Regional blood flows in the goat latissimus dorsi muscle before and after chronic stimulation

Klabunde, Richard E., William A. Anderson, Marius Locke,Sigrid E. Ianuzzo, and C. David Ianuzzo. Regional blood flows inthe goat latissimus dorsi muscle before and after chronic stimulation. J. Appl. Physiol. 81(6):2365-2372, 1996.Latissimus dorsi muscle (LDM) regional bloodflows were determined in anesthetized goats by using coloredmicrospheres under noncontracting and contracting conditions, eitherbefore or after 8-10 wk of chronic muscle stimulation. Surgicaldissection of the LDM, leaving only the thoracodorsal artery to supplythe muscle, did not alter regional noncontracting blood flows butsignificantly reduced the normal hyperemic response to musclecontraction in muscle regions (posterior-medial) furthest from theentrance of the thoracodorsal artery. Eight to 10 wk after acute muscledissection, posterior-medial hyperemic flows were restored. Chronicstimulation of the LDM for 8-10 wk, in either dissected ornondissected muscles, did not alter regional blood flows innoncontracting muscle; however, it significantly reduced hyperemicflows in all muscle regions, although capillary density was increasedand the muscle was transformed into a predominantly type I fiber type.These results, coupled with data from previous experiments, suggestthat the muscle damage observed in the posterior-medial regions of theLDM after surgical dissection and chronic stimulation may be related toreduced hyperemic flow responses caused by surgical isolation of themuscle.

     

Variable-frequency train stimulation of canine latissimus dorsi muscle during shortening contractions

George, David T., Stuart A. Binder-Macleod, Thomas N. Delosso, and William P. Santamore. Variable-frequency trainstimulation of canine latissimus dorsi muscle during shorteningcontractions. J. Appl. Physiol. 83(3):994-1001, 1997.In cardiomyoplasty, the latissimus dorsi muscle(LDM) is wrapped around the heart ventricles and electrically activatedwith a constant-frequency train (CFT). This study tested the hypothesesthat increased mechanical performance from the LDM could be achieved byactivating the muscle with variable-frequency trains (VFTs) of shorterduration or containing fewer stimulus pulses than the CFT now used. Themechanical performance of the canine LDM (n = 7) during shortening contractionswas measured while the muscle was stimulated with 5- and 6-pulse CFTs(of duration 132 and 165 ms, respectively) and 5- and 6-pulse VFTs (ofduration 104 and 143 ms, respectively) that were designed to takeadvantage of the catchlike property of skeletal muscle. Measurementswere made from fresh and fatigued muscles. For the fresh muscles, the VFTs elicited significantly greater peak power than did the 6-pulse CFT. When the muscles were fatigued, VFT stimulation significantly improved both the peak and mean power produced compared withstimulation by CFTs. These results show that stimulation of the LDMwith shorter duration VFTs is potentially useful for application incardiomyoplasty.

     

Induction of neuronal type nitric oxide synthase in skeletal muscle by chronic electrical stimulation in vivo

Reiser, Peter J., William O. Kline, and Pal L. Vaghy.Induction of neuronal type nitric oxide synthase in skeletal muscle by chronic electrical stimulation in vivo. J. Appl. Physiol. 82(4): 1250-1255, 1997.Fast-twitch skeletal muscles contain more neuronal-type nitricoxide synthase (nNOS) than slow-twitch muscles because nNOS is presentonly in fast (type II) muscle fibers. Chronic in vivo electricalstimulation of tibialis anterior and extensor digitorum longus musclesof rabbits was used as a method of inducing fast-to-slow fiber typetransformation. We have studied whether an increase in musclecontractile activity induced by electrical stimulation alters nNOSexpression, and if so, whether the nNOS expression decreases to thelevels present in slow muscles. Changes in the expression of myosinheavy chain isoforms and maximum velocity of shortening of skinnedfibers indicated characteristic fast-to-slow fiber type transformationafter 3 wk of stimulation. At the same time, activity of NOS doubled inthe stimulated muscles, and this correlated with an increase in theexpression of nNOS shown by immunoblot analysis. These data suggestthat nNOS expression in skeletal muscle is regulated by muscle activityand that this regulation does not necessarily follow the fast-twitchand slow-twitch pattern during the dynamic phase of phenotypetransformation.

