Cadaver validation of skeletal muscle measurement by magnetic resonance imaging and computerized tomography

Magnetic resonance imaging (MRI) and computerizedtomography (CT) are promising reference methods for quantifying wholebody and regional skeletal muscle mass. Earlier MRI and CTvalidation studies used data-acquisition techniques and data-analysisprocedures now outdated, evaluated anatomic rather than adiposetissue-free skeletal muscle (ATFSM), studied only the relatively largethigh, or found unduly large estimation errors. The aim ofthe present study was to compare arm and leg ATFSM cross-sectional areaestimates (cm²) by usingstandard MRI and CT acquisition and image-analysis methods withcorresponding cadaver estimates. A second objective was to validate MRIand CT measurements of adipose tissue embedded within muscle(interstitial adipose tissue) and surrounding muscle (subcutaneousadipose tissue). ATFSM area (n = 119)by MRI [38.9 ± 22.3 (SD)cm²], CT (39.7 ± 22.8 cm²), and cadaver (39.5 ± 23.0 cm²) were not different(P > 0.001), and both MRI and CTestimates of ATFSM were highly correlated with corresponding cadavervalues [MRI: r = 0.99, SE of estimate (SEE) 3.9 cm²,P < 0.001; and CT:r = 0.99, SEE = 3.8 cm²,P < 0.001].Similarly good results were observed between MRI- and CT-measured vs.cadaver-measured interstitial and subcutaneous adipose tissue. ForMRI-ATFSM the intraobserver correlation for duplicate measurements invivo was 0.99 [SEE = 8.7 cm²(2.9%), P < 0.001]. Thesefindings strongly support the use of MRI and CT as reference methodsfor appendicular skeletal muscle, interstitial and subcutaneous adiposetissue measurement in vivo.

     

Muscle quality. I.Age-associated differences between arm and leg muscle groups

To determine the differences between armand leg muscle quality (MQ) across the adult life span in men andwomen, concentric (Con) and eccentric (Ecc) peak torque (PT) weremeasured in 703 subjects (364 men and 339 women, age range 19-93yr) and appendicular skeletal muscle mass (MM) was determined in thearm and leg in a subgroup of 502 of these subjects (224 men and 278 women). Regression analysis showed that MQ, defined as PT per unit ofMM, was significantly higher in the arm (~30%) than in the legacross age in both genders (P < 0.01). Arm and leg MQ declined at a similar rate with age in men,whereas leg MQ declined ~20% more than arm MQ with increasing age inwomen (P  0.01 andP < 0.05 for Con and Ecc PT,respectively). Moreover, the age-associated decrease in arm MQ wassteeper in men than in women whether Con or Ecc PT was used (bothP < 0.05). Arm MQ as determined byCon PT showed a linear age-related decline in men and women (28 and20%, respectively, P < 0.001),whereas arm MQ as determined by Ecc PT showed a linear age-relateddecline in men (25%, P < 0.001) butnot in women (not significant). In contrast, both genders exhibited anage-related quadratic decline in leg MQ as determined by Con PT(~40%) and Ecc PT (~25%; both P < 0.001), and the rate of decline was similar for men and women. ThusMQ is affected by age and gender, but the magnitude of this effectdepends on the muscle group studied and the type of muscle action (Convs. Ecc) used to assess strength.

     

Effects of resistance training and chromium picolinate on body composition and skeletal muscle in older men

The effects of chromium picolinate (CrPic)supplementation and resistance training (RT) on skeletal muscle size,strength, and power and whole body composition were examined in 18 men(age range 56-69 yr). The men were randomly assigned(double-blind) to groups (n = 9) thatconsumed either 17.8 µmol Cr/day (924 µg Cr/day) as CrPic or alow-Cr placebo for 12 wk while participating twice weekly in ahigh-intensity RT program. CrPic increased urinary Cr excretion~50-fold (P < 0.001). RT-inducedincreases in muscle strength (P < 0.001) were not enhanced by CrPic. Arm-pull muscle power increased withRT at 20% (P = 0.016) but not at 40, 60, or 80% of the one repetition maximum, independent of CrPic.Knee-extension muscle power increased with RT at 20, 40, and 60%(P < 0.001) but not at 80% of onerepetition maximum, and the placebo group gained more muscle power thandid the CrPic group (RT by supplemental interaction,P < 0.05). Fat-free mass(P < 0.001), whole body muscle mass(P < 0.001), and vastus lateralistype II fiber area (P < 0.05)increased with RT in these body-weight-stable men, independent ofCrPic. In conclusion, high-dose CrPic supplementation did not enhancemuscle size, strength, or power development or lean body mass accretionin older men during a RT program, which had significant, independenteffects on these measurements.

