Enzyme activities of FT and ST muscle fibers in heavy-resistance trained athletes

Tissue samples were obtained from the vastus lateralis muscle of elite olympic weight and power lifters (OL/PL, n = 6), bodybuilders (BB, n = 7), and sedentary men (n = 7). Enzyme activities of citrate synthase (CS), lactate dehydrogenase (LD), 3-OH-acyl-CoA-dehydrogenase (HAD), and myokinase (MK) were assayed on freeze-dried dissected pools of slow-twitch (ST) and fast-twitch (FT) fiber fragments by fluorometric means. Histochemical analyses were carried out to assess fiber type composition and fiber area. CS and HAD activities were lower (P less than 0.05), and LD and MK were higher (P less than 0.05) in FT than ST fibers in the entire subject pool (n = 20). CS of FT fibers and HAD of ST fibers were lower in athletes (P less than 0.05-0.01) compared with nonathletes, whereas LD of both fiber types was higher (P less than 0.05-0.001) in athletes. CS activity of ST fibers and MK activity of FT fibers were higher (P less than 0.05) in BB compared with OL/PL. FT and ST fiber area was greater (P less than 0.05) in athletes than in nonathletes. BB displayed greater (P less than 0.05) fiber size than OL/PL. FT/ST area was greater (P less than 0.05) in OL/PL than BB. It is suggested that long-term heavy-resistance training results in specific metabolic adaptations of FT and ST fiber types. These changes appear to be influenced by the type of resistance training.     

Development of rat muscle during short- and long-term hindlimb suspension

Histochemical and contractile properties of developing rat soleus (Sol) and plantaris (P) muscles were studied after hindlimb suspension to determine the effects of reduced activity levels on muscle development. Suspension (S) began at age 18 days and lasted for 14, 28, and 206 days, and results were compared with age-matched controls. Body weights were normal until 14 days and Sol growth was inhibited more than P, weighing 38 and 47% of controls at 46 and 224 days compared with 68 and 59% in P. The Sol did not develop into a slow-twitch (ST) muscle as evidenced by faster times to peak tension and half-relaxation times, faster times to develop 50% of maximum tetanic tension (Po) and a mean of 33% fewer ST fibers. Twitch tension and Po were lower in S-Sol and S-P, but force/cross-sectional area was unchanged. Fiber areas were smaller, but no structural changes characteristic of disuse atrophy were found. Fiber type populations were unchanged in P, and contractile properties were only minimally affected, demonstrating the greater importance of activity for ST muscles during development.     

Differential effects of mild therapeutic exercise during a period of inactivity on power generation in soleus type I single fibers with age

The purpose of this study was to investigate the effects of mild therapeutic exercise (treadmill) in preventing the inactivity-induced alterations in contractile properties (e.g., power, force, and velocity) of type I soleus single fibers in three different age groups. Young adult (5- to 12-mo-old), middle-aged (24- to 31-mo-old), and old (32- to 40-mo-old) F344BNF1 rats were randomly assigned to three experimental groups: weight-bearing control (CON), non-weight bearing (NWB), and NWB with exercise (NWBX). NWB rats were hindlimb suspended for 2 wk, representing inactivity. The NWBX rats were hindlimb suspended for 2 wk and received therapeutic exercise on a treadmill four times a day for 15 min each. Peak power and isometric maximal force were reduced following hindlimb suspension (HS) in all three age groups. HS decreased fiber diameter in young adult and old rats (-21 and -12%, respectively). Specific tension (isometric maximal force/cross-sectional area) was significantly reduced in both the middle-aged (-36%) and old (-23%) rats. The effects of the mild therapeutic exercise program on fiber diameter and contractile properties were age specific. Mild treadmill therapeutic exercise attenuated the HS-induced reduction in fiber diameter (+17%, 93% level of CON group) and peak power (μN·fiber length·s(-1)) (+46%, 63% level of CON group) in young adult rats. In the middle-aged animals, this exercise protocol improved peak power (+60%, 100% level of CON group) and normalized power (kN·m(-2)·fiber length·s(-1)) (+45%, 108% level of CON group). Interestingly, treadmill exercise resulted in a further reduction in shortening velocity (-42%, 67% level of CON group) and specific tension (-29%, 55% level of CON group) in the old animals. These results suggest that mild treadmill exercise is beneficial in attenuating and preventing inactivity-induced decline in peak power of type I soleus single fibers in young adult and middle-aged animals, respectively. However, this exercise program does not prevent the HS-induced decline in muscle function in the old animals.     

