Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action

Ryschon, T. W., Fowler, R. E. Wysong, A.-R. Anthony, and R. S. Balaban. Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action. J. Appl. Physiol. 83(3): 867-874, 1997.The purpose of this study was to estimate the efficiency of ATPutilization for concentric, eccentric, and isometric muscle action inthe human tibialis anterior and extensor digitorum longus in vivo. Adynamometer was used to quantitate muscle work, or tension, whilesimultaneous ³¹P-nuclear magneticresonance data were collected to monitor ATP, phosphocreatine,inorganic phosphate, and pH. The relative efficiency of the actions wasestimated in two ways: steady-state effects on high-energyphosphates and a direct comparison of ATP synthesis rates with work. Inthe steady state, the cytosolic free energy dropped to the lowest valuewith concentric activity, followed by eccentric and isometric actionfor comparative muscle tensions. Estimates of ATP synthesis ratesrevealed a mechanochemical efficiency [i.e., ATP productionrate/work (both in J/s)] of 15.0 ± 1.3% in concentric and34.7 ± 6.1% in eccentric activity. The estimated maximum ATPproduction rate was highest in concentric action, suggesting anactivation of energy metabolism under these conditions. By using directmeasures of metabolic strain and ATP turnover, these data demonstrate adecreasing metabolic efficiency in human muscle action from isometric,to eccentric, to concentric action.

     

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.

     

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.

     

Stretch-shorten cycle compared with isometric preload: contributions to enhanced muscular performance

Walshe, Andrew D., Greg J. Wilson, and Gertjan J. C. Ettema.Stretch-shorten cycle compared with isometric preload: contributions to enhanced muscular performance. J. Appl. Physiol. 84(1): 97-106, 1998.To isolateany difference muscular contraction history may have on concentric workoutput, 40 trained male subjects performed three separate isokineticconcentric squats that involved differing contraction histories:1) a concentric-only (CO) squat, 2) a concentric squat preceded by anisometric preload (IS), and 3) astretch-shorten cycle (SSC) squat. Over the first 300 ms of theconcentric movement, work output for both the SSC and IS conditions wassignificantly greater (154.8 ± 39.8 and 147.9 ± 34.7 J, respectively; P < 0.001) comparedwith the CO squat (129.7 ± 34.4 J). In addition, work output afterthe SSC test over the first 300 ms was also significantly larger thanthat for the corresponding period after the IS protocol(P < 0.05). There was no difference in normalized, integrated electromyogram among any of the conditions. It was concluded that concentric performance enhancement derived from apreceding stretch of the muscle-tendon complex was largely due to theattainment of a higher active muscle state before the start of theconcentric movement. However, it was also hypothesized that contractileelement potentiation was a significant contributor to stretch-inducedmuscular performance under these conditions.

     

Effects of transverse fiber stiffness and central tendon on displacement and shape of a simple diaphragm model