     

Prior eccentric contractions impair maximal insulin action on muscle glucose uptake in the conscious rat

Asp, Sven, Allan Watkinson, Nicholas D. Oakes, and Edward W. Kraegen. Prior eccentric contractions impair maximal insulin action on muscle glucose uptake in the conscious rat.J. Appl. Physiol. 82(4):1327-1332, 1997.Our aim was to examine the effect of prioreccentric contractions on insulin action locally in muscle in theintact conscious rat. Anesthetized rats performed one-leg eccentriccontractions through the use of calf muscle electrical stimulationfollowed by stretch of the active muscles. Two days later, basal andeuglycemic clamp studies were conducted with the rats in the awakefasted state. Muscle glucose metabolism was estimated from2-[¹⁴C(U)]deoxy-D-glucoseandD-[3-³H]glucose administration, and comparisons were made between the eccentrically stimulated and nonstimulated (control) calfmuscles. At midphysiological insulin levels, effects ofprior eccentric exercise on muscle glucose uptake were notstatistically significant. Maximal insulin stimulation revealed reducedincremental glucose uptake above basal(P < 0.05 in the red gastrocnemius;P < 0.1 in the white gastrocnemiusand soleus) and impaired net glycogen synthesis in all eccentricallystimulated muscles (P < 0.05). Weconclude that prior eccentric contractions impair maximal insulin action (responsiveness) on local muscle glucose uptake and glycogen synthesis in the conscious rat.

     

Time course of recovery from nerve injury in skeletal muscle: energy state and local circulation

Hayashi, Yoshihiro, Takaaki Ikata, Hiroaki Takai, ShinjiroTakata, Takayuki Sogabe, and Keiko Koga. Time course of recoveryfrom nerve injury in skeletal muscle: energy state and localcirculation. J. Appl. Physiol. 82(3):732-737, 1997.This study examined the time course of recoveryfrom nerve injury on energy state assessed by phosphorus-31 magneticresonance spectroscopy and local circulation dynamics by fluorine-19magnetic resonance spectroscopy in skeletal muscles ofrats. The hindlimb muscles that had undergone unilateralsciatic nerve compression for 2 wk (CN) were compared withsham-operated (SO) muscles and with muscles that had the compressionremoved after 2 wk and were allowed to recover for 4 wk (R4) or for 6 wk (R6). The energy state and local circulation dynamics of CN muscleswere less than those of SO muscles (P < 0.01). The energy state of R4 muscles remained at levels similar toCN muscles, whereas the local circulation dynamics improved but notback to SO values. In R6 muscles, both parameters returned to SOvalues. These results showed that the recovery processes of circulationprecede those of energy state in skeletal muscles.

     

Endurance training affects myosin heavy chain phenotype in regenerating fast-twitch muscle

Bigard, Xavier A., Chantal Janmot, Danièle Merino,Françoise Lienhard, Yannick C. Guezennec, and Anne D'Albis.Endurance training affects myosin heavy chain phenotype inregenerating fast-twitch muscle. J. Appl.Physiol. 81(6): 2658-2665, 1996.The aim of thisstudy was to analyze the effects of treadmill training (2 h/day, 5 days/wk, 30 m/min, 7% grade for 5 wk) on the expression of myosinheavy chain (MHC) isoforms during and after regeneration of afast-twitch white muscle [extensor digitorum longus (EDL)]. Male Wistar rats were randomly assigned to a sedentary(n = 10) or an endurance-trained (ET;n = 10) group. EDL muscle degeneration and regeneration were induced by two subcutaneous injections of a snaketoxin. Five days after induction of muscle injury, animals were trainedover a 5-wk period. It was verified that ~40 days after venomtreatment, central nuclei were present in the treated EDL muscles fromsedentary and ET rats. The changes in the expression of MHCs in EDLmuscles were detected by using a combination of biochemical andimmunocytochemical approaches. Compared with contralateral nondegenerated muscles, relative concentrations of types I, IIa, andIIx MHC isoforms in ET rats were greater in regenerated EDL muscles(146%, P < 0.05; 76%,P < 0.01; 87%,P < 0.01, respectively). Their elevation corresponded to a decreasein the relative concentration of type IIb MHC (36%,P < 0.01). Although type I accountedfor only 3.2% of total myosin in regenerated muscles from the ETgroup, the cytochemical analysis showed that the proportion of positive staining with the slow MHC antibody was markedly greater in regenerated muscles than in contralateral ones. Collectively, these results demonstrate that the regenerated EDL muscle is sensitive to endurance training and suggest that the training-induced shift in MHC isoforms observed in these muscles resulted from an additive effect of regeneration and repeated exercise.