Effect of hindlimb suspension on the functional properties of slow and fast soleus fibers from three strains of mice.

Cross-sectional area (CSA), peak Ca2+-activated force (Po), fiber specific force (Po/CSA), and unloaded shortening velocity (Vo) were measured in slow-twitch [containing type I myosin heavy chain (MHC)] and fast-twitch (containing type II MHC) chemically skinned soleus muscle fiber segments obtained from three strains of weight-bearing and 7-day hindlimb-suspended (HS) mice. HS reduced soleus slow MHC content (from approximately 50 to approximately 33%) in CBA/J and ICR strains without affecting slow MHC content in C57BL/6 mice ( approximately 20% of total MHC). Two-way ANOVA revealed HS-induced reductions in CSA, Po, and Po/CSA of slow and fast fibers from all strains. Fiber Vo was elevated post-HS, but not consistently across strains. No MHC x HS treatment interactions were observed for any variable for C57BL/6 and CBA/J mice, and the two significant interactions found for the ICR strain (CSA, Po) appeared related to inherent pre-HS differences in slow vs. fast fiber CSA. In the mouse HS models studied here, fiber atrophy and contractile dysfunction were partially dependent on animal strain and generally independent of fiber MHC isoform content.     

Recovery in skeletal muscle contractile function after prolonged hindlimb immobilization

Contractile properties of slow-twitch soleus (SOL), fast-twitch extensor digitorum longus (EDL), and fast-twitch superficial region of the vastus lateralis were determined in vitro (22 degrees C) in rats remobilized after prolonged (3 mo) hindlimb immobilization (IM). For all muscles the muscle-to-body weight ratio was significantly depressed by IM, and the ratios failed to completely recover even after 90 days. The contractile properties of the fast-twitch muscles were less affected by IM than the slow-twitch SOL. The IM shortened the SOL isometric twitch duration due to a reduced contraction and half-relaxation time. These parameters returned to control levels by the 14th day of recovery. Peak tetanic tension (Po, g/cm2) declined with IM by 46% in the SOL but showed no significant change in the fast-twitch muscles. After IM the SOL Po (g/cm2) recovered to control values by 28 days. The recovery of Po in absolute units (g) was considerably slower and did not return to control levels until 60 (SOL) to 90 (EDL) days. The maximum shortening velocity was not altered by IM in any of the muscles studied. These results demonstrate that both fast- and slow-twitch skeletal muscles possess the ability to completely recover normal contractile function following prolonged periods of hindlimb IM.     

Adaptation of rat lateral gastrocnemius muscle motor units during hindlimb unloading

Contractile and fatigue-resistance properties of 71 lateral gastrocnemius muscle (LG) motor units (MU) following 14 days of hindlimb unloading (HU) were compared to those of 60 LG MU from control rats. The MU properties were assessed from isolated and stimulated individual motor axons. The MU were classified using standard criteria (shape of unfused tetani and fatigue resistance). The HU did not affect LG MU composition, but diminished the maximal tetanic tension (Po) of all MU types: P0 was significantly reduced by about 40% for the slow and fast-resistant MU, and by 18% for the fast-fatigable ones. The speed-related properties of fast-resistant MU became more similar to those of slower MU. The fatigue properties of MU were evaluated during a 5-min exercise test, using two fatigue indexes, FI2 and FI5, which expressed the relative capacity of MU to generate tension after 2 and 5 min, respectively. Results showed that 14 days of HU did not change the fatigue sensitivity of the LG MU. However, when F15 was compared to FI2, a greater decrease was observed after HU than in control conditions for the fast-resistant and fast-intermediate MU. It was concluded that a prolonged fatigue test may show changes in metabolic properties of muscle fibres during 14 days of HU. Specific adaptations of LG MU as well as comparisons with those of the soleus muscle under the same conditions are discussed.     