     

Fiber type and citrate synthase activity in the human gastrocnemius and vastus lateralis with aging

The purpose ofthis study was to determine whether enzymatic and histochemicalcharacteristics of human skeletal muscle are altered with aging.Tissues from the vastus lateralis (VL) and gastrocnemius were analyzedfor citrate synthase (CS) activity and fiber type in 55 sedentary men(age range 18-80 yr). In this population, CS activity in thegastrocnemius was negatively related to age(r = 0.32,P < 0.05); there was no relationshipin the VL. Treadmill-determined maximal oxygen consumption waspositively related (r = 0.40, P < 0.05) to CS in the gastrocnemiusbut not in the VL. CS activity in the gastrocnemius was 24% lower inthe oldest (60 yr, n = 10) vs. theyoungest (30 yr; n = 12) men; therewas no change in CS activity in the VL with aging. No changes in fibertype were evident with age in either muscle. These data suggest areduction in oxidative enzyme activity in human skeletal muscle withthe aging process; this relationship may be muscle-group specific.

     

Skeletal muscle mass and the reduction of VO2 max in trained older subjects

Proctor, David N., and Michael J. Joyner. Skeletalmuscle mass and the reduction ofO_{2 max} in trainedolder subjects. J. Appl. Physiol.82(5): 1411-1415, 1997.The role of skeletal muscle mass in theage-associated decline in maximalO₂ uptake (O_{2 max}) is poorlydefined because of confounding changes in muscle oxidative capacity andin body fat and the difficulty of quantifying active muscle mass duringexercise. We attempted to clarify these issues byexamining the relationship between several indexes of muscle mass, asestimated by using dual-energy X-ray absorptiometry and treadmillO_{2 max} in 32 chronically endurance-trained subjects from four groups(n = 8/group): young men(20-30 yr), older men (56-72 yr), young women(19-31 yr), and older women (51-72 yr).O_{2 max} per kilogrambody mass was 26 and 22% lower in the older men (45.9 vs. 62.0 ml · kg¹ · min¹)and older women (40.0 vs. 51.5 ml · kg¹ · min¹).These age differences were reduced to 14 and 13%, respectively, whenO_{2 max} was expressedper kilogram of appendicular muscle. When appropriately adjusted forage and gender differences in appendicular muscle mass by analysis ofcovariance, whole body O_{2 max} was 0.50 ± 0.09 l/min less (P < 0.001) in theolder subjects. This effect was similar in both genders.These findings suggest that the reducedO_{2 max} seen in highlytrained older men and women relative to their younger counterparts isdue, in part, to a reduced aerobic capacity per kilogram of activemuscle independent of age-associated changes in body composition, i.e.,replacement of muscle tissue by fat. Because skeletal muscleadaptations to endurance training can be well maintained in oldersubjects, the reduced aerobic capacity per kilogram of muscle likelyresults from age-associated reductions in maximalO₂ delivery (cardiac outputand/or muscle blood flow).

     

Phosphorylation of p70S6k correlates with increased skeletal muscle mass following resistance exercise

High-resistance exercise training results in an increase inmuscle wet mass and protein content. To begin to address the acute changes following a single bout of high-resistance exercise, a newmodel has been developed. Training rats twice a week for 6 wk resultedin 13.9 and 14.4% hypertrophy in the extensor digitorum longus (EDL)and tibialis anterior (TA) muscles, respectively. Polysome profilesafter high-resistance lengthening contractions suggest that the rate ofinitiation is increased. The activity of the 70-kDa S6 protein kinase(p70^S6k), a regulator oftranslation initiation, is also increased following high-resistancelengthening contractions (TA, 363 ± 29%; EDL, 353 ± 39%).Furthermore, the increase inp70^S6k activity 6 h after exercisecorrelates with the percent change in muscle mass after 6 wk oftraining (r = 0.998). The tightcorrelation between the activation ofp70^S6k and the long-term increasein muscle mass suggests thatp70^S6k phosphorylation may be agood marker for the phenotypic changes that characterize musclehypertrophy and may play a role in load-induced skeletal muscle growth.