     

Effects of 4wk of hindlimb suspension on skeletal muscle mitochondrial respiration in rats

Yajid, Fatima, Jacques G. Mercier, Béatrice M. Mercier, Hervé Dubouchaud, and Christian Préfaut.Effects of 4 wk of hindlimb suspension on skeletal musclemitochondrial respiration in rats. J. Appl.Physiol. 84(2): 479-485, 1998.We investigated inrats the effect of 4 wk of hypodynamia on the respiration of mitochondria isolated from four distinct muscles [soleus,extensor digitorum longus, tibial anterior, and gastrocnemius(Gas)] and from subsarcolemmal (SS) and intermyofibrillar (IMF)regions of mixed hindlimb muscles that mainly contained the four citedmuscles. With pyruvate plus malate as respiratory substrate, 4 wk ofhindlimb suspension produced an 18% decrease in state3 respiration for IMF mitochondria compared with thosein the control group (P < 0.05). TheSS mitochondria state 3 were notsignificantly changed. Concerning the four single muscles, themitochondrial respiration was significantly decreased in the Gasmuscle, which showed a 59% decrease in state3 with pyruvate + malate(P < 0.05). The other musclespresented no significant decrease in respiratory rate in comparisonwith the control group. With succinate + rotenone, there was nosignificant difference in the respiratory rate compared with therespective control group, whatever the mitochondrial origin (SS, orIMF, or from single muscle). We conclude that 4 wk of hindlimbsuspension alters the respiration of IMF mitochondria in hindlimbskeletal muscles and seems to act negatively on complex I of theelectron-transport chain or prior sites. The muscle mitochondria mostaffected are those isolated from the Gas muscle.

     

Voluntary exercise training enhances glucose transport in muscle stimulated by insulin-like growth factor I

Hokama, Jason Y., Ryan S. Streeper, and Erik J. Henriksen.Voluntary exercise training enhances glucose transport in muscle stimulated by insulin-like growth factor I. J. Appl. Physiol. 82(2): 508-512, 1997.Skeletal muscle glucosetransport can be regulated by hormonal factors such as insulin andinsulin-like growth factor I (IGF-I). Although it is well establishedthat exercise training increases insulin action on muscle glucosetransport, it is currently unknown whether exercise training leads toan enhancement of IGF-I-stimulated glucose transport in skeletal muscle. Therefore, we measured glucose transport activity [by using 2-deoxy-D-glucose (2-DG)uptake] in the isolated rat epitrochlearis muscle stimulated bysubmaximally and maximally effective concentrations of insulin (0.2 and13.3 nM) or IGF-I (5 and 50 nM) after 1, 2, and 3 wk of voluntary wheelrunning (WR). After 1 wk of WR, both submaximal andmaximal insulin-stimulated 2-DG uptake rates were significantly(P < 0.05) enhanced (43 and 31%)compared with those of sedentary controls, and these variables werefurther increased after 2 (86 and 57%) and 3 wk (71 and 70%) ofWR. Submaximal and maximal IGF-I-stimulated 2-DG uptakerates were significantly enhanced after 1 wk of WR (82 and 61%), andthese increases did not expand substantially after 2 (71 and 58%) and3 wk (96 and 70%) of WR. This enhancement of hormone-stimulated 2-DGuptake in WR muscles preceded any alteration in glucose transporter(GLUT-4) protein level, which increased only after 2 (24%) and 3 wk(54%) of WR. Increases in GLUT-4 protein were significantly correlated (r = 0.844) with increases in citratesynthase. These results indicate that exercise training can enhanceboth insulin-stimulated and IGF-I-stimulated muscle glucose transportactivity and that these improvements can develop without an increase inGLUT-4 protein.

