 days of ATP-E ingestion (14.9 +/- 2.8 mmol.L-1) compared to placebo (16.3 +/- 1.6 mmol.L-1). These data suggested that the improvement in 30-s Wingate test performance in this group may be related to the increased dose of ATP-E.

Endurance and resistance exercise induce muscle fiber type specific responses in androgen binding capacity

This study examined the effects of different exercise training programs on androgen receptor content and receptor affinity to dihydrotestosterone in fast glycolytic (FG) and slow oxidative (SO) skeletal muscle fibers in rats. Twenty-four male Sprague-Dawley rats were equally divided into three groups: control, endurance exercise trained and resistance exercise trained. After the exercise programs were completed, the extensor digitorum longus (EDL), predominantly a FG muscle, and the soleus, predominantly a SO muscle, were isolated, weighted and both androgen receptor content and affinity to dihydrotestosterone were determined. Resistance training evoked a significant (P < 0.05) hypertrophic response in the soleus but not the EDL. Endurance training was not associated with any significant hypertrophy in either the soleus or the EDL. Neither the endurance nor the resistance training program resulted in changes in androgen receptor affinity to dihydrotestosterone. However, alterations in androgen receptor content were noted. The endurance training program resulted in a significant increase in androgen receptor content in the soleus, but no significant difference in the EDL. The resistance training program elicited a significant decrease in androgen receptor content in the soleus, and a significant increase in the EDL. These results indicate that different exercise stimuli induce changes in androgen receptor content that are specific to skeletal muscle fiber type.

The neuromuscular junction. Muscle fibre type differences, plasticity and adaptability to increased and decreased activity

The neuromuscular junction (NMJ) of adult mammalian muscle is the site of the transduction of electrical stimuli, generated by the nervous system, to the underlying muscle fibres, resulting in muscle action. It has been demonstrated that, in some ways, the morphology of the NMJ is specific to muscle fibre type. It is also known that while the structure of the NMJ generally remains stable in young, healthy adults, a subtle form of remodelling continuously occurs at this synapse. The morphology and physiology of the NMJ have been shown to adapt to both increased, and decreased use. Indeed, morphological changes of the NMJ are associated with functional alterations in neuromuscular transmission. Increased activity of the myoneural synapse results in adaptations that enhance neuromuscular transmission and, thus, muscle performance. Similarly to increased usage, decreased neuromuscular activity results in structural alterations of the NMJ. However, unlike those responses observed with enhanced activity, decreased recruitment of the myoneural synapse can impair neuromuscular transmission and muscle performance. Thus, the NMJ demonstrates both anatomical and physiological adaptations following substantial changes in its pattern of activity. These NMJ adaptations can affect the functional capacity of skeletal muscle in vivo.

Perceived exertion during hypobaric hypoxia in low- and moderate-altitude natives

Ratings of perceived exertion (RPE) were examined in six low- (LAN) and eight moderate- (MAN) altitude natives during exercise at their residence (home) altitude (366 m and 2,200 m, respectively) and 1-4 wk later following 2-d decompression to 4,270 m (447 mm Hg). Cardiorespiratory, plasma lactate, and differentiated RPE measures were obtained at exercise intensities representing 35, 55, 75, 85, and 100% VO2peak. In general, cardiorespiratory and plasma lactate values were similar in LAN and MAN at their residence altitudes and during hypobaric hypoxia. However, the decrease in VCO2 was greater (P < 0.05) in LAN than MAN. At their residence altitudes, both LAN and MAN reported local RPE values that were greater (P < 0.05) than central ratings at the moderate to high exercise intensities. At 447 mm Hg, central and local RPE were similar in LAN. However, there was a significant correlation between acute mountain sickness (AMS) symptoms and central RPE (r = 0.875) across the five exercise intensities in LAN. The differences between the central and local RPE noted in MAN during their residence testing also persisted at 447 mm Hg. Thus, differentiated ratings of perceived exertion were similar in MAN at their residence altitude and at 4,270 m, but not in LAN subjects. Several factors, including AMS, may have contributed to this group difference in the RPE response.