     

Human skeletal muscle carnitine palmitoyltransferase I activity determined in isolated intact mitochondria

This study was designed to compare theactivity of skeletal muscle carnitine palmitoyltransferase I (CPT I) intrained and inactive men (n = 14) andwomen (n = 12). CPT Iactivity was measured in intact mitochondria, isolated from needlebiopsy vastus lateralis muscle samples (~60 mg). The variability ofCPT I activity determined on two biopsy samples from the same leg onthe same day was 4.4, whereas it was 7.0% on two biopsy samples fromthe same leg on different days. The method was sensitive to the CPT Iinhibitor malonyl-CoA (88% inhibition) and therefore specific for CPTI activity. The mean CPT I activity for all 26 subjects was 141.1 ± 10.6 µmol · min¹ · kgwet muscle (wm)¹ and wasnot different when all men vs. all women (140.5 ± 15.7 and 142.2 ± 14.5 µmol · min¹ · kgwm¹, respectively) were compared. However, CPT Iactivity was significantly higher in trained vs. inactive subjects forboth men (176.2 ± 21.1 vs. 104.1 ± 13.6 µmol · min¹ · kgwm¹) and women (167.6 ± 14.1 vs. 91.2 ± 9.5 µmol · min¹ · kgwm¹). CPT I activity was also significantly correlatedwith citrate synthase activity (all subjects,r = 0.76) and maximal oxygen consumption expressed in milliliters per kilogram per minute (all subjects, r = 0.69). Theresults of this study suggest that CPT I activity can be accurately andreliably measured in intact mitochondria isolated from human musclebiopsy samples. CPT I activity was not affected by gender, and higheractivities in aerobically trained subjects appeared to be the result ofincreased mitochondrial content in both men and women.

     

Resistance exercise training reduces hypertriglyceridemia in HIV-infected men treated with antiviral therapy

Hypertriglyceridemia, peripheral insulin resistance,and trunk adiposity are metabolic complications recently recognized in people infected with human immunodeficiency virus (HIV) and treated with highly active antiretroviral therapy (HAART). These complications may respond favorably to exercise training. Using a paired design, wedetermined whether 16 wk of weight-lifting exercise increased musclemass and strength and decreased fasting serum triglycerides and adiposetissue mass in 18 HIV-infected men. The resistance exercise regimenconsisted of three upper and four lower body exercises done for1-1.5 h/day, 4 days/wk for 64 sessions. Dual-energy X-rayabsorptiometry indicated that exercise training increased whole bodylean mass 1.4 kg (P = 0.005) but did not reduce adipose tissue mass (P = NS). Axial proton-magnetic resonanceimaging indicated that thigh muscle cross-sectional area increased5-7 cm² (P < 0.005). Muscle strengthincreased 23-38% (P < 0.0001) on all exercises.Fasting serum triglycerides were decreased at the end of training(281-204 mg/dl; P = 0.02). These findings imply that resistance exercise training-induced muscle hypertrophy may promote triglyceride clearance from the circulation ofhypertriglyceridemic HIV-infected men treated with antiviral therapy.

     

Comparisons of two-, three-, and four-compartment models of body composition analysis in men and women

This study compared the traditionaltwo-compartment (fat mass or FM; fat free mass or FFM)hydrodensitometric method of body composition measurement, which isbased on body density, with three (FM, total body water or TBW, fatfree dry mass)- and four (FM, TBW, bone mineral mass or BMM,residual)-compartment models in highly trained men(n = 12), sedentary men(n = 12), highly trained women(n = 12), and sedentary women(n = 12). The means andvariances for the relative body fat (%BF) differences between the two-and three-compartment models [2.2 ± 1.6 (SD) % BF;n = 48] were significantlygreater (P  0.02) than those between the three- and four-compartment models (0.2 ± 0.3% BF;n = 48) for all four groups. Thethree-compartment model is more valid than the two-compartmenthydrodensitometric model because it controls for biological variabilityin TBW, but additional control for interindividual variability in BMMvia the four-compartment model achieves little extra accuracy. Thecombined group (n = 48) exhibited greater (P < 0.001) FFM densities(1.1075 ± 0.0049 g/cm³) thanthe hydrodensitometric assumption of 1.1000 g/cm³, which is based on analysesof three male cadavers aged 25, 35, and 46 yr. This was primarilybecause their FFM hydration (72.4 ± 1.1%;n = 48) was lower(P  0.001) than thehydrodensitometric assumption of 73.72%.