     

Long-term creatine intake is beneficial to muscle performance during resistance training

Vandenberghe, K., M. Goris, P. Van Hecke, M. Van Leemputte,L. Vangerven, and P. Hespel. Long-term creatine intake isbeneficial to muscle performance during resistance training. J. Appl. Physiol. 83(6):2055-2063, 1997.The effects of oral creatine supplementation onmuscle phosphocreatine (PCr) concentration, muscle strength, and bodycomposition were investigated in young female volunteers(n = 19) during 10 wk ofresistance training (3 h/wk). Compared with placebo, 4 days ofhigh-dose creatine intake (20 g/day) increased(P < 0.05) muscle PCr concentration by 6%. Thereafter, this increase was maintained during 10 wk of training associated with low-dose creatine intake (5 g/day).Compared with placebo, maximal strength of the muscle groups trained,maximal intermittent exercise capacity of the arm flexors, and fat-free mass were increased 20-25, 10-25, and 60% more(P < 0.05), respectively, duringcreatine supplementation. Muscle PCr and strength, intermittent exercise capacity, and fat-free mass subsequently remained at a higherlevel in the creatine group than in the placebo group during 10 wk ofdetraining while low-dose creatine was continued. Finally, on cessationof creatine intake, muscle PCr in the creatine group returned to normalwithin 4 wk. It is concluded that long-term creatine supplementationenhances the progress of muscle strength during resistance training insedentary females.

     

Long-term facilitation of upper airway muscle activities in vagotomized and vagally intact cats

Mateika, J. H., and R. F. Fregosi. Long-termfacilitation of upper airway muscle activities in vagotomized andvagally intact cats. J. Appl. Physiol.82(2): 419-425, 1997.The primary purpose of the presentinvestigation was to determine whether long-term facilitation (LTF) ofupper airway muscle activities occurs in vagotomized and vagally intactcats. Tidal volume and diaphragm, genioglossus, and nasal dilatormuscle activities were recorded before, during, and after one carotidsinus nerve was stimulated five times with 2-min trains of constantcurrent. Sixty minutes after stimulation, nasal dilator andgenioglossus muscle activities were significantly greater than controlin the vagotomized cats but not in the vagally intact cats. Tidalvolume recorded from the vagotomized and vagally intact cats wassignificantly greater than control during the poststimulation period.In contrast, diaphragm activities were not significantly elevated inthe poststimulation period in either group of animals. We conclude that1) LTF of genioglossus and nasaldilator muscle activities can be evoked in vagotomized cats;2) vagal mechanisms inhibit LTF inupper airway muscles; and 3) LTF canbe evoked in accessory inspiratory muscles because LTF of inspiredtidal volume was greater than LTF of diaphragm activity.

     

Modulation of myosin isoform expression by mechanical loading: role of stimulation frequency

Caiozzo, Vincent J., Michael J. Baker, and Kenneth M. Baldwin. Modulation of myosin isoform expression by mechanical loading: role of stimulation frequency. J. Appl.Physiol. 82(1): 211-218, 1997.This study testedthe hypothesis that mechanical loading, not stimulation frequency perse, plays a key role in determining the plasticity of myosin heavychain (MHC) protein isoform expression in muscle undergoing resistancetraining. Female Sprague-Dawley rats were randomly assigned toresistance-training programs that employed active1) shortening(n = 7) or2) lengthening contractions(n = 8). The medial gastrocnemius (MG)muscles in each group trained under loading conditions thatapproximated 90-95% of maximum isometric tetanictension but were stimulated at frequencies of 100 and~25 Hz, respectively. Lengthening and shortening contractions wereproduced by using a Cambridge ergometer system. The MG muscles trainedevery other day, performing a total of 16 training sessions. Bothtraining programs produced significant (P < 0.01) and similar reductions inthe fast type IIB MHC protein isoform in the white MG muscle, reducingits relative content to ~50% of the total MHC protein isoform pool.These changes were accompanied by increases in the relative content ofthe fast type IIX MHC protein isoform that were of similar magnitudefor both groups. The results of this study clearly demonstrate thatstimulation frequency does not play a key role in modulating MHCisoform alterations that result from high-resistance training.

     

Effects of nandrolone decanoate on respiratory and peripheral muscles in male and female rats

