Determination of fascicle length and pennation in a contracting human muscle in vivo

Fukunaga, Tetsuo, Yoshiho Ichinose, Masamitsu Ito, YasuoKawakami, and Senshi Fukashiro. Determination of fascicle lengthand pennation in a contracting human muscle in vivo.J. Appl. Physiol. 82(1): 354-358, 1997.We have developed a technique to determine fascicle length inhuman vastus lateralis muscle in vivo by using ultrasonography. Whenthe subjects had the knee fully extended passively from a position of110° flexion (relaxed condition), the fascicle length decreasedfrom 133 to 97 mm on average. During static contractions at 10% ofmaximal voluntary contraction strength (tensed condition), fascicleshortening was more pronounced (from 126 to 67 mm), especially when theknee was closer to full extension. Similarly, as the knee was extended, the angle of pennation (fascicle angle, defined as the angle between fascicles and aponeurosis) increased (relaxed, from 14 to 18°; tensed, from 14 to 21°), and a greater increase in the pennation angle was observed in the tensed than in the relaxed condition when theknee was close to extension (<40°). We conclude that there aredifferences in fascicle lengths and pennation angles when the muscle isin a relaxed and isometrically tensed conditions and that thedifferences are affected by joint angles, at least at thesubmaximal contraction level.

     

Mechanical properties of the patellar tendon in adults and children

It is not currently known how the mechanical properties of human tendons change with maturation in the two sexes. To address this, the stiffness and Young's modulus of the patellar tendon were measured in men, women, boys and girls (each group, n=10). Patellar tendon force (F_pt) was calculated from the measured joint moment during a ramped voluntary isometric knee extension contraction, the antagonist knee extensor muscle co-activation quantified from its electromyographical activity, and the patellar tendon moment arm measured from magnetic resonance images. Tendon elongation was imaged using the sagittal-plane ultrasound scans throughout the contraction. Tendon cross-sectional area was measured at rest from ultrasound scans in the transverse plane. Maximal F_pt and tendon elongation were (mean±SE) 5453±307 N and 5±0.5 mm for men, 3877±307 N and 4.9±0.6 mm for women, 2017±170 N and 6.2±0.5 mm for boys and 2169±182 N and 5.9±0.7 mm for girls. In all groups, tendon stiffness and Young's modulus were examined at the level that corresponded to the maximal 30% of the weakest participant's F_pt and stress, respectively; these were 925–1321 N and 11.5–16.5 MPa, respectively. Stiffness was 94% greater in men than boys and 84% greater in women than girls (p<0.01), with no differences between men and women, or boys and girls (men 1076±87 N/mm; women 1030±139 N/mm; boys 555±71 N/mm and girls 561.5±57.4 N/mm). Young's modulus was 99% greater in men than boys (p<0.01), and 66% greater in women than girls (p<0.05). There were no differences in modulus between men and women, or boys and girls (men 597±49 MPa; women 549±70 MPa; boys 255±42 MPa and girls 302±33 MPa). These findings indicate that the mechanical stiffness of tendon increases with maturation due to an increased Young's modulus and, in females due to a greater increase in tendon cross-sectional area than tendon length.     

In vivo determination of fascicle curvature in contracting human skeletal muscles.

Fascicle curvature of human medial gastrocnemius muscle (MG) was determined in vivo by ultrasonography during isometric contractions at three (distal, central, and proximal) locations (n = 7) and at three ankle angles (n = 7). The curvature significantly (P < 0.05) increased from rest to maximum voluntary contraction (MVC) (0.4-5.2 m(-1)). In addition, the curvature at MVC became larger in the order dorsiflexed, neutral, plantar flexed (P < 0.05). Thus both contraction levels and muscle length affected the curvature. Intramuscular differences in neither the curvature nor the fascicle length were found. The direction of curving was consistent along the muscle: fascicles were concave in the proximal side. Fascicle length estimated from the pennation angle and muscle thickness, under the assumption that the fascicle was straight, was underestimated by ~6%. In addition, the curvature was significantly correlated to pennation angle and muscle thickness. These findings are particularly important for understanding the mechanical functions of human skeletal muscle in vivo.     