     

Oxidation of lactate and acetate in rat skeletal muscle: analysis by 13C-nuclear magnetic resonance spectroscopy

Bertocci, Loren A., John G. Jones, Craig R. Malloy, RonaldG. Victor, and Gail D. Thomas. Oxidation of lactateand acetate in rat skeletal muscle: analysis by¹³C-nuclear magnetic resonancespectroscopy. J. Appl. Physiol. 83(1): 32-39, 1997.The balance between carbohydrate and fatty acidutilization in skeletal muscle previously has been studied in vivo byusing a variety of methods such as arteriovenous concentrationdifferences and radioactive isotope tracer techniques. However, thesemethodologies provide only indirect estimates of substrate oxidation.We used ¹³C-nuclear magneticresonance (NMR) spectroscopy and non-steady-state isotopomer analysisto directly quantify the relative oxidation of two competing exogenoussubstrates in rat skeletal muscles. We infused[1,2-¹³C]acetate and[3-¹³C]lactateintravenously in anesthetized rats during the final 30 min of 35 (n = 10) or 95 (n = 10) min of intense, unilateral, rhythmic hindlimb contractions.¹³C-NMR spectroscopy andisotopomer analysis were performed on extracts of gastrocnemius andsoleus muscles from both the contracting and contralateralresting hindlimbs. We found that1)[¹³C]lactate and[¹³C]acetate were taken up and oxidized by both restingand contracting skeletal muscles; and2) high-intensity musclecontractions altered the pattern of substrate utilization such that therelative oxidation of acetate decreased while that of lactate remainedunchanged or increased. Based on these findings, we propose that¹³C-NMR spectroscopy incombination with isotopomer analysis can be used to study the generaldynamics of substrate competition between carbohydrates and fats in ratskeletal muscle.

     

Strength, skeletal muscle composition, and enzyme activity in multiple sclerosis

Kent-Braun, J. A., A. V. Ng, M. Castro, M. W. Weiner, D. Gelinas, G. A. Dudley, and R. G. Miller. Strength, skeletal musclecomposition and enzyme activity in multiple sclerosis. J. Appl. Physiol. 83(6):1998-2004, 1997.This study examined functional, biochemical, andmorphological characteristics of skeletal muscle in nine multiplesclerosis (MS) patients and eight healthy controls in an effort toascertain whether intramuscular adaptations could account for excessivefatigue in this disease. Analyses of biopsies of the tibialis anteriormuscle showed that there were fewer type I fibers (66 ± 6 vs. 76 ± 6%), and that fibers of all types were smaller (average26%) and had lower succinic dehydrogenase (SDH; average40%) and SDH/-glycerol-phosphate dehydrogenase (GPDH) butnot GPDH activities in MS vs. control subjects, suggesting that musclein this disease is smaller and relies more on anaerobic thanaerobic-oxidative energy supply than does muscle of healthyindividuals. Maximal voluntary isometric force fordorsiflexion was associated with both average fiber cross-sectionalarea (r = 0.71, P = 0.005) and muscle fat-free cross-sectional area by magnetic resonance imaging(r = 0.80, P < 0.001). Physical activity,assessed by accelerometer, was associated with average fiber SDH/GPDH(r = 0.78, P = 0.008). There was a tendency forsymptomatic fatigue to be inversely associated with average fiber SDHactivity (r = 0.57,P = 0.068). The results of thisstudy suggest that the inherent characteristics of skeletal musclefibers per se and of skeletal muscle as a whole are altered in thedirection of disuse in MS. They also suggest that changes in skeletalmuscle in MS may significantly affect function.