Bisschop, Anja, Ghislaine Gayan-Ramirez, HélèneRollier, P. N. Richard Dekhuijzen, René Dom, Vera de Bock, andMarc Decramer. Effects of nandrolone decanoate on respiratory and peripheral muscles in male and female rats. J. Appl.Physiol. 82(4): 1112-1118, 1997.Thirty maleand 18 female adult rats received weekly an intramuscular injection ofeither saline (control; C), 1.5 mg/kg (low-dose; LD) nandrolonedecanoate or 7.5 mg/kg (high-dose; HD) nandrolone decanoate during 5 wk. Compared with respective C, growth rate was stunted in male HD ratsfrom 2 wk of treatment on, whereas it was enhanced in female LD and HDrats after 1 wk. Mass of all muscles studied varied proportionally tobody weight, except for the gastrocnemius (males: 0.49 ± 0.04 vs. C: 0.52 ± 0.03%, not significant; females: 0.17 ± 0.01 vs. C: 0.15 ± 0.01%, P < 0.05). In vitro contractile andfatigue properties of the diaphragm remained unchanged, except for adecrease in twitch kinetics (time to peak tension: C, 21 ± 2; LD,19 ± 1; HD, 19 ± 2 ms, P < 0.05; half-relaxation time: C, 26 ± 5, LD, 25 ± 5, HD, 23 ± 3 ms, P < 0.01).Histochemistry of the diaphragm and the gastrocnemius revealed asignificant increase in type IIx/b dimensions. In the gastrocnemius,type I fiber dimensions also increased. A pair-fed study, includinganother 24 female rats, showed that the changes in oral food intakeonly partly accounted for the observed anabolic effects.

     

Effect of endurance exercise training on muscle glycogen supercompensation in rats

Nakatani, Akira, Dong-Ho Han, Polly A. Hansen, Lorraine A. Nolte, Helen H. Host, Robert C. Hickner, and John O. Holloszy. Effect of endurance exercise training on muscle glycogensupercompensation in rats. J. Appl.Physiol. 82(2): 711-715, 1997.The purpose of this study was to test the hypothesis that the rate and extent ofglycogen supercompensation in skeletal muscle are increased byendurance exercise training. Rats were trained by using a 5-wk-long swimming program in which the duration of swimming was gradually increased to 6 h/day over 3 wk and then maintained at 6 h/day for anadditional 2 wk. Glycogen repletion was measured in trained anduntrained rats after a glycogen-depleting bout of exercise. The ratswere given a rodent chow diet plus 5% sucrose in their drinking waterad libitum during the recovery period. There were remarkabledifferences in both the rates of glycogen accumulation and the glycogenconcentrations attained in the two groups. The concentration ofglycogen in epitrochlearis muscle averaged 13.1 ± 0.9 mg/g wet wtin the untrained group and 31.7 ± 2.7 mg/g in the trained group(P < 0.001) 24 h after the exercise.This difference could not be explained by a training effect on glycogensynthase. The training induced ~50% increases in muscle GLUT-4glucose transporter protein and in hexokinase activity inepitrochlearis muscles. We conclude that endurance exercise trainingresults in increases in both the rate and magnitude of muscle glycogensupercompensation in rats.

     

Responses of group III and IV muscle afferents to dynamic exercise

Adreani, Christine M., Janeen M. Hill, and Marc P. Kaufman.Responses of group III and IV muscle afferents to dynamic exercise. J. Appl. Physiol. 82(6):1811-1817, 1997.Tetanic contraction of hindlimb skeletal muscle,induced by electrical stimulation of either ventral roots or peripheralnerves, is well known to activate group III and IV afferents.Nevertheless, the effect of dynamic exercise on the discharge of thesethin fiber afferents is unknown. To shed some light on this question,we recorded in decerebrate cats the discharge of 24 group III and 10 group IV afferents while the mesencephalic locomotor region (MLR) wasstimulated electrically. Each of the 34 afferents had their receptivefields in the triceps surae muscles. Stimulation of the MLR for 1 min caused the triceps surae muscles to contract rhythmically, an effectinduced by an -motoneuron discharge pattern and recruitment orderalmost identical to that occurring during dynamic exercise. Eighteen ofthe 24 group III and 8 of the 10 group IV muscle afferents werestimulated by MLR stimulation. The oxygen consumption of thedynamically exercising triceps surae muscles was increased by 2.5-foldover their resting levels. We conclude that low levels of dynamicexercise stimulate group III and IV muscle afferents.