Boriek, Aladin M., and Joseph R. Rodarte. Effects oftransverse fiber stiffness and central tendon on displacement and shapeof a simple diaphragm model. J. Appl. Physiol. 82(5): 1626-1636, 1997.Our previous experimental results (A. M. Boriek, S. Lui, and J. R. Rodarte. J. Appl. Physiol. 75:527-533, 1993 and A. M. Boriek, T. A. Wilson, and J. R. Rodarte.J. Appl. Physiol. 76: 223-229, 1994) showed that1) costal diaphragm shape is similar at functional residualcapacity and end inspiration regardless of whether the diaphragm muscleshortens actively (increased tension) or passively (decreased tension);2) diaphragmatic muscle length changes minimally in thedirection transverse to the muscle fibers, suggesting the diaphragm maybe inextensible in that direction; and 3) the central tendon isnot stretched by physiological stresses. A two-dimensional orthotropicmaterial has two different stiffnesses in orthogonal directions. In theplane tangent to the muscle surface, these directions are along thefibers and transverse to the fibers. We wondered whether orthotropicmaterial properties in the muscular region of the diaphragm andinextensibility of the central tendon might contribute to the constancyof diaphragm shape. Therefore, in the present study, we examined theeffects of stiffness transverse to muscle fibers and inextensibility ofthe central tendon on diaphragmatic displacement and shape. Finiteelement hemispherical models of the diaphragm were developed by usingpressurized isotropic and orthotropic membranes with a wide range ofstiffness ratios. We also tested heterogeneous models, in which themuscle sheet was an orthotropic material, having transverse fiberstiffness greater than that along the fibers, with the central tendonbeing an inextensible isotropic cap. These models revealed thatincreased transverse stiffness limits the shape change of thediaphragm. Furthermore, an inextensible cap simulating the centraltendon dramatically limits the change in shape as well as the membrane displacement in response to pressure. These findings provide a plausible mechanism by which the diaphragm maintains similar shapes despite different physiological loads. This study suggests that changesof diaphragm shape are restricted because the central tendon isessentially inextensible and stiffness in the direction transverse tothe muscle fibers is greater than stiffness along the fibers.

     

Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions

Potvin, J. R. Effects of muscle kinematics on surfaceEMG amplitude and frequency during fatiguing dynamic contractions. J. Appl. Physiol. 82(1): 144-151, 1997.Fifteen male subjects performed a repetitive elbowflexion/extension task with a 7-kg mass until exhaustion. Average jointangle, angular velocity, and biceps brachii surface electromyographic(EMG) amplitude (aEMG) and mean powerfrequency (MPF) were calculated with each consecutive 250-ms segment ofdata during the entire trial. Data were separated into concentric oreccentric phases and into seven 20°-ranges from 0 to 140° ofelbow flexion. A regression analysis was used to estimate the restedand fatigued aEMG and MPF values. aEMG values were expressed as apercentage of amplitudes from maximum voluntary contractions (MVC).Under rested dynamic conditions, the average concentric aEMG amplitudewas 10% MVC higher than average eccentric values. Rested MPF valueswere similar for concentric and eccentric phases, although valuesincreased ~20 Hz from the most extended to flexed joint angles.Fatigue resulted in an average increase in concentric and eccentricaEMG of 35 and 10% MVC, respectively. The largest concentric aEMGincreases (up to 58% MVC) were observed at higher joint velocities,whereas eccentric increases appeared to be related to decreases invelocity. Fatigue had a similar effect on MPF during both concentricand eccentric phases. Larger MPF decreases were observed at shortermuscle lengths such that values within each angle range were verysimilar by the end of the trial. It was hypothesized that this findingmay reflect a biological minimum in conduction velocity beforepropagation failure occurs.

     

High-energy phosphates and tension production in rabbit tibialis anterior/extensor digitorum longus muscles

Ryschon, T. W., J. C. Jarvis, S. Salmons, and R. S. Balaban.High-energy phosphates and tension production in rabbit tibialisanterior/extensor digitorum longus muscles. J. Appl. Physiol. 82(3): 1024-1029, 1997.The effects ofrepetitive muscle contraction on energy state and tension productionwere studied in rabbit tibialis anterior/extensor digitorum longusmuscles that had been subjected to 90 days of continuous indirectelectrical stimulation at 10 Hz. Anesthetized chronically stimulatedand control rabbits were challenged with 15 min of stimulation at 4 and15 tetani/min.P_i-to-phosphocreatine (PCr) ratio(P_i/PCr) was measured in vivo before, during, andafter acute stimulation by³¹P-magnetic resonancespectroscopy, and tension was recorded at the same time. AlthoughP_i/PCr was low at rest, it wassignificantly higher in chronically stimulated muscle than in controlmuscle (0.20 ± 0.02 vs. 0.05 ± 0.01, P < 0.05). Stimulation of control muscle for 15 min at both 4 and 15 tetani/min induced a significant rise in P_i/PCr, whereas the sameconditions in chronically stimulated muscle did not produce anysignificant departure from initial levels. The tension produced bycontrol muscle fell to 93 ± 3% of its initial value duringstimulation at 4 tetani/min and to 61 ± 7% at 15 tetani/min,respectively. In chronically stimulated muscle, on the other hand,tension was potentiated above its initial level at both stimulationrates (135 ± 15 and 138 ± 11%, respectively) and remainedsignificantly elevated throughout each trial. The ability ofchronically stimulated muscle to sustain high levels of activity withminimal perturbations in P_i/PCr ordecrement in tension is attributable to cellular adaptations thatinclude a well-documented increase in oxidative capacity.