     

Lung resistance and elastance in spontaneously breathing preterm infants: effects of breathing pattern and demographics

Reported values of lung resistance(RL) and elastance (EL) in spontaneouslybreathing preterm neonates vary widely. We hypothesized that thisvariability in lung properties can be largely explained by both inter-and intrasubject variability in breathing pattern and demographics.Thirty-three neonates receiving nasal continuous positive airwaypressure [weight 606-1,792 g, gestational age (GA) of25-33 wk, 2-49 days old] were studied. Transpulmonary pressure was measured by esophageal manometry and airway flow by facemask pneumotachography. Breath-to-breath changes in RL andEL in each infant were estimated by Fourier analysis ofimpedance (Z) and by multiple linear regression (MLR).RL_MLR (RL_MLR = 0.85 × RL_Z 0.43; r² = 0.95) and EL_MLR(EL_MLR = 0.97 × EL_Z + 8.4; r² = 0.98) werehighly correlated to RL_Z andEL_Z, respectively. Both RL(mean ± SD; RL_Z = 70 ± 38, RL_MLR = 59 ± 36 cmH₂O · s · l¹)and EL (EL_Z = 434 ± 212, EL_MLR = 436 ± 210 cmH₂O/l)exhibited wide intra- and intersubject variability.Regardless of computation method, RL was found to decreaseas a function of weight, age, respiratory rate (RR), and tidal volume(VT) whereas it increased as a function ofRR · VT and inspiratory-to-expiratorytime ratio (TI/TE). EL decreasedwith increasing weight, age, VT and female gender andincreased as RR and TI/TE increased. Weconclude that accounting for the effects of breathing patternvariability and demographic parameters on estimates of RLand EL is essential if they are to be of clinical value.Multivariate statistical models of RL and ELmay facilitate the interpretation of lung mechanics measurements inspontaneously breathing infants.

     

Normalized metabolic stress for 31P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume

Fowler, M. D., T. W. Ryschon, R. E. Wysong, C. A. Combs, andR. S. Balaban. Normalized metabolic stress for³¹P-MR spectroscopy studies ofhuman skeletal muscle: MVC vs. muscle volume. J. Appl.Physiol. 83(3): 875-883, 1997.A criticalrequirement of submaximal exercise tests is the comparability ofworkload and associated metabolic stress between subjects. In thisstudy, ³¹P-magnetic resonancespectroscopy was used to estimate metabolic strain in the soleus muscleduring dynamic, submaximal plantar flexion in which target torque was10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy,normally active adults, (PCr + P_i)/PCr, where PCr isphosphocreatine, was highly correlated with power output normalized tothe volume of muscle in the plantar flexor compartment(r = 0.89, P < 0.001). The same variable was also correlated, although less strongly(r = 0.78, P < 0.001), with power normalized toplantar flexor cross-sectional area. These findings suggest thatcomparable levels of metabolic strain can be obtained in subjects ofdifferent size when the power output, or stress, for dynamic plantarflexion is selected as a function of plantar flexor muscle volume. Incontrast, selecting power output as a function of MVC resulted in apositive linear relationship between (PCr + P_i)/PCr and thetorque produced, indicating that metabolic strain was increasing ratherthan achieving constancy as a function of MVC. These findings providenew insight into the design of dynamic muscle contraction protocolsaimed at detecting metabolic differences between subjects of differentbody size but having similar blood flow capacity and mitochondrialvolume per unit of muscle.

     

Muscle quality. II. Effects of strength training in 65-to 75-yr-old men and women

To determine theeffects of strength training (ST) on muscle quality (MQ,strength/muscle volume of the trained muscle group), 12 healthy oldermen (69 ± 3 yr, range 65-75 yr) and 11 healthy older women (68 ± 3 yr, range 65-73 yr) were studied before and after aunilateral leg ST program. After a warm-up set, four sets ofheavy-resistance knee extensor ST exercise were performed 3 days/wk for9 wk on the Keiser K-300 leg extension machine. The men exhibitedgreater absolute increases in the knee extension one-repetition maximum(1-RM) strength test (75 ± 2 and 94 ± 3 kg before andafter training, respectively) and in quadriceps muscle volume measuredby magnetic resonance imaging (1,753 ± 44 and 1,955 ± 43 cm³) than the women (42 ± 2 and 55 ± 3 kg for the 1-RM test and 1,125 ± 53 vs.1,261 ± 65 cm³ forquadriceps muscle volume before and after training, respectively, inwomen; both P < 0.05). However,percent increases were similar for men and women in the 1-RM test (27 and 29% for men and women, respectively), muscle volume (12% forboth), and MQ (14 and 16% for men and women, respectively).Significant increases in MQ were observed in both groups in the trainedleg (both P < 0.05) and in the 1-RMtest for the untrained leg (both P < 0.05), but no significant differences were observed between groups,suggesting neuromuscular adaptations in both gender groups. Thus,although older men appear to have a greater capacity for absolutestrength and muscle mass gains than older women in response to ST, the relative contribution of neuromuscular and hypertrophic factors to theincrease in strength appears to be similar between genders.

