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.

     

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.

     

Rapid determination of glomerular filtration rate by single-bolus inulin: a comparison of estimation analyses

Rapid measurement of glomerular filtration rate(GFR) by an inulin single-bolus technique would be useful, but itsaccuracy has been questioned. We hypothesized that reportedinaccuracies reflect the use of inappropriate mathematical models. GFRwas measured in 14 intact and 5 unilaterally nephrectomized conscious male Sprague-Dawley rats (mean weight 368 ± 12 g) by bothsingle-bolus (25 mg/kg) and constant-infusion techniques (0.693 mg · kg¹ · min¹).The temporal decline in plasma inulin concentration was analyzed through biexponential curve fitting, which accounted for renal inulinloss before complete vascular and interstitial mixing. We compared ourmathematical model based on empirical rationale with those of otherinvestigators whose studies suggest inaccuracy of single-bolus methods.Our mathematical model yielded GFR values by single bolus that agreedwith those obtained by constant infusion [slope = 0.94 ± 0.16 (SE); y intercept = 0.23 ± 0.64; r = 0.82]. Incomparison to the data obtained by constant inulin infusion, thismethod yielded a very small bias of 0.0041 ± 0.19 ml/min. Two previously reported models yielded unsatisfactory values (slope = 1.46 ± 0.34, y intercept = 0.47 ± 1.5, r = 0.72; and slope = 0.17 ± 1.26, y intercept = 17.15 ± 5.14, r = 0.03). The biases obtained byusing these methods were 2.21 ± 0.42 and 13.90 ± 1.44 ml/min, respectively. The data indicate that when appropriate mathematical models are used, inulin clearance after single-bolus delivery can be used to measure GFR equivalent to that obtained byconstant infusion of inulin. Attempts to use methods of analysis forsimplicity or expediency can result in unacceptable measurements relative to the clinical range of values seen.

     

Beat-to-beat modulation of heart rate is coupled to coronary perfusion pressure in the isolated heart

A goal ofclinicians caring for heart transplant recipients has been to use heartrate variability as a noninvasive means of diagnosing graftrejection. The determinants of beat-to-beat variability inthe surgically denervated heart have yet to be elucidated. We used anisolated, blood buffer-perfused porcine heart preparation toquantitatively assess the relationship between coronary perfusion andsinus node automaticity. Hearts(n = 9) were suspended in aLangendorff preparation, and heart rate (HR) fluctuations werequantified while perfusion pressure was modulated between 70/50, 80/60,90/70, and 100/80 mmHg at 0.067 Hz. In 32 of 32 recordings, the crossspectrum of perfusion pressure vs. HR showed the largest peak centeredat 0.067 Hz. In eight of nine experiments during nonpulsatileperfusion, HR accelerated as perfusion pressure was increased from 40 to 110 mmHg (mean increase 24.2 ± 3.0 beats/min). HR increased 0.34 beats/min per mmHg increase in perfusion pressure (least squares linearregression y = 25.8 mmHg + 0.34x;r = 0.88, P < 0.0001). Administration of low-and high-dose nitroglycerin (Ntg) resulted in a modest increase in flowbut produced a significant decrease in HR and blunted the response ofHR to changes in perfusion pressure (HR increase 0.26 beats · min¹ · mmHg¹,r = 0.87, P < 0.0001 after low-dose Ntg; 0.25 beats · min¹ · mmHg¹,r = 0.78, P < 0.0001 after high-dose Ntg).These experiments suggest that sinus node discharge in the isolatedperfused heart is mechanically coupled to perfusion pressure on abeat-to-beat basis.     

Expanded blood volumes contribute to the increased cardiovascular performance of endurance-trained older men