     

Airway-parenchymal interdependence after airway contraction in rat lung explants

The constrictionof pulmonary airways is limited by the tethering effect exerted byparenchymal attachments. To characterize this tethering effect at thescale of intraparenchymal airways, we studied the pattern ofparenchymal distortion due to bronchoconstriction in a rat lung explantsystem. First, we measured the elastic modulus under tension for 2%(wt/vol) agarose alone (37.6 ± 1.5 kPa) and for agarose-filled lung(5.7 ± 1.3 kPa). The latter is similar to the elastic modulus ofair-filled lung at total lung capacity (4.5-6 kPa) (S. J. Lai-Fook, T. A. Wilson, R. E. Hyatt, and J. R. Rodarte.J. Appl. Physiol. 40: 508-513,1976), suggesting that explants can be used as a model of lung tissuedistortion. Subsequently, confocal microscopic images of fluorescentlylabeled 0.5-mm-thick explants prepared from agarose-filled rat lungsinflated to total lung capacity (48 ml/kg) were acquired. Images weretaken before and after airway constriction was induced by directapplication of 10 mM methacholine, and the pattern of parenchymaldistortion was measured from the displacement of tissue landmarksidentified in each image for 14 explants. The magnitude of the radialcomponent of tissue displacement was calculated as a function ofdistance from the airway wall and characterized by a parameter,b, describing the rate at which tissuemovement decreased with radial distance. The parameterb was 0.994 ± 0.19 (SE), which isclose to the prediction of b = 1 ofmicromechanical modeling (T. A. Wilson. J. Appl.Physiol. 33: 472-478, 1972). There was significantvariability in b, however, which wascorrelated with the fractional reduction in airway diameter (r = 0.496). Additionally, parenchymaldistortion showed significant torsion with respect to the radialdirection. This torsion was similar in concentric zones around theairway, suggesting that it originates from inhomogeneity in theparenchyma rather than inhomogeneous airway constriction. Our resultsdemonstrate the significance of the nonlinear mechanical properties ofalveolar walls and the anisotropy of the parenchyma in determining the nature of airway-parenchymal interdependence.

     

Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers

Talmadge, Robert J., Roland R. Roy, and V. Reggie Edgerton.Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers. J. Appl.Physiol. 81(6): 2540-2546, 1996.The effects of14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044)on myosin heavy chain (MHC) isoform content of the rat soleus muscleand single muscle fibers were determined. On the basis ofelectrophoretic analyses, there was a de novo synthesis of type IIx MHCbut no change in either type I or IIa MHC isoform proportions aftereither SF or HS compared with controls. The percentage of fiberscontaining only type I MHC decreased by 26 and 23%, and the percentageof fibers with multiple MHCs increased from 6% in controls to 32% inHS and 34% in SF rats. Type IIx MHC was always found in combinationwith another MHC or combination of MHCs; i.e., no fibers contained typeIIx MHC exclusively. These data suggest that the expression of thenormal complement of MHC isoforms in the adult rat soleus muscle isdependent, in part, on normal weight bearing and that the absence ofweight bearing induces a shift toward type IIx MHC protein expression in the preexisting type I and IIa fibers of the soleus.