     

Resistance exercise maintains skeletal muscle protein synthesis during bed rest

Ferrando, Arny A., Kevin D. Tipton, Marcas M. Bamman, andRobert R. Wolfe. Resistance exercise maintains skeletal muscle protein synthesis during bed rest. J. Appl.Physiol. 82(3): 807-810, 1997.Spaceflightresults in a loss of lean body mass and muscular strength. Aground-based model for microgravity, bed rest, results in a loss oflean body mass due to a decrease in muscle protein synthesis (MPS).Resistance training is suggested as a proposed countermeasure forspaceflight-induced atrophy because it is known to increase both MPSand skeletal muscle strength. We therefore hypothesized that scheduledresistance training throughout bed rest would ameliorate the decreasein MPS. Two groups of healthy volunteers were studied during 14 days ofsimulated microgravity. One group adhered to strict bed rest (BR;n = 5), whereas a second group engagedin leg resistance exercise every other day throughout bed rest (BREx;n = 6). MPS was determined directly bythe incorporation of infusedL-[ring-¹³C₆]phenylalanineinto vastus lateralis protein. After 14 days of bed rest, MPS in theBREx group did not change and was significantly greater than in the BRgroup. Thus moderate-resistance exercise can counteract the decrease inMPS during bed rest.

     

Predicting body cell mass with bioimpedance by using theoretical methods: a technological review

De Lorenzo, A., A. Andreoli, J. Matthie, and P. Withers.Predicting body cell mass with bioimpedance by using theoretical methods: a technological review. J. Appl.Physiol. 82(5): 1542-1558, 1997.The body cellmass (BCM), defined as intracellular water (ICW), was estimated in 73 healthy men and women by total body potassium (TBK) and by bioimpedancespectroscopy (BIS). In 14 other subjects, extracellular water (ECW) andtotal body water (TBW) were measured by bromide dilution and deuteriumoxide dilution, respectively. For all subjects, impedance spectral datawere fit to the Cole model, and ECW and ICW volumes were predicted byusing model electrical resistance terms R_E andR_I in an equation derived from Hanai mixture theory,respectively. The BIS ECW prediction bromide dilution wasr = 0.91, standard error of theestimate (SEE) 0.90 liter. The BIS TBW prediction of deuterium spacewas r = 0.95, SEE 1.33 liters. The BISICW prediction of the dilution-determined ICW wasr = 0.87, SEE 1.69 liters. The BIS ICWprediction of the TBK-determined ICW for the 73 subjects wasr = 0.85, SEE = 2.22 liters. Theseresults add further support to the validity of the Hanai theory, theequation used, and the conclusion that ECW and ICW volume can bepredicted by an approach based solely on fundamental principles.

     

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.

     

Training, muscle volume, and energy expenditure in nonobese American girls

Little is known about the relationship among training,energy expenditure, muscle volume, and fitness in prepubertalgirls. Because physical activity is high in prepubertalchildren, we hypothesized that there would be no effect of training.Forty pre- and early pubertal (mean age 9.1 ± 0.1 yr) nonobesegirls enrolled in a 5 day/wk summer school program for 5 wk and were randomized to control (n = 20) or training groups(n = 20; 1.5 h/day, endurance-type exercise). Totalenergy expenditure (TEE) was measured using doubly labeled water, thighmuscle volume using magnetic resonance imaging, and peak O₂uptake (O_2 peak) using cycle ergometry.TEE was significantly greater (17%, P < 0.02) in thetraining girls. Training increased thigh muscle volume (+4.3 ± 0.9%, P < 0.005) andO_2 peak (+9.5 ± 6%,P < 0.05), effects surprisingly similar to thoseobserved in adolescent girls using the same protocol (Eliakim A,Barstow TJ, Brasel JA, Ajie H, Lee W-NP, Renslo R, Berman N, and CooperDM, J Pediatr 129: 537-543, 1996). We furthercompared these two sample populations: thigh muscle volume per weightwas much lower in adolescent compared with prepubertal girls (17.0 ± 0.3 vs. 27.8 ± 0.6 ml/kg body mass; P < 0.001), and allometric analysis revealed remarkably low scaling factorsrelating muscle volume (0.34 ± 0.05, P < 0.0001), TEE (0.24 ± 0.06, P < 0.0004), andO_2 peak (0.28 ± 0.07, P < 0.0001) to body mass in all subjects. Muscle andcardiorespiratory functions were quite responsive to brief training inprepubertal girls. Moreover, a retrospective, cross-sectional analysissuggests that increases in muscle mass andO_2 peak may be depressed in nonobeseAmerican girls as they mature.