To determine whether expanded intravascular volumes contributeto the older athlete's higher exercise stroke volume and maximal oxygen consumption(O_{2 max}),we measured peak upright cycle ergometry cardiac volumes(^99mTc ventriculography) andplasma (¹²⁵I-labeled albumin) andred cell (NaCr⁵¹) volumes in 7 endurance-trained and 12 age-matched lean sedentary men. The athleteshad ~40% higherO_{2 max} values thandid the sedentary men and larger relative plasma (46 vs. 38 ml/kg), red cell (30 vs. 26 ml/kg), and total blood volumes (76 vs. 64 ml/kg) (allP < 0.05). Athletes hadlarger peak cycle ergometer exercise stroke volume indexes (75 vs. 57 ml/m²,P < 0.05) and 17% largerend-diastolic volume indexes. In the total group,O_{2 max}correlated with plasma, red cell, and total blood volumes(r = 0.61-0.70,P < 0.01). Peakexercise stroke volume was correlated directly with the blood volumevariables (r = 0.59-0.67,P < 0.01). Multiple regressionanalyses showed that fat-free mass and plasma or total blood volume,but not red cell volume, were independent determinants ofO_{2 max} andpeak exercise stroke volume. Plasma and total blood volumes correlated with the stroke volume and end-diastolic volume changes from rest topeak exercise. This suggests that expanded intravascular volumes, particularly plasma and total blood volumes, contribute to the higherpeak exercise left ventricular end-diastolic volume, stroke volume, andcardiac output and hence the higherO_{2 max} in master athletes by eliciting both chronic volume overload and increased utilization of the Frank-Starling effect during exercise.

     

Cardiovascular adaptations to 10days of cycle exercise

Mier, Constance M., Michael J. Turner, Ali A. Ehsani, andRobert J. Spina. Cardiovascular adaptations to 10 days of cycleexercise. J. Appl. Physiol. 83(6):1900-1906, 1997.We hypothesized that 10 days of training wouldenhance cardiac output (CO) and stroke volume (SV) during peak exerciseand increase the inotropic response to -adrenergic stimulation. Tensubjects [age 26 ± 2 (SE) yr] trained on a cycleergometer for 10 days. At peak exercise, training increasedO₂ uptake, CO, and SV(P < 0.001). Left ventricular (LV)size and function at rest were assessed with two-dimensional echocardiography before (baseline) and after atropine injection (1.0 mg) and during four graded doses of dobutamine. LV end-diastolic diameter increased with training (P < 0.02), whereas LV wall thickness was unchanged. LV contractileperformance was assessed by relating fractional shortening (FS) to theestimated end-systolic wall stress(_ES). Training increased theslope of the FS-_ES relationship (P < 0.05), indicating enhancedsystolic function. The increase in slope correlated with increases inCO (r = 0.71,P < 0.05) and SV(r = 0.70,P < 0.05). The increase in bloodvolume also correlated with increases in CO(r = 0.80, P < 0.01) and SV (r = 0.85, P < 0.004). These datashow that 10 days of training enhance the inotropic response to-adrenergic stimulation, associated with increases in CO and SVduring peak exercise.

     

Testosterone and cortisol in relationship to dietary nutrients and resistance exercise

Volek, Jeff S., William J. Kraemer, Jill A. Bush, ThomasIncledon, and Mark Boetes. Testosterone and cortisol inrelationship to dietary nutrients and resistance exercise.J. Appl. Physiol. 82(1): 49-54, 1997.Manipulation of resistance exercise variables (i.e., intensity,volume, and rest periods) affects the endocrine response to exercise;however, the influence of dietary nutrients on basal andexercise-induced concentrations of hormones is less understood. Thepresent study examined the relationship between dietary nutrients andresting and exercise-induced blood concentrations of testosterone (T)and cortisol (C). Twelve men performed a bench press exercise protocol(5 sets to failure using a 10-repetitions maximum load) and a jumpsquat protocol (5 sets of 10 repetitions using 30% of each subject's1-repetition maximum squat) with 2 min of rest between all sets. Ablood sample was obtained at preexercise and 5 min postexercise fordetermination of serum T and C. Subjects also completed detaileddietary food records for a total of 17 days. There was a significant(P  0.05) increase in postexercise Tcompared with preexercise values for both the bench press (7.4%) andjump squat (15.1%) protocols; however, C was not significantly different from preexercise concentrations. Significantcorrelations were observed between preexercise T and percent energyprotein (r = 0.71), percentenergy fat (r = 0.72), saturated fattyacids (g ^· 1,000 kcal^{1 ·} day¹;r = 0.77), monounsaturated fatty acids(g ^· 1,000 kcal^{1 ·} day¹;r = 0.79), the polyunsaturatedfat-to-saturated fat ratio (r = 0.63), and the protein-to-carbohydrate ratio (r = 0.59). There were nosignificant correlations observed between any nutritional variables andpreexercise C or the absolute increase in T and C after exercise. Thesedata confirm that high-intensity resistance exercise results inelevated postexercise T concentrations. A more impressive finding wasthat dietary nutrients may be capable of modulating restingconcentrations of T.