     

Frequency dependence of forced oscillatory respiratory mechanics in horses with heaves

Young, S. S., D. Tesarowski, and L. Viel. Frequencydependence of forced oscillatory respiratory mechanics in horses withheaves. J. Appl. Physiol. 82(3):983-987, 1997.The effect of measurement frequency on respiratorymechanics was investigated in six horses with reversible allergicairway disease. Total respiratory impedance was measured at 1.5, 2.0, 3.0, and 5.0 Hz by using the forced oscillation technique with thehorses in remission, after acute antigenic challenge producing clinicalheaves, and with heaves but after the administration of 2 mg fenoterolby inhalation. The slopes of the magnitude(|Zrs|) and real part (R) of total respiratoryimpedance over the frequency range 1.5-3 Hz changed significantlyafter antigenic challenge and fenoterol. The ratio of R at 2 Hz to R at3 Hz, however, discriminated better among the three conditions.Compliance and resonant frequency (calculated by using a three-elementmodel) changed significantly after antigenic challenge and fenoterol,but inertance did not. We concluded that horses with heaves showedfrequency dependence of R and |Zrs| atfrequencies up to 3 Hz and that parameters derived from a three-element model were useful indicators of small airway obstruction in the horse.

     

Doublet potentiation during eccentric and concentric contractions of cat soleus muscle

Sandercock, Thomas G., and C. J. Heckman. Doubletpotentiation during eccentric and concentric contractions of cat soleusmuscle. J. Appl. Physiol. 82(4):1219-1228, 1997.The addition of an extra stimulus pulse, ordoublet, at the beginning of a low-frequency train has been shown tosubstantially increase isometric force. This study examined the effectsof muscle movement on this doublet potentiation. The soleus muscles ofanesthetized cats were stimulated at 10 Hz for 1 s, with and without anadded doublet (0.01-s interval). Isovelocity releases reduced but didnot eliminate peak and early doublet potentiation (average 0.0-0.5s after the doublet). Large releases, >0.4 s after the doublet,completely abolished sustained doublet potentiation (average0.5-1.0 s after the doublet). In contrast, early isovelocitystretches boosted peak doublet potentiation. Yet, large stretches laterin the stimulus almost completely eliminated sustained doubletpotentiation. This suggests that a different mechanism is responsiblefor early and sustained doublet potentiations. Because peak and averageinitial doublet potentiation were not strongly affected by movement,doublets still offer a viable control strategy to increase force during movement while minimizing the number of stimulus pulses.

     

Age and gender comparisons of muscle strength in 654women and men aged 20-93 yr

Lindle, R. S., E. J. Metter, N. A. Lynch, J. L. Fleg, J. L. Fozard, J. Tobin, T. A. Roy, and B. F. Hurley. Age and gendercomparisons of muscle strength in 654 women and men aged 20-93 yr.J. Appl. Physiol. 83(5): 1581-1587, 1997.To assess ageand gender differences in muscle strength, isometric, concentric (Con),and eccentric (Ecc) peak torque was measured in the knee extensors at aslow (0.52 rad/s) and fast (3.14 rad/s) velocity in 654 subjects (346 men and 308 women, aged 20-93 yr) from the Baltimore LongitudinalStudy of Aging. Regression analysis revealed significant(P < 0.001) age-related reductions in Con and Ecc peaktorque for men and women at both velocities, but no differences wereobserved between the gender groups or velocities. Age explained lossesin Con better than Ecc peak torque, accounting for 30% (Con) vs. 19%(Ecc) of the variance in men and 28% (Con) vs. 11% (Ecc) in women. Toassess age and gender differences in the ability to store and utilizeelastic energy, the stretch-shortening cycle was determined in a subsetof subjects (n = 47). The older women (mean age = 70 yr)showed a significantly greater enhancement in the stretch-shorteningcycle, compared with men of similar age (P < 0.01) andcompared with younger men and women (each P < 0.05). Bothmen and women showed significant declines in muscle quality for Conpeak torque (P < 0.01), but no gender differences were observed. Only the men showed a significant decline in muscle quality(P < 0.001) for Ecc peak torque. Thus both men and women experience age-related losses in isometric, Con, and Ecc knee extensorpeak torque; however, age accounted for less of the variance in Eccpeak torque in women, and women tend to better preserve muscle qualitywith age for Ecc peak torque. In addition, older women have an enhancedcapacity to store and utilize elastic energy compared with similarlyaged men as well as with younger women and men.