     

Interrelationship of oxidative metabolism and local perfusion demonstrated by NMR in human skeletal muscle

Toussaint, Jean-François, Kenneth K. Kwong, FidelisM'Kparu, Robert M. Weisskoff, Paul J. LaRaia, and Howard L. Kantor. Interrelationship of oxidative metabolism and local perfusion demonstrated by NMR in human skeletal muscle. J. Appl.Physiol. 81(5): 2221-2228, 1996.Using nuclearmagnetic resonance (NMR), we have examined the relationship ofhigh-energy phosphate metabolism and perfusion in human soleus andgastrocnemius muscles. With ³¹P-NMR spectroscopy, we monitoredphosphocreatine (PCr) decay and recovery in eight normal volunteers andfour heart failure patients performing ischemic plantar flexion. Byusing echo-planar imaging, perfusion was independently measured by alocal [inversion-recovery (T1-flow)] and a regionaltechnique (NMR-plethysmography). After correction for its pHdependence, PCr recovery time constant is 27.5 ± 8.0 s innormal volunteers, with mean flow 118 ± 75 (soleus andgastrocnemius T1-flow) and 30.2 ± 9.7 ml ^· 100 ml^{1 ·} min¹(NMR-plethysmography-flow). We demonstrate a positive correlation between PCr time constant and local perfusion given byy = 50  0.15x(r² = 0.68, P = 0.01) for the 8 normal subjects,and y = 64  0.24x (r² = 0.83, P = 0.0001) for the 12 subjectsrecruited in the study. Regional perfusion techniques also show asignificant but weaker correlation. Using this totally noninvasivemethod, we conclude that aerobic ATP resynthesis is related to themagnitude of perfusion, i.e., O₂availability, and demonstrate that magnetic resonance imaging andmagnetic resonance spectroscopy together can accurately assess musclefunctional status.

     

Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training

McCall, G. E., W. C. Byrnes, A. Dickinson, P. M. Pattany,and S. J. Fleck. Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training.J. Appl. Physiol. 81(5):2004-2012, 1996.Twelve male subjects with recreationalresistance training backgrounds completed 12 wk of intensifiedresistance training (3 sessions/wk; 8 exercises/session; 3 sets/exercise; 10 repetitions maximum/set). All major muscle groupswere trained, with four exercises emphasizing the forearm flexors.After training, strength (1-repetition maximum preacher curl) increasedby 25% (P < 0.05). Magneticresonance imaging scans revealed an increase in the biceps brachiimuscle cross-sectional area (CSA) (from 11.8 ± 2.7 to 13.3 ± 2.6 cm²;n = 8;P < 0.05). Muscle biopsies of thebiceps brachii revealed increases(P < 0.05) in fiber areas for type I(from 4,196 ± 859 to 4,617 ± 1,116 µm²;n = 11) and II fibers (from 6,378 ± 1,552 to 7,474 ± 2,017 µm²;n = 11). Fiber number estimated fromthe above measurements did not change after training (293.2 ± 61.5 × 10³ pretraining; 297.5 ± 69.5 × 10³ posttraining;n = 8). However, the magnitude ofmuscle fiber hypertrophy may influence this response because thosesubjects with less relative muscle fiber hypertrophy, but similarincreases in muscle CSA, showed evidence of an increase in fibernumber. Capillaries per fiber increased significantly(P < 0.05) for both type I(from 4.9 ± 0.6 to 5.5 ± 0.7;n = 10) and II fibers (from 5.1 ± 0.8 to 6.2 ± 0.7; n = 10). Nochanges occurred in capillaries per fiber area or muscle area. Inconclusion, resistance training resulted in hypertrophy of the totalmuscle CSA and fiber areas with no change in estimated fiber number,whereas capillary changes were proportional to muscle fiber growth.