     

Association of chest wall motion and tidal volume responses during CO2 rebreathing

Yan, Sheng, Pawel Sliwinski, and Peter T. Macklem.Association of chest wall motion and tidal volume responses during CO₂ rebreathing.J. Appl. Physiol. 81(4):1528-1534, 1996.The purpose of this study is to investigate theeffect of chest wall configuration at end expiration on tidal volume(VT) response duringCO₂ rebreathing. In a group of 11 healthy male subjects, the changes in end-expiratory andend-inspiratory volume of the rib cage (Vrc,E andVrc,I, respectively) and abdomen (Vab,E and Vab,I, respectively) measured by linearizedmagnetometers were expressed as a function of end-tidalPCO₂(PET_CO2). The changes inend-expiratory and end-inspiratory volumes of the chest wall(Vcw,E and Vcw,I,respectively) were calculated as the sum of the respectiverib cage and abdominal volumes. The magnetometer coils were placed atthe level of the nipples and 1-2 cm above the umbilicus andcalibrated during quiet breathing against theVT measured from apneumotachograph. TheVrc,E/PET_CO2 slope was quite variable among subjects. It was significantly positive (P < 0.05) in fivesubjects, significantly negative in four subjects(P < 0.05), and not different fromzero in the remaining two subjects. TheVab,E/PET_CO2slope was significantly negative in all subjects(P < 0.05) with a much smallerintersubject variation, probably suggesting a relatively more uniformrecruitment of abdominal expiratory muscles and a variable recruitmentof rib cage muscles during CO₂rebreathing in different subjects. As a group, the meanVrc,E/PET_CO2,Vab,E/PET_CO2, andVcw,E/PET_CO2slopes were 0.010 ± 0.034, 0.030 ± 0.007, and0.020 ± 0.032 l / Torr, respectively;only theVab,E/PET_CO2 slope was significantly different from zero. More interestingly, theindividualVT/PET_CO2slope was negatively associated with theVrc,E/PET_CO2(r = 0.68,P = 0.021) and Vcw,E/PET_CO2slopes (r = 0.63,P = 0.037) but was not associated withtheVab,E/PET_CO2slope (r = 0.40, P = 0.223). There was no correlation oftheVrc,E/PET_CO2 andVcw,E/PET_CO2slopes with age, body size, forced expiratory volume in 1 s, orexpiratory time. The groupVab,I/PET_CO2 slope (0.004 ± 0.014 l / Torr) was not significantlydifferent from zero despite theVT nearly being tripled at theend of CO₂ rebreathing. Inconclusion, the individual VTresponse to CO₂, althoughindependent of Vab,E, is a function ofVrc,E to the extent that as theVrc,E/PET_CO2slope increases (more positive) among subjects, theVT response toCO₂ decreases. These results maybe explained on the basis of the respiratory muscle actions andinteractions on the rib cage.

     

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.

     

Endothelial cell dilatory pathways link flow and wall shear stress in an intact arteriolar network