     

Abdominal muscle fatigue after maximal ventilation in humans

Kyroussis, Dimitris, Gary H. Mills, Michael I. Polkey,Carl-Hugo Hamnegard, Nicholaos Koulouris, Malcolm Green, and John Moxham. Abdominal muscle fatigue after maximal ventilation inhumans. J. Appl. Physiol. 81(4):1477-1483, 1996.Abdominal muscles are the principal muscles ofactive expiration. To investigate the possibility of abdominal musclelow-frequency fatigue after maximal ventilation in humans, westimulated the nerve roots supplying the abdominal muscles. We used amagnetic stimulator (Magstim 200) powering a 90-mm circular coil andstudied six normal subjects. To assess the optimum level of stimulationand posture, we stimulated at each intervertebral level betweenT₇ andL₁ in the prone, supine, andseated positions. At T₁₀, we usedincreasing power outputs to assess the pressure-power relationship.Care was taken to avoid muscle potentiation. Twitch gastric pressure(Pga) was recorded with a balloon-tipped catheter. Mean (±SD)baseline twitch Pga measured with the subjects in the prone position atT₁₀ was 23.5 ± 5.4 cmH₂O. Within-occasion mean twitchPga coefficient of variation was 4.6 ± 1.1%. Twitch Pga wasmeasured with the subjects in the prone position with stimulation overT₁₀ before and after 2 min ofmaximal isocapnic ventilation (MIV). Twenty minutes after MIV, meantwitch Pga fell by 17 ± 9.1%(P = 0.03) and remained low 90 minafter MIV. We conclude that after maximal ventilation in humans thereis a reduction of twitch Pga and, therefore, of low-frequency fatiguein abdominal muscles.

     

Decreased insulin action on muscle glucose transport after eccentric contractions in rats

Asp, Sven, and Erik A. Richter. Decreased insulinaction on muscle glucose transport after eccentric contractions in rats. J. Appl. Physiol. 81(5):1924-1928, 1996.We have recently shown that eccentriccontractions (Ecc) of rat calf muscles cause muscle damage anddecreased glycogen and glucose transporter GLUT-4 protein content inthe white (WG) and red gastrocnemius (RG) but not in the soleus (S) (S. Asp, S. Kristiansen, and E. A. Richter. J. Appl.Physiol. 79: 1338-1345, 1995). To study whetherthese changes affect insulin action, hindlimbs were perfused at three different insulin concentrations (0, 200, and 20,000 µU/ml) 2 daysafter one-legged eccentric contractions of the calf muscles. Comparedwith control, basal glucose transport was slightly higher (P < 0.05) in Ecc-WG and -RG,whereas it was lower (P < 0.05) atboth submaximal and maximal insulin concentrations in the Ecc-WG and atmaximal concentrations in the Ecc-RG. In the Ecc-S, the glucosetransport was unchanged in hindquarters perfused in the absence orpresence of a submaximal stimulating concentration of insulin, whereasit was slightly (P < 0.05) higherduring maximal insulin stimulation compared with control S. At the endof perfusion the glycogen concentrations were lower in bothEcc-gastrocnemius muscles compared with control muscles at all insulinconcentrations. Fractional velocity of glycogen synthase increasedsimilarly with increasing insulin concentrations in Ecc- and control WGand RG. We conclude that insulin action on glucose transport but notglycogen synthase activity is impaired in perfused muscle exposed toprior eccentric contractions.

     

Pliometric contraction-induced injury of mouse skeletal muscle: effect of initial length

Hunter, Kam D., and John A. Faulkner. Pliometriccontraction-induced injury of mouse skeletal muscle: effect of initial length. J. Appl. Physiol. 82(1):278-283, 1997.For single pliometric (lengthening) contractionsinitiated from optimal fiber length (L_f), the mostimportant factor determining the subsequent force deficit is the workinput during the stretch. We tested the hypothesis that regardless ofthe initial length, the force deficit is primarily a function of thework input. Extensor digitorum longus muscles of mice were maximallyactivated in situ and lengthened at 2 L_f /s from oneof three initial fiber lengths (90, 100, or 120% of L_f) to one ofthree final fiber lengths (150, 160, or 170% of L_f). Maximalisometric force production was assessed before and after the pliometriccontraction. No single mechanical factor, including thework input(r² = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit appears to arisefrom a complex interaction of mechanicalevents. With the data grouped by initial fiber length,the correlation between the average work and the average force deficitwas high(r² = 0.97-0.99). Consequently, differences in force deficits among groups were best explained on the basis of the initial fiber length andthe work input during the stretch.