Interaction of compensatory overload and hindlimb suspension on myosin isoform expression

The purpose of this study was to investigate the role of chronic weight-bearing activity as the primary inducer of compensatory muscle growth and changes in myosin isoform expression in rodent fast-twitch plantaris muscle. Thus, female rats were subjected to the independent and simultaneous exposure of functional overload (induced via synergist removal) and hindlimb unweighting (suspension) for 6 wk. Groups (n = 7/group) consisted of normal-control (NC); overload (OV); normal-suspension (N-SUS); and overload-suspension (OV-SUS). Body weight of both suspension groups was significantly less than both the NC and OV groups (P less than 0.001). Compared with the NC group, normalized plantaris weight (mg/g body wt) of both the OV and OV-SUS groups was greater, whereas that of the N-SUS was lower (P less than 0.001). However, normalized plantaris weight was greater in OV compared with OV-SUS by 35% (P less than 0.001). Myofibril protein content (mg/g) and Ca2+-regulated myofibril adenosinetriphosphatase (ATPase) specific activity were similar for all groups except that ATPase was lower in the OV group compared with the other groups (P less than 0.05). Native myosin isoform analysis revealed a significant increase in the expression of slow and intermediate myosin and the repression of fast myosin 1 (Fm1) in OV compared with NC. This shift in expression was not as pronounced in the OV-SUS group. Interestingly, only traces of slow myosin were observed in the N-SUS group compared with the other groups. These results suggest that weight bearing is an essential component of the overload model for inducing significant increases in both muscle mass and slow myosin isoform expression. Second, lack of weight bearing, while not markedly affecting fast myosins, appears to repress the expression of slow myosin.     

Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.

The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.     

Thermal adaptations in lizard muscle function

Summary This study was undertaken to investigate thermal adaptations in muscle contractile properties in closely-related lizards with different preferred body temperatures (PBT). The species examined all belong to theSphenomorphus group of Australian skinks (Scincidae: Lygosominae). Preferred body temperatures are conservative at the generic level as follows:Ctenotus, 35°C;Sphenomorphus, 30°C;Eremiascincus, 25°C. Contractile properties of the fast glycolytic portion of the iliofibularis muscle were measured. Translational adaptations are evident in several isometric factors, including tetanic tension (Po), twitch tension (Pt), twitch time to peak tension (TPT), and twitch half-relaxation time (1/2 RT). Capacity adaptations are not evident in rates of tetanic tension development (dPo/dt) or in maximal velocities of isotonic shortening (V _max). Rotational adaptations are not evident in any contractile properties. Thermal limits on upper response temperatures are about 5°C warmer inCtenotus than in the more cryophilic species, indicative of resistance adaptation in muscle performance. Despite these adaptive shifts, there is little indication that muscle functional capacities are optimized or equalized at PBT in these lizards.Abbreviations FG fast glycolytic - IF iliofibularis muscle - PBT preferred body temperature - Po tetanic tension - Pt twitch tension - 1/2RT twitch half relaxation time - TPT twitch time to peak tension     

兔膈肌力学模式对慢性电刺激的适应性改变和细胞外Ca …

目的：研究兔膈肌肌条力学对不同频率慢性电刺激（ＣＥＳ）的适应性变化特征和细胞外Ｃａ＾２＋变化夺其力学特征的影响。方法：测定正常对照组和ＣＥＳ组的颤搐收缩张力（Ｐｔ）、峰值张力时间（ＴＰＴ）、１／２松弛时间（１／２ＲＴ）、强直颤搐收缩张力（Ｐｏ）、疲劳指数（ＦＩ）和疲劳恢复指数（ＦＲＩ）;观察在无Ｃａ＾２＋Ｈａｎｋ’ｓ液和标准Ｈａｎｋ’ｓ液时肌条收缩张力消失和恢复的时间差异。结果：①同对照组作比较,     

Correlation between Ribosome Biogenesis and the Magnitude of Hypertrophy in Overloaded Skeletal Muscle