     

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.

     

Effect of time-varying load on degree of bronchoconstriction in the dog

It is wellestablished that the degree of airway smooth muscle shortening producedby a given dose of bronchial agonist is greatly affected by lungvolume. The airways are tethered by parenchymal attachments, thetension of which increases progressively with lung volume, therebypresenting a commensurately increasing hindrance to smooth musclecontraction. Earlier studies (P. F. Dillon, M. O. Aksoy, S. P. Driska,and R. A. Murphy. Science 211:495-497, 1981) presented evidence that smooth muscle contractioninitially involves rapidly cycling cross bridges, whichthen change to noncycling (latch) bridges. They also suggested thatmost of the muscle shortening occurs during the early rapidcross-bridge phase. This implies that smooth muscle subject to a givenload early in contraction should shorten less than when it is subjectto the same load later on. An in vitro study (W. Li and N. L. Stephens.Can. J. Physiol. Pharmacol. 72:1458-1463, 1994) obtained support for this notion. To test thishypothesis in vivo, we measured the changes in lung impedance at 1 and6 Hz produced in dogs by a bolus intravenous injection of methacholinewhen lung volume was increased for 10 s at different times afterinjection. We found that the changes in mechanics were greatlyinhibited, whereas lung volume was elevated. However, when lung volumewas returned to its initial level, the lung mechanics continued tochange at a rate unaffected by the preceding volume change. We concludethat temporary mechanical inhibition of airway smooth muscle shorteningin the normal dog in vivo merely delays an otherwise normal course ofcontraction.

     

Greater initial adaptations to submaximal muscle lengthening than maximal shortening

Hortobágyi, Tibor, Jason Barrier, David Beard, JohnBraspennincx, Peter Koens, Paul Devita, Line Dempsey, and Jean Lambert. Greater initial adaptations to submaximal muscle lengthening thanmaximal shortening. J. Appl. Physiol.81(4): 1677-1682, 1996.The purpose of this study was tocompare the short-term strength and neural adaptations to eccentric andconcentric training at equal force levels. Forty-two sedentary women(age = 21.5 yr) were ranked based on the initial quadriceps strengthscore, and trios of subjects were randomly assigned to either aneccentric (n = 14), a concentric (n = 14), or a nonexercising controlgroup (n = 14). Training involved atotal of 824 eccentric or concentric quadriceps actions at 1.05 rad ^· s¹administered in four sets of 6-10 repetitions, four times per weekfor 6 wk. Before and after training, all subjects were tested forunilateral maximal isometric and eccentric and concentric actions at1.05 rad ^· s¹and for a 40-repetition eccentric and concentric fatigue series of theleft and right quadriceps. Surface electromyographic activity of thevastus lateralis and medialis was monitored during testing. Concentrictraining increased concentric (36%, P < 0.05), isometric (18%, P < 0.05), and eccentric strength (13%), and eccentric training increasedeccentric (42%, P < 0.05),isometric (30%, P < 0.05), andconcentric (13%) strength. Eccentric training improved eccentric andisometric strength more (P < 0.05)than did concentric training. The electromyographic adaptations weregreater with eccentric training. Cross-education was 6%, and neithertraining mode modified fatigability. The data suggest that training ofthe quadriceps muscle with submaximal eccentric actions brings aboutgreater strength adaptations faster than does training withmaximal-level concentric actions in women. This greater adaptation islikely to be mediated by both mechanical and neural factors.