Frame, Mary D. S., and Ingrid H. Sarelius. Endothelialcell dilatory pathways link flow and wall shear stress in an intactarteriolar network. J. Appl. Physiol.81(5): 2105-2114, 1996.Our purpose was to determine whether theendothelial cell-dependent dilatory pathways contribute to theregulation of flow distribution in an intact arteriolar network. Cellflow, wall shear stress (T),diameter, and bifurcation angle were determined for four sequentialbranches of a transverse arteriole in the superfused cremaster muscleof pentobaribtal sodium (Nembutal, 70 mg/kg)-anesthetized hamsters(n = 51). Control cell flow wassignificantly greater into upstream than into downstream branches[1,561 ± 315 vs. 971 ± 200 (SE) cells/s,n = 12]. Tissue exposure to 50 µMN-nitro-L-arginine + 50 µM indomethacin (L-NNA + Indo) produced arteriolar constriction of 14 ± 4% and decreasedflow into the transverse arteriole. More of the available cell flow wasdiverted to downstream branches, yet flow distribution remainedunequal. Control T was higherupstream than downstream (31.3 ± 6.8 vs. 9.8 ± 1.5 dyn/cm²).L-NNA + Indo decreasedT upstream and increasedT downstream to become equal inall branches, in contrast to flow. To determine whether constriction ingeneral induced the same changes, 5%O₂ (8 ± 4% constriction) or10⁹ M norepinephrine (NE;4 ± 3% constriction) was added to the tissue (n = 7). WithO₂, flow was redistributed tobecome equal into each branch. With NE, flow decreased progressivelymore into the first three branches. The changes in flow distributionwere thus predictable and dependent on the agonist. WithO₂ or NE, the spatial changes inflow were mirrored by spatial changes inT. Changes in diameter and incell flux were not related forL-NNA + Indo (r = 0.45),O₂(r = 0.07), or NE(r = 0.36). For all agonists, when thebifurcation angle increased, cell flow to the branch decreasedsignificantly, whereas if the angle decreased, flow was relativelypreserved; thus active changes in bifurcation angle may influence redcell distribution at arteriolar bifurcations. Thus, when theendothelial cell dilatory pathways were blocked, the changes in flowand in T were uncoupled; yet when they were intact, flowand T changed together.

     

"Living high-training low": effect of moderate-altitude acclimatization with low-altitude training on performance

Levine, Benjamin D., and James Stray-Gundersen."Living high-training low": effect of moderate-altitudeacclimatization with low-altitude training on performance.J. Appl. Physiol. 83(1): 102-112, 1997.The principal objective of this study was to test the hypothesisthat acclimatization to moderate altitude (2,500 m) plus training atlow altitude (1,250 m), "living high-training low," improvessea-level performance in well-trained runners more than an equivalentsea-level or altitude control. Thirty-nine competitive runners (27 men,12 women) completed 1) a 2-wklead-in phase, followed by 2) 4 wkof supervised training at sea level; and3) 4 wk of field training camprandomized to three groups: "high-low"(n = 13), living at moderate altitude(2,500 m) and training at low altitude (1,250 m); "high-high"(n = 13), living and training atmoderate altitude (2,500 m); or "low-low"(n = 13), living and training in amountain environment at sea level (150 m). A 5,000-m time trial was theprimary measure of performance; laboratory outcomes included maximalO₂ uptake(O_{2 max}), anaerobic capacity (accumulated O₂ deficit),maximal steady state (MSS; ventilatory threshold), running economy,velocity at O_{2 max}, and blood compartment volumes. Both altitude groups significantly increased O_{2 max}(5%) in direct proportion to an increase in red cell mass volume(9%; r = 0.37, P < 0.05), neither of which changedin the control. Five-kilometer time was improved by the field trainingcamp only in the high-low group (13.4 ± 10 s), in directproportion to the increase inO_{2 max}(r = 0.65, P < 0.01). Velocity atO_{2 max} andMSS also improved only in the high-low group. Four weeks of livinghigh-training low improves sea-level running performance in trainedrunners due to altitude acclimatization (increase in red cell massvolume and O_{2 max}) and maintenance of sea-level training velocities, mostlikely accounting for the increase in velocity atO_{2 max} and MSS.

     

Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise

Barstow, Thomas J., Andrew M. Jones, Paul H. Nguyen, andRichard Casaburi. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise.J. Appl. Physiol. 81(4):1642-1650, 1996.We tested the hypothesis that the amplitude ofthe additional slow component ofO₂ uptake(O₂) during heavy exerciseis correlated with the percentage of type II (fast-twitch) fibers inthe contracting muscles. Ten subjects performed transitions to a workrate calculated to require aO₂ equal to 50% betweenthe estimated lactate (Lac) threshold and maximalO₂ (50%).Nine subjects consented to a muscle biopsy of the vastus lateralis. Toenhance the influence of differences in fiber type among subjects,transitions were made while subjects were pedaling at 45, 60, 75, and90 rpm in different trials. Baseline O₂ was designed to besimilar at the different pedal rates by adjusting baseline work ratewhile the absolute increase in work rate above the baseline was thesame. The O₂ response after the onset of exercise was described by a three-exponential model. Therelative magnitude of the slow component at the end of 8-min exercisewas significantly negatively correlated with %type I fibers at everypedal rate (r = 0.64 to 0.83, P < 0.05-0.01). Furthermore,the gain of the fast component forO₂ (asml ^· min^{1 ·} W¹)was positively correlated with the %type I fibers across pedal rates(r = 0.69-0.83). Increase inpedal rate was associated with decreased relative stress of theexercise but did not affect the relationships between%fiber type and O₂parameters. The relative contribution of the slow component was alsosignificantly negatively correlated with maximalO₂(r = 0.65), whereas the gainfor the fast component was positively associated(r = 0.68-0.71 across rpm). Theamplitude of the slow component was significantly correlated with netend-exercise Lac at all four pedal rates(r = 0.64-0.84), but Lac was notcorrelated with %type I (P > 0.05).We conclude that fiber type distribution significantly affects both thefast and slow components ofO₂ during heavy exerciseand that fiber type and fitness may have both codependent andindependent influences on the metabolic and gas-exchange responses toheavy exercise.