     

Lower limb skeletal muscle function after 6wk of bed rest

Berg, H. E., L. Larsson, and P. A. Tesch. Lower limbskeletal muscle function after 6 wk of bed rest. J. Appl. Physiol. 82(1): 182-188, 1997.Force,electromyographic (EMG) activity, muscle mass, and fibercharacteristics were studied in seven healthy men before and after 6 wkof bed rest. Maximum voluntary isometric and concentric knee extensortorque decreased (P < 0.05)uniformly across angular velocities by 25-30% after bed rest.Maximum quadricep rectified EMG decreased by 19 ± 23%, whereassubmaximum (100-Nm isometric action) EMG increased by 44 ± 28%.Knee extensor muscle cross-sectional area (CSA), assessed by usingmagnetic resonance imaging, decreased by 14 ± 4%. Maximum torqueper knee extensor CSA decreased by 13 ± 9%. Vastus lateralis fiberCSA decreased 18 ± 14%. Neither type I, IIA, and IIB fiberpercentages nor their relative proportions of myosin heavy chain (MHC)isoforms were altered after bed rest. Because the decline in strengthcould not be entirely accounted for by decreased muscle CSA, it issuggested that the strength loss is also due to factors resulting indecreased neural input to muscle and/or reduced specifictension of muscle, as evidenced by a decreased torque/EMG ratio.Additionally, it is concluded that muscle unloading in humans does notinduce important changes in fiber type or MHC composition or in vivomuscle contractile properties.

     

Sprint training, in vitro and in vivo muscle function, and myosin heavy chain expression

Harridge, S. D. R., R. Bottinelli, M. Canepari, M. Pellegrino, C. Reggiani, M. Esbjörnsson, P. D. Balsom, and B. Saltin. Sprint training, in vitro and in vivo muscle function, and myosin heavy chain expression. J. Appl.Physiol. 84(2): 442-449, 1998.Sprint trainingrepresents the condition in which increases in muscle shortening speed,as well as in strength, might play a significant role in improvingpower generation. This study therefore aimed to determine the effectsof sprint training on 1) thecoupling between myosin heavy chain (MHC) isoform expression andfunction in single fibers, 2) thedistribution of MHC isoforms across a whole muscle, and3) in vivo muscle function. Sevenyoung male subjects completed 6 wk of training (3-s sprints) on a cycleergometer. Training was without effect on maximum shortening velocityin single fibers or in the relative distribution of MHC isoforms ineither the soleus or the vastus lateralis muscles. Electrically evokedand voluntary isometric torque generation increased(P < 0.05) after training in boththe plantar flexors (+8% at 50 Hz and +16% maximal voluntarycontraction) and knee extensors (+8% at 50 Hz and +7% maximalvoluntary contraction). With the shortening potential of the musclesapparently unchanged, the increased strength of the major lower limbmuscles is likely to have contributed to the 7% increase(P < 0.05) in peak pedal frequency during cycling.

     

Cytochrome c mRNA in skeletal muscles of immobilized limbs

Booth, Frank W., Wei Lou, Marc T. Hamilton, and Zhen Yan.Cytochrome c mRNA in skeletalmuscles of immobilized limbs. J. Appl.Physiol. 81(5): 1941-1945, 1996.Even thoughimmobilization of a slow skeletal muscle in a lengthened positionprevents muscle atrophy, it is unknown whether this treatment wouldprevent a decrease in mitochondrial quantity. We found that, regardless of muscle length in immobilized limbs, the mRNA of a marker for mitochondrial quantity, cytochrome c,decreased. Cytochrome c mRNA permilligram of muscle was 62 and 72% less 1 wk after fixation of thesoleus muscle in shortened and lengthened positions, respectively, thanage-matched controls. Cytochrome cmRNA per milligram wet weight was 36 and 32% less in the tibialisanterior muscle fixed for 1 wk in the shortened and lengthenedpositions, respectively, compared with age-matched controls. Recently,in the 3-untranslated region of cytochromec mRNA a novel RNA-protein interactionthat decreases in chronically stimulated rat skeletal musclewas identified.[Z. Yan, S. Salmons, Y. L. Dang, M. T. Hamilton, and F. W. Booth. Am. J. Physiol. 271 (CellPhysiol. 40): C1157- C1166,1996]. The RNA-protein interaction inthe 3-untranslated region of cytochrome c mRNA in soleus and tibialis anteriormuscles was unaffected by fixation in either shortened or lengthenedposition. We conclude that, whereas lengthening muscle during limbfixation abates the loss of total muscle protein, the percentagedecrease in cytochrome c mRNA isproportionally greater than total protein. This suggests that thedesign of countermeasures to muscle atrophy should include differentexercises to maintain total protein and mitochondria.