Influence of 90-day simulated microgravity on human tendon mechanical properties and the effect of resistive countermeasures.

While microgravity exposure is known to cause deterioration of skeletal muscle performance, little is known regarding its effect on tendon structure and function. Hence, the aims of this study were to investigate the effects of simulated microgravity on the mechanical properties of human tendon and to assess the effectiveness of resistive countermeasures in preventing any detrimental effects. Eighteen men (aged 25-45 yr) underwent 90 days of bed rest: nine performed resistive exercise during this period (BREx group), and nine underwent bed rest only (BR group). Calf-raise and leg-press exercises were performed every third day using a gravity-independent flywheel device. Isometric plantar flexion contractions were performed by using a custom-built dynamometer, and ultrasound imaging was used to determine the tensile deformation of the gastrocnemius tendon during contraction. In the BR group, tendon stiffness estimated from the gradient of the tendon force-deformation relation decreased by 58% (preintervention: 124 +/- 67 N/mm; postintervention: 52 +/- 28 N/mm; P < 0.01), and the tendon Young's modulus decreased by 57% postintervention (P < 0.01). In the BREx group, tendon stiffness decreased by 37% (preintervention: 136 +/- 66 N/mm; postintervention: 86 +/- 47 N/mm; P < 0.01), and the tendon Young's modulus decreased by 38% postintervention (P < 0.01). The relative decline in tendon stiffness and Young's modulus was significantly (P < 0.01) greater in the BR group compared with the BREx group. Unloading decreased gastrocnemius tendon stiffness due to a change in tendon material properties, and, although the exercise countermeasures did attenuate these effects, they did not completely prevent them. It is suggested that the total loading volume was not sufficient to completely prevent alterations in tendon mechanical properties.     

Effects of isometric training on the elasticity of human tendon structures in vivo.

The present study aimed to investigate the effect of isometric training on the elasticity of human tendon structures. Eight subjects completed 12 wk (4 days/wk) of isometric training that consisted of unilateral knee extension at 70% of maximal voluntary contraction (MVC) for 20 s per set (4 sets/day). Before and after training, the elongation of the tendon structures in the vastus lateralis muscle was directly measured using ultrasonography while the subjects performed ramp isometric knee extension up to MVC. The relationship between the estimated muscle force and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness of the tendon structures. The training increased significantly the volume (7.6+/-4.3%) and MVC torque (33.9+/-14.4%) of quadriceps femoris muscle. The L values at force production levels beyond 550 N were significantly shorter after training. The stiffness increased significantly from 67.5+/-21.3 to 106.2+/-33.4 N/mm. Furthermore, the training significantly increased the rate of torque development (35.8 +/- 20.4%) and decreased electromechanical delay (-18.4+/-3.8%). Thus the present results indicate that isometric training increases the stiffness and Young's modulus of human tendon structures as well as muscle strength and size. This change in the tendon structures would be assumed to be an advantage for increasing the rate of torque development and shortening the electromechanical delay.     

Atrial distension in humans during microgravity induced by parabolic flights

Videbaek, Regitze, and Peter Norsk. Atrialdistension in humans during microgravity induced by parabolic flights.J. Appl. Physiol. 83(6):1862-1866, 1997.The hypothesis was tested that human cardiacfilling pressures increase and the left atrium is distended during 20-speriods of microgravity (µG) created by parabolic flights, comparedwith values of the 1-G supine position. Left atrial diameter(n = 8, echocardiography) increasedsignificantly during µG from 26.8 ± 1.2 to 30.4 ± 0.7 mm(P < 0.05). Simultaneously, centralvenous pressure (CVP; n = 6, transducer-tipped catheter) decreased from 5.8 ± 1.5 to 4.5 ± 1.1 mmHg (P < 0.05), and esophageal pressure (EP; n = 6) decreased from1.5 ± 1.6 to 4.1 ± 1.7 mmHg (P < 0.05). Thus transmural CVP(TCVP = CVP  EP; n = 4)increased during µG from 6.1 ± 3.2 to 10.4 ± 2.7 mmHg(P < 0.05). It is concluded thatshort periods of µG during parabolic flights induce an increase inTCVP and left atrial diameter in humans, compared with the resultsobtained in the 1-G horizontal supine position, despite a decrease inCVP.