     

Muscle fiber architecture of the dog diaphragm

Boriek, Aladin M., Charles C. Miller III, and Joseph R. Rodarte. Muscle fiber architecture of the dog diaphragm.J. Appl. Physiol. 84(1): 318-326, 1998.Previous measurements of muscle thickness and length ratio ofcostal diaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte.J. Appl. Physiol. 77: 2065-2070,1994) suggested, but did not prove, discontinuous muscle fiberarchitecture. We examined diaphragmatic muscle fiber architecture usingmorphological and histochemical methods. In 15 mongrel dogs, transversesections along the length of the muscle fibers were analyzedmorphometrically at ×20, by using the BioQuant System IVsoftware. We measured fiber diameters, cross-sectional fiber shapes,and cross-sectional area distributions of fibers. We also determinednumbers of muscle fibers per cross-sectional area and ratio ofconnective tissue to muscle fibers along a course of the muscle fromnear the chest wall (CW) to near the central tendon (CT) for midcostalleft and right hemidiaphragms, as well as ventral, middle, and dorsalregions of the left costal hemidiaphragm. In six other mongrel dogs,the macroscopic distribution of neuromuscular junctions (NMJ) onthoracic and abdominal diaphragm surfaces was determined by stainingthe intact diaphragmatic muscle for acetylcholinesterase activity. Theaverage major diameter of muscle fibers was significantly smaller, andthe number of fibers was significantly larger midspan between CT and CWthan near the insertions. The ratio of connective tissues to musclefibers was largest at CW compared with other regions along the lengthof the muscle. The diaphragm is transversely crossed by multiplescattered NMJ bands with fairly regular intervals offset in adjacentstrips. Muscle fascicles traverse two to five NMJ, consistent withfibers that do not span the entire fascicle from CT to CW. Theseresults suggest that the diaphragm has a discontinuous fiberarchitecture in which contractile forces may be transmitted among themuscle fibers through the connective tissue adjacent to the fibers.

     

PKC-dependent regulation of transepithelial resistance: roles of MLC and MLC kinase

The mechanisms by which protein kinase C (PKC) activationresults in increased transepithelial resistance (TER) are unknown [G. Hecht, B. Robinson, and A. Koutsouris. Am.J. Physiol. 266 (Gastrointest. LiverPhysiol. 29): G214-G221, 1994]. We havepreviously shown that phosphorylation of the regulatory light chain ofmyosin II (MLC) is associated with decreases in TER and have suggested that contraction of the perijunctional actomyosin ring (PAMR) increasestight junction (TJ) permeability [J. R. Turner, B. K. Rill, S. L. Carlson, D. Carnes, R. Kerner, R. J. Mrsny, and J. L. Madara.Am. J. Physiol. 273 (Cell Physiol. 42): C1378-C1385, 1997]. We therefore hypothesized that PKC activation alters TER via relaxation of the PAMR. Activation of PKC by the phorbol ester phorbol 12-myristate 13-acetate (PMA) resulted in a progressive dose-dependent increase in TER that was apparent within 15 min (111%of controls) and maximal within 2 h (142% of controls). Similarincreases were induced by a diacylglycerol analog, and the effects ofboth PMA and the diacylglycerol analog were prevented by the PKCinhibitor bisindolylmaleimide I. PMA treatment caused progressivedecreases in MLC phosphorylation, by 12% at 15 min and 41% at 2 h.Phosphorylation of MLC kinase (MLCK) increased by 64% within 15 min ofPMA treatment and was stable over 2 h (51% greater than that ofcontrols). Thus increases in MLCK phosphorylation preceded decreases inMLC phosphorylation. These data suggest that PKC regulates TER viadecreased phosphorylation of MLC, possibly due to inhibitoryphosphorylation of MLCK. The decreased phosphorylation of MLC likelyreduces PAMR tension, leading to decreased TJ permeability.