     

Age and gender dependency of baroreflex sensitivity in healthy subjects

Laitinen, Tomi, Juha Hartikainen, Esko Vanninen, LeoNiskanen, Ghislaine Geelen, and Esko Länsimies. Age andgender dependency of baroreflex sensitivity in healthy subjects.J. Appl. Physiol. 84(2): 576-583, 1998.We evaluated the correlates of baroreflex sensitivity (BRS) inhealthy subjects. The study consisted of 117 healthy, normal-weight,nonsmoking male and female subjects aged 23-77 yr. Baroreflexcontrol of heart rate was measured by using the phenylephrinebolus-injection technique. Frequency- and time-domain analysis of heartrate variability and an exercise test were performed. Plasmanorepinephrine, epinephrine, insulin, and arginine vasopressinconcentrations and plasma renin activity were measured. In theunivariate analysis, BRS correlated with age(r = 0.65,P < 0.001), diastolic blood pressure(r = 0.47, P < 0.001), exercise capacity(r = 0.60, P < 0.001), and the high-frequency component of heart rate variability (r = 0.64, P < 0.001). There was also asignificant correlation between BRS and plasma norepinephrine concentration (r = 0.22,P < 0.05) and plasma renin activity (r = 0.32, P < 0.001). According to themultivariate analysis, age and gender were the most importantphysiological correlates of BRS. They accounted for 52% ofinterindividual BRS variation. In addition, diastolic blood pressureand high-frequency component of heart rate variability were significantindependent correlates of BRS. BRS was significantly higher in men thanin women (15.0 ± 1.2 vs. 10.2 ± 1.1 ms/mmHg, respectively;P < 0.01). Twenty-four percent ofwomen >40 yr old and 18% of men >60 yr old had markedly depressedBRS (<3 ms/mmHg). We conclude that physiological factors, particularly age and gender, have significant impact on BRS in healthysubjects. In addition, we demonstrate that BRS values that have beenproposed to be useful in identifying postinfarction patients at highrisk of sudden death are frequently found in healthy subjects.

     

Chest wall mechanics in sustained microgravity

We assessed theeffects of sustained weightlessness on chest wall mechanics in fiveastronauts who were studied before, during, and after the 10-daySpacelab D-2 mission (n = 3)and the 180-day Euromir-95 mission (n = 2). We measured flow and pressure at the mouth and rib cage andabdominal volumes during resting breathing and during a relaxationmaneuver from midinspiratory capacity to functional residual capacity.Microgravity produced marked and consistent changes () in thecontribution of the abdomen to tidal volume [Vab/(Vab + Vrc), where Vab is abdominal volume and Vrc is rib cagevolume], which increased from 30.7 ± 3.5 (SE)% at1 G head-to-foot acceleration to 58.3 ± 5.7% at 0 G head-to-foot acceleration (P < 0.005). Values ofVab/(Vab + Vrc) did not change significantly during the 180 days of the Euromir mission, but in the two subjects Vab/(Vab + Vrc) was greater on postflight day1 than on subsequent postflight days or preflight. Inthe two subjects who produced satisfactory relaxation maneuvers, the slope of the Konno-Mead plot decreased in microgravity; this decrease was entirely accounted for by an increase in abdominal compliance because rib cage compliance did not change. These alterations aresimilar to those previously reported during short periods ofweightlessness inside aircrafts flying parabolic trajectories. They arealso qualitatively similar to those observed on going from upright tosupine posture; however, in contrast to microgravity, such posturalchange reduces rib cage compliance.