The effects of exercise training of different intensities on neuromuscular junction morphology

Little is known about the effects of exercise training on neuromuscular junction morphology in skeletal muscle. The objectives of this investigation were: 1) to determine if exercise training would elicit changes in neuromuscular junction morphology, 2) to determine if exercise training of different intensities would evoke specific changes in neuromuscular junction morphology, and 3) to determine whether changes in neuromuscular junction structure occur independently of changes in muscle fibre type and size. Twenty-four age and size matched male Sprague-Dawley rats were randomly assigned to three groups: high-intensity trained (HIT), low-intensity trained (LIT), or untrained. Neuromuscular junction morphology of the soleus muscle was determined via immunofluorescent staining. Presynaptic acetylcholine vesicles were visualized with SV-2 antibody in conjunction with fluorescein isothiocyanate labelled secondary secondary antibody. Postsynaptic acetylcholine receptors were identified with rhodamine labelled alpha-bungarotoxin. Laser scanning microscopy was used to produce images of synapses, which were used to quantitate the following: total area of SV-2 and alpha-bungarotoxin staining, density of acetylcholine vesicles and receptors, structural complexity, and synaptic coupling. To visualize nerve terminal branching, a smaller number of neuromuscular junctions were stained with C-2 antibody, which reacts with a neurofilament epitope, in conjunction with fluorescein isothiocyanate labelled secondary antibody. Total length of branching, number of branches, average length of branches, and ratio of secondary to primary branches per neuromuscular junction were determined. Citrate synthase activity, fibre type composition and fibre cross-sectional areas of the soleus muscle were assessed to determine the presence of a training effect in that muscle. Results indicate that training did induce hypertrophy of the neuromuscular junction that was independent of muscle hypertrophy. Although the HIT and LIT groups exhibited similar hypertrophic responses of the neuromuscular junction, the HIT group displayed more dispersed synapses than the LIT group. Neither exercise training program, however, resulted in altered densities of acetylcholine vesicles or receptors, nor did training significantly change synaptic coupling. Nerve terminal branching was also affected by exercise training. Neuromuscular junctions from the HIT group demonstrated a greater total length of branching, average length per branch, and number of finer, or secondary, branches than those of the LIT group.

Oxygen consumption following exercise of moderate intensity and duration

To study the effects of exercise intensity and duration on excess postexercise oxygen consumption (EPOC), 8 men [age = 27.6 (SD 3.8) years, VO2max = 46.1 (SD 8.5) ml min-1 kg-1] performed four randomly assigned cycle-ergometer tests (20 min at 60% VO2max, 40 min at 60% VO2max, 20 min at 70% VO2max, and 40 min at 70% VO2max). O2 uptake, heart rate and rectal temperature were measured before, during, and for 1 h following the exercise tests. Blood for plasma lactate measurements was obtained via cannulae before, and at selected times, during and following exercise. VO2 rapidly declined to preexercise levels following each of the four testing sessions, and there were no differences in EPOC between the sessions. Blood lactate and rectal temperature increased (P < 0.05) with exercise, but had returned to preexercise levels by 40 min of recovery. The results indicate that VO2 returned to resting levels within 40 min after the end of exercise, regardless of the intensity (60% and 70% VO2max) or duration (20 min and 40 min) of the exercise, in men with a moderate aerobic fitness level.

Anaerobic power responses to amino acid nutritional supplementation

To examine the effect of a nutritional supplement (ATP-E) on high intensity exercise performance, 23 physically active males volunteered to perform six Wingate Anaerobic Power tests. Tests were performed prior to and at 14 and 21 days during ATP-E or placebo ingestion. The experiment followed a double-blind and random-order design. Twelve subjects (responders, R) showed an increase in preexercise blood ATP on Day 14 of ATP-E ingestion compared to control measures. The remaining 11 subjects (nonresponders, NR) had no change in preexercise blood ATP. Peak power and mean power were unchanged for both R and NR subjects across the exercise tests, but R experienced a decrease (p < 0.05) in immediate post-exercise plasma lactate on Day 14 of ATP-E testing compared to their control measures. NR had no change in peak plasma lactate at any time during the study. The results suggest that short-term high intensity exercise performance was maintained in R with less reliance on anaerobic metabolism, and that response was evident following 14 days of ATP-E ingestion.

Physiological adaptations to resistance exercise. Implications for athletic conditioning

Resistance training results in a wide spectrum of adaptations in various physiological systems. Increases in muscle size and strength, changes in body composition, neuroendocrine function and cardiovascular responses have been observed following resistance training. Additionally, resistance training may be an effective means by which the incidence of sports injuries can be reduced. The physiological alterations induced by resistance training appear to be specific to the number of sets and repetitions and exercises performed. Thus, special attention is required when developing the exercise prescription for resistance training.