External loads applied to skeletal muscle cause increases in the protein translation rate, which leads to muscle hypertrophy. Although some studies have demonstrated that increases in the capacity and efficiency of translation are involved in this process, it remains unclear how these two factors are related to the magnitude of muscle hypertrophy. The present study aimed to clarify the roles played by the capacity and efficiency of translation in muscle hypertrophy. We used an improved synergist ablation in which the magnitude of compensatory hypertrophy could be controlled by partial removal of synergist muscles. Male rats were assigned to four groups in which the plantaris muscle was unilaterally subjected to weak (WK), moderate (MO), middle (MI), and strong (ST) overloading by four types of synergist ablation. Fourteen days after surgery, the weight of the plantaris muscle per body weight increased by 8%, 22%, 32% and 45%, in the WK, MO, MI and ST groups, respectively. Five days after surgery, 18+28S rRNA content (an indicator of translational capacity) increased with increasing overload, with increases of 1.8-fold (MO), 2.2-fold (MI), and 2.5-fold (ST), respectively, relative to non-overloaded muscle (NL) in the WK group. rRNA content showed a strong correlation with relative muscle weight measured 14 days after surgery (r = 0.98). The phosphorylated form of p70S6K (a positive regulator of translational efficiency) showed a marked increase in the MO group, but no further increase was observed with further increase in overload (increases of 22.6-fold (MO), 17.4-fold (MI), and 18.2-fold (ST), respectively, relative to NL in the WK group). These results indicate that increases in ribosome biogenesis at the early phase of overloading are strongly dependent on the amount of overloading, and may play an important role in increasing the translational capacity for further gain of muscular size.     

Lack of skeletal muscle hypertrophy in very aged male Fischer 344 x Brown Norway rats.

To examine the effect of extreme old age on muscle plasticity, 6- (adult) and 36-mo-old (old) male Fischer 344 x Brown Norway hybrid rats underwent bilateral surgical ablation of the gastrocnemius muscle to functionally overload (OV) the fast-twitch plantaris muscle for 8 wk. Plantaris muscle wet weight, muscle cross-sectional area (CSA), and average fiber CSA decreased by 44, 42, and 40%, respectively, in old compared with adult rats, and peak isometric tetanic tension decreased by 83%. Compared with muscles in age-matched controls, plantaris muscle mass increased by 53% and type I, IIA, and IIX/IIB CSA increased by 91, 76, and 103%, respectively, in adult-OV rats, but neither wet mass nor fiber CSA increased in old-OV rats. OV decreased type I, IIA, and IIX/IIB mean fiber CSA by 31, 35, and 30%, respectively, in old-OV rats. Collectively, our data indicate that in extreme old age the plantaris muscle undergoes significant loss of mass, fiber CSA, and contractile function, as well as its capacity to undergo hypertrophy in response to a chronic increase in mechanical load.     

Models of disuse: a comparison of hindlimb suspension and immobilization

The effects of 1 and 2 wk of hindlimb suspension (HS) on rat skeletal muscle function were determined and the results compared with those obtained previously with hindlimb immobilization (HI). Both models of disuse (HS and HI) primarily affected slow-twitch muscle. Each decreased the isometric twitch duration in the slow-twitch soleus; however, the HS-mediated effect was entirely a result of a shortened contraction time (CT), whereas HI reduced one-half relaxation time (1/2 RT) as well as CT. Soleus muscle mass and peak tetanic tension (Po) declined with disuse. The HS effect on muscle mass and Po was variable, however, for all experiments HS produced atrophy equal to or greater than HI. A major difference existed in the effects of HS and HI on the maximal speed of soleus muscle shortening (Vmax). One and 2 wk of HS produced increases in Vmax to 4.45 +/- 0.34 and 6.83 +/- 0.74 fiber lengths/s, respectively, compared with control velocities of 3.05 +/- 0.08. By contrast over a similar time period, HI had no significant effect on soleus Vmax. The increase in Vmax at 14 days of HS was associated with, and perhaps caused by, the increased expression of a second faster migrating isozyme of myosin. The new native isozyme comigrated with fast myosin, but its light chain subunits contained only LC1s and LC2s. The mechanism responsible for the increase is unknown. One plausible explanation is that the apparent HS-mediated modification in muscle fiber type is dependent on the elimination of loadbearing or isometric contractions, a condition that does not exist during HI.     

Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading.

The role of satellite cells and DNA unit size in determining muscle size was examined by inhibiting postnatal skeletal muscle development by using hindlimb suspension. Satellite cell mitotic activity and DNA unit size were determined in the soleus muscles from hindlimb-suspended and age-matched weight-bearing rats before the initiation of hindlimb suspension, at the conclusion of a 28-day hindlimb-suspension period, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-suspended rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb suspension, but they were the same (P > 0. 05) as those of weight-bearing rats 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, nuclei per millimeter, and DNA unit size for the hindlimb-suspended rats were significantly (P < 0.05) smaller than for the weight-bearing rats at all recovery times. Satellite cell mitotic activity was significantly (P < 0.05) higher in the soleus muscles from hindlimb-suspended rats 2 wk after reloading, but it was the same (P > 0.05) as in weight-bearing rats 9 wk after reloading. Juvenile soleus muscles failed to achieve normal muscle size 9 wk after reloading because there was incomplete compensation for the hindlimb-suspension-induced interruptions in myonuclear accretion and DNA unit size expansion.     