     

Posttetanic potentiation of human dorsiflexors

O'Leary, Deborah D., Karen Hope, and Digby G. Sale.Posttetanic potentiation of human dorsiflexors.J. Appl. Physiol. 83(6):2131-2138, 1997.Twitch contractions of the ankle dorsiflexors were evoked before and after applied 7-s tetanic stimulation at 100 Hzin 20 young adults. Torque decreased 15% during the tetanus. At 5 safter tetanus, twitch peak torque had potentiated 45%. Potentiationdeclined to 28% after 1 min, rose slightly to 33% at 2 min, anddeclined slowly with potentiation still 25% after 5 min. There waslarge intersubject variation in the amount of potentiation(5-140%) and its persistence (5 to 20 min). The muscle compoundaction potential (M wave) did not change significantly (from pretetanicvalue) at 5 s after tetanus but increased sharply (26%) at 2 min andthen subsided. Twitch half relaxation time (23%) decreasedsignificantly more than twitch rise time (13%) 5 s after tetanus andrecovered more slowly. Twitch rates of torque development (75%) andrelaxation (71%) increased similarly 5 s after tetanus and were stillelevated (~25%) at 5 min. The extent of twitch torque potentiationwas significantly inversely correlated with pretetanic twitch rise time(r = 0.69), half relaxation time (r = 0.61), andtwitch-to-tetanus ratio (r = 0.66). The data indicate that posttetanic potentiation has agreater effect on twitch half relaxation time than on time to peaktorque and is more prominent in muscles with a short twitch time courseand small twitch-to-tetanus ratio.

     

In vivo validation of whole body composition estimates from dual-energy X-ray absorptiometry

Prior, Barry M., Kirk J. Cureton, Christopher M. Modlesky,Ellen M. Evans, Mark A. Sloniger, Michael Saunders, and Richard D. Lewis. In vivo validation of whole body composition estimates fromdual-energy X-ray absorptiometry. J. Appl.Physiol. 83(2): 623-630, 1997.We validated wholebody composition estimates from dual-energy X-ray absorptiometry (DEXA)against estimates from a four-component model to determine whetheraccuracy is affected by gender, race, athletic status, ormusculoskeletal development in young adults. Measurements of bodydensity by hydrostatic weighing, body water by deuterium dilution, andbone mineral by whole body DEXA were obtained in 172 young men(n = 91) and women(n = 81). Estimates of body fat(%Fat) from DEXA (%Fat_DEXA)were highly correlated with estimates of body fat from thefour-component model [body density, total body water, and totalbody mineral (%Fat_d,w,m);r = 0.94, standard error of theestimante (SEE) = 2.8% body mass (BM)] with no significantdifference between methods [mean of the difference ± SD ofthe difference = 0.4 ± 2.9 (SD) % BM,P = 0.10] in women and men. Onthe basis of the comparison with%Fat_d,w,m, estimates of%Fat_DEXA were slightly moreaccurate than those from body density(r = 0.91, SEE = 3.4%; mean of the difference ± SD of the difference = 1.2 ± 3.4% BM).Differences between %Fat_DEXA and%Fat_d,w,m were weakly related tobody thickness, as reflected by BMI (r = 0.34), and to the percentage of water in the fat-free mass(r = 0.51), but were notaffected by race, athletic status, or musculoskeletal development. Weconclude that body composition estimates from DEXA are accuratecompared with those from a four-component model in young adults whovary in gender, race, athletic status, body size, musculoskeletaldevelopment, and body fatness.