     

Kinematics and mechanics of midcostal diaphragm of dog

Boriek, Aladin M., Joseph R. Rodarte, and Theodore A. Wilson. Kinematics and mechanics of midcostal diaphragm of dog. J. Appl. Physiol. 83(4):1068-1075, 1997.Radiopaque markers were attached to theperitoneal surface of three neighboring muscle bundles in the midcostaldiaphragm of four dogs, and the locations of the markers were trackedby biplanar video fluoroscopy during quiet spontaneous breathing andduring inspiratory efforts against an occluded airway at three lungvolumes from functional residual capacity to total lung capacity inboth the prone and supine postures. Length and curvature of the musclebundles were determined from the data on marker location. Musclelengths for the inspiratory states, as a fraction of length atfunctional residual capacity, ranged from 0.89 ± 0.04 at endinspiration during spontaneous breathing down to 0.68 ± 0.07 duringinspiratory efforts at total lung capacity. The muscle bundles werefound to have the shape of circular arcs, with the three bundlesforming a section of a right circular cylinder. With increasing lungvolume and diaphragm displacement, the circular arcs rotate around theline of insertion on the chest wall, the arcs shorten, but the radiusof curvature remains nearly constant. Maximal transdiaphragmaticpressure was calculated from muscle curvature and maximaltension-length data from the literature. The calculated maximaltransdiaphragmatic pressure-length curve agrees well with the data ofRoad et al. (J. Appl. Physiol. 60:63-67, 1986).

     

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.

     

Impact of resistance exercise during bed rest on skeletal muscle sarcopenia and myosin isoform distribution

Bamman, Marcas M., Mark S. F. Clarke, Daniel L. Feeback,Robert J. Talmadge, Bruce R. Stevens, Steven A. Lieberman, and MichaelC. Greenisen. Impact of resistance exercise during bed rest onskeletal muscle sarcopenia and myosin isoform distribution. J. Appl. Physiol. 84(1): 157-163, 1998.Because resistance exercise (REx) and bed-rest unloading (BRU)are associated with opposing adaptations, our purpose was to test theefficacy of REx against the effects of 14 days of BRU on theknee-extensor muscle group. Sixteen healthy men were randomly assignedto no exercise (NoEx; n = 8) or REx(n = 8). REx performed five sets ofleg press exercise with 80-85% of one repetition maximum (1 RM)every other day during BRU. Muscle samples were removed from the vastuslateralis muscle by percutaneous needle biopsy. Myofiber distributionwas determined immunohistochemically with three monoclonal antibodiesagainst myosin heavy chain (MHC) isoforms (I, IIa, IIx). MHCdistribution was further assessed by quantitative gel electrophoresis.Dynamic 1-RM leg press and unilateral maximum voluntary isometriccontraction (MVC) were determined. Maximal neural activation (root meansquared electromyogram) and rate of torque development (RTD) weremeasured during MVC. Reductions(P < 0.05) in type I (15%) and typeII (17%) myofiber cross-sectional areas were found in NoEx but not inREx. Electrophoresis revealed no changes in MHC isoform distribution. The percentage of type IIx myofibers decreased(P < 0.05) in REx from 9 to 2% anddid not change in NoEx. 1 RM was reduced(P < 0.05) by 9% in NoEx but wasunchanged in REx. MVC fell by 15 and 13% in NoEx and REx,respectively. The agonist-to-antagonist root mean squaredelectromyogram ratio decreased (P < 0.05) 19% in REx. RTD slowed (P < 0.05) by 54% in NoEx only. Results indicate that REx preventedBRU-induced myofiber atrophy and also maintained training-specificstrength. Unlike spaceflight, BRU did not induce shifts in myosinphenotype. The reported benefits of REx may prove useful in prescribingexercise for astronauts in microgravity.