Biochemical and physiological changes in overloaded rat fast- and slow-twitch ankle extensors

The rat soleus (SOL) or medial gastrocnemius (MG) were chronically overloaded by removing their major synergists bilaterally. After 12-14 wks the overloaded SOL (OS) and overloaded MG (OMG) muscles had approximately 50% greater cross-sectional areas (CSA) than the controls. Maximum twitch (Pt) and tetanic (Po) tensions were approximately 46% larger in the OS compared with the normal SOL. The OMG produced 10 and 37% higher Pt and Po, respectively. Specific tension (Po/CSA) was not altered in either group (P greater than 0.05). Contraction times and half-relaxation times were unchanged. Myofibrillar and myosin ATPase specific activities indicated a shift toward that resembling a slower muscle in both the OS and the red portion but not the white portion of the OMG. Generally, markers of glycogen metabolism were reduced (P less than 0.05) in the same muscle areas that showed reduced ATPase activity. These biochemical results were consistent with the apparent histochemical conversion of fibers from fast-twitch, glycolytic----fast-twitch, oxidative-glycolytic----slow-twitch, oxidative types in these muscle areas. These results suggest that overloading either a fast- or slow-twitch plantarflexor results in an increase in muscle mass and maximum tension and in metabolic shifts that generally resemble those observed in a slower muscle. Further, the degree of adaptation appears to be related to the initial fiber type composition of the muscle and/or of the muscle region.     

Electromechanical stimulation ameliorates inactivity-induced adaptations in the medial gastrocnemius of adult rats.

The efficacy of high-load, short-duration isometric contractions, delivered as one vs. two sessions per day, on blunting inactivity-induced adaptations in the medial gastrocnemius (MG) were compared. Adult rats were assigned to a control (Con) or spinal cord-isolated (SI) group where one limb was stimulated (SI-Stim) while the other served as a SI control (SI-C). One bout of stimulation (BION microstimulator) consisted of a 100-Hz, 1-s stimulus, delivered every 30 s for 5 min with a 5-min rest period. This bout was repeated six times consecutively (SI-Stim1) or with a 9-h rest interval after the third bout (SI-Stim2) for 30 consecutive days. MG weights (relative to body weight) were 63, 72, and 79% of Con in SI-C, SI-Stim1, and SI-Stim2, respectively. Mean fiber size was 56% smaller in SI-C than in Con, and it was 19 and 31% larger in SI-Stim1 and SI-Stim2, respectively, compared with SI-C. Maximum tetanic tension was 42, 60, and 73% of Con in SI-C, SI-Stim1, and SI-Stim2, respectively. Specific tension was 77% of Con in SI-C, and at Con levels in both SI-Stim groups. SI increased the percent IIb myosin heavy chain composition (from 49 to 77%) and IIb+ fibers (from 63 to 79%): these adaptations were prevented by both Stim paradigms. These results demonstrate that 1) brief periods of high-load isometric contractions are effective in reducing inactivity-induced atrophy, functional deficits, and phenotypic adaptations in a fast hindlimb extensor, and 2) the same amount of stimulation distributed in two compared with one session per day is more effective in ameliorating inactivity-related adaptations.     

Resistive breathing activates the glutathione redox cycle and impairs performance of rat diaphragm.

Free radical activation and lipid peroxidation have been described in skeletal muscle during strenuous exercise. We hypothesized that oxygen radicals could also be formed in the diaphragm muscle during strenuous resistive breathing and that these radicals might affect diaphragm function. Seven control and 12 experimental male Sprague-Dawley rats were studied. Six experimental animals were subjected to resistive breathing (RB) alone and six animals received 15 min of mechanical ventilatory support (MV) after the resistive breathing period. Inspiratory resistance was adjusted to maintain airway opening pressure at 70% maximum in both groups until exhaustion. Diaphragm samples were obtained for analysis of thiobarbituric acid-reactive substances (TBAR), reduced glutathione (GSH), and glutathione disulfide (GSSG). In vitro isometric contraction times, twitch (Pt) tension and maximum tetanic (Po) tension, force-frequency curves, fatigue index, and recovery index were measured. In RB and MV compared with controls, there were significant decreases in Pt and Po. Diaphragm TBAR concentrations were increased in MV compared with controls or RB. GSSG-to-total glutathione ratio was increased in RB and MV compared with controls. Production of free radicals during RB and MV may represent an important mechanism of diaphragmatic injury that could contribute to the decline in contractility.     