     

Crural diaphragm activation during dynamic contractions at various inspiratory flow rates

The purpose of this study was to evaluate the influence ofvelocity of shortening on the relationship between diaphragm activation and pressure generation in humans. This was achieved by relating theroot mean square (RMS) of the diaphragm electromyogram to thetransdiaphragmatic pressure (Pdi) generated during dynamic contractionsat different inspiratory flow rates. Five healthy subjects inspiredfrom functional residual capacity to total lung capacity at differentflow rates while reproducing identical Pdi and chest wall configurationprofiles. To change the inspiratory flow rate, subjects performed theinspirations while breathing across two different inspiratoryresistances (10 and 100 cmH₂O · l¹ · s),at mouth pressure targets of 10, 20, 40, and60 cmH₂O. The diaphragmelectromyogram was recorded and analyzed with control of signalcontamination and electrode positioning. RMS values obtained forinspirations with identical Pdi and chest wall configuration profileswere compared at the same percentage of inspiratory duration. Atinspiratory flows ranging between 0.1 and 1.4 l/s, there was nodifference in the RMS for the inspirations from functional residualcapacity to total lung capacity when Pdi and chest wall configurationprofiles were reproduced (n = 4). Athigher inspiratory flow rates, subjects were not able to reproducetheir chest wall displacements and adopted different recruitmentpatterns. In conclusion, there was no evidence for increased demand ofdiaphragm activation when healthy subjects breathe with similar chestwall configuration and Pdi profiles, at increasing flow rates up to 1.4 l/s.

     

Energy requirements and physical activity in free-living older women and men: a doubly labeled water study

Determinants of daily energy needs and physicalactivity are unknown in free-living elderly. This study examineddeterminants of daily total energy expenditure (TEE) andfree-living physical activity in older women(n = 51; age = 67 ± 6 yr) and men(n = 48; age = 70 ± 7 yr) by usingdoubly labeled water and indirect calorimetry. Usingmultiple-regression analyses, we predicted TEE by using anthropometric,physiological, and physical activity indexes. Data were collected onresting metabolic rate (RMR), body composition, peak oxygen consumption(O_{2 peak}),leisure time activity, and plasma thyroid hormone. Data adjusted forbody composition were not different between older women and men,respectively (in kcal/day): TEE, 2,306 ± 647 vs. 2,456 ± 666;RMR, 1,463 ± 244 vs. 1,378 ± 249; and physical activity energyexpenditure, 612 ± 570 vs. 832 ± 581. In a subgroup of 70 womenand men, RMR andO_{2 peak}explained approximately two-thirds of the variance in TEE(R² = 0.62;standard error of the estimate = ±348 kcal/day). Crossvalidation ofthis equation in the remaining 29 women and men was successful, with nodifference between predicted and measured TEE (2,364 ± 398 and2,406 ± 571 kcal/day, respectively). The strongest predictors ofphysical activity energy expenditure(P < 0.05) for womenand men were O_{2 peak}(r = 0.43), fat-free mass(r = 0.39), and body mass(r = 0.34). In summary, RMR andO_{2 peak} are importantindependent predictors of energy requirements in the elderly.Furthermore, cardiovascular fitness and fat-free mass are moderatepredictors of physical activity in free-living elderly.

     

Oxygen transport in conscious newborn dogs during hypoxic hypometabolism

We questioned whether the decrease inO₂ consumption(O₂) during hypoxia innewborns is a regulated response or reflects a limitation inO₂ availability. Experiments wereconducted on previously instrumented conscious newborn dogs.O₂ was measured at a warmambient temperature (30°C, n = 7)or in the cold (20°C, n = 6),while the animals breathed air or were sequentially exposed to 15 minof fractional inspired O₂(FI_O2): 21, 18, 15, 12, 10, 8, and 6%. In normoxia,O₂ averaged 15 ± 1 (SE)and 25 ± 1 ml · kg¹ · min¹in warm and cold conditions, respectively. In the warmcondition, hypometabolism (i.e., hypoxicO₂ < normoxicO₂) occurred at FI_O2 10%, whereas in thecold condition, hypometabolism occurred atFI_O2 12%. The sameresults were obtained in a separate group(n = 14) of noninstrumented puppies.For all levels of FI_O2 withhypometabolism, the relationships between measures ofO₂ availability (arterialO₂ saturation or content, venousPO₂ or saturation,x-axis) vs.O₂(y-axis) had lower slopes in warm than in coldconditions. Hence, O₂ during hypometabolism in the warm condition was not the maximal attainable for the level of oxygenation. The results do not support thepossibility that the hypoxic drop inO₂ in the newborn reflects a limitation in O₂availability. The results are compatible with the ideathat the phenomenon is one of "regulated conformism" tohypoxia.

     

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.