Short vs. long rest period between the sets in hypertrophic resistance training: influence on muscle strength, size, and hormonal adaptations in trained men

Acute and long-term hormonal and neuromuscular adaptations to hypertrophic strength training were studied in 13 recreationally strength-trained men. The experimental design comprised a 6-month hypertrophic strength-training period including 2 separate 3-month training periods with the crossover design, a training protocol of short rest (SR, 2 minutes) as compared with long rest (LR, 5 minutes) between the sets. Basal hormonal concentrations of serum total testosterone (T), free testosterone (FT), and cortisol (C), maximal isometric strength of the leg extensors, right leg 1 repetition maximum (1RM), dietary analysis, and muscle cross-sectional area (CSA) of the quadriceps femoris by magnetic resonance imaging (MRI) were measured at months 0, 3, and 6. The 2 hypertrophic training protocols used in training for the leg extensors (leg presses and squats with 10RM sets) were also examined in the laboratory conditions at months 0, 3, and 6. The exercise protocols were similar with regard to the total volume of work (loads x sets x reps), but differed with regard to the intensity and the length of rest between the sets (higher intensity and longer rest of 5 minutes vs. somewhat lower intensity but shorter rest of 2 minutes). Before and immediately after the protocols, maximal isometric force and electromyographic (EMG) activity of the leg extensors were measured and blood samples were drawn for determination of serum T, FT, C, and growth hormone (GH) concentrations and blood lactate. Both protocols before the experimental training period (month 0) led to large acute increases (p < 0.05-0.001) in serum T, FT, C , and GH concentrations, as well as to large acute decreases (p < 0.05-0.001) in maximal isometric force and EMG activity. However, no significant differences were observed between the protocols. Significant increases of 7% in maximal isometric force, 16% in the right leg 1RM, and 4% in the muscle CSA of the quadriceps femoris were observed during the 6-month strength-training period. However, both 3-month training periods performed with either the longer or the shorter rest periods between the sets resulted in similar gains in muscle mass and strength. No statistically significant changes were observed in basal hormone concentrations or in the profiles of acute hormonal responses during the entire 6-month experimental training period. The present study indicated that, within typical hypertrophic strength-training protocols used in the present study, the length of the recovery times between the sets (2 vs. 5 minutes) did not have an influence on the magnitude of acute hormonal and neuromuscular responses or long-term training adaptations in muscle strength and mass in previously strength-trained men.     

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.     

Skeletal muscle responses to lower limb suspension in humans.

Eight subjects participated in a 6-wk unilateral lower limb suspension (ULLS) study to determine the influence of reduced weight bearing on human skeletal muscle morphology. The right shoe was outfitted with a platform sole that prevented the left foot from bearing weight while walking with crutches, yet it allowed freedom of movement about the ankle, knee, and hip. Magnetic resonance images pre- and post-ULLS showed that thigh muscle cross-sectional area (CSA) decreased (P less than 0.05) 12% in the suspended left lower limb, whereas right thigh muscle CSA did not change. Likewise, magnetic resonance images collected post-ULLS showed that muscle CSA was 14% smaller (P less than 0.05) in the left than in the right leg. The decrease in muscle CSA of the thigh was due to a twofold greater response of the knee extensors (-16%, P less than 0.05) than knee flexors (-7%, P less than 0.05). The rectus femoris muscle of the knee extensors showed no change in CSA, whereas the three vastus muscles showed similar decreases of approximately 16% (P less than 0.05). The apparent atrophy in the leg was due mainly to reductions in CSA of the soleus (-17%) and gastrocnemius muscles (-26%). Biopsies of the left vastus lateralis pre- and post-ULLS showed a 14% decrease (P less than 0.05) in average fiber CSA. The decrease was evident in both type I (-12%) and II (-15%) fibers. The number of capillaries surrounding the different fiber types was unchanged after ULLS.(ABSTRACT TRUNCATED AT 250 WORDS)