     

Contribution of abdomen and legs to central blood volume expansion in humans during immersion

Johansen, Lars Bo, Thomas Ulrik Skram Jensen, Bettina Pump,and Peter Norsk. Contribution of abdomen and legs to central bloodvolume expansion in humans during immersion. J. Appl.Physiol. 83(3): 695-699, 1997.The hypothesis wastested that the abdominal area constitutes an important reservoir forcentral blood volume expansion (CBVE) during water immersion inhumans. Six men underwent 1) water immersion for 30 min (WI),2) water immersion for 30 min withthigh cuff inflation (250 mmHg) during initial 15 min to exclude legsfrom contributing to CBVE (WI+Occl), and3) a seated nonimmersed control with15 min of thigh cuff inflation (Occl). Plasma protein concentration andhematocrit decreased from 68 ± 1 to 64 ± 1 g/l and from 46.7 ± 0.3 to 45.5 ± 0.4%(P < 0.05), respectively, during WIbut were unchanged during WI+Occl. Left atrial diameter increased from27 ± 2 to 36 ± 1 mm (P < 0.05) during WI and increased similarly during WI+Occl from 27 ± 2 to 35 ± 1 mm (P < 0.05). Centralvenous pressure increased from 3.7 ± 1.0 to 10.4 ± 0.8 mmHg during WI (P < 0.05) butonly increased to 7.0 ± 0.8 mmHg during WI+Occl(P < 0.05). In conclusion, the dilution of blood induced by WI to the neck is caused by fluid from thelegs, whereas the CBVE is caused mainly by blood from theabdomen.

     

Tensile properties of the in vivo human gastrocnemius tendon

In the present experiment we obtained the tensile properties of the human gastrocnemius tendon, a high-stressed tendon suitable for spring-like action during locomotion. Measurements were taken in vivo in six men. The gastrocnemius tendon elongation during tendon loading−unloading induced by muscle contraction−relaxation was measured using real-time ultrasonography. Tendon forces were calculated from the moment generated during isometric plantarflexion contraction, using tendon moment arm length data obtained in vivo with the tendon travel method. Tendon stiffness data were calculated from the slope of the tendon force−elongation curve, and were then normalized to the tendon's original dimensions, obtained from morphometric analysis of sonographs, to estimate the tendon Young's modulus. Mechanical hysteresis values were obtained from area calculations by numerical integration. The elongation of the tendon increased curvilinearly with the force acting upon it, from 1.7±1 mm (0.8±0.3% strain) at 87.5±8.5 N to 11.1±3.1 mm (4.9±1% strain) at 875±85 N. The tendon Young's modulus and mechanical hysteresis were 1.16±0.15 GPa and 18±3%, respectively. These values fall within the range of values obtained from in vitro experiments and are very similar to the respective values recently obtained from in vivo measurements in the less highly stressed human tibialis anterior tendon (1.2 GPa and 19%), thus indicating that the material properties of tendon are independent of physiological loading and function. Combining the present tendon force−elongation data with previously reported Achilles tendon force data recorded during walking indicates that the gastrocnemius tendon would provide 6% of the total external work produced by the locomotor system. This estimate illustrates the contribution of passive elastic mechanisms on the economy and efficiency of walking. The contributions would be greater in more active exercise such as running.     

Muscle fatigue and exhaustion during dynamic leg exercise in normoxia and hypobaric hypoxia

Fulco, Charles S., Steven F. Lewis, Peter N. Frykman, RobertBoushel, Sinclair Smith, Everett A. Harman, Allen Cymerman, and Kent B. Pandolf. Muscle fatigue and exhaustion during dynamic leg exercisein normoxia and hypobaric hypoxia. J. Appl. Physiol. 81(5): 1891-1900, 1996.Using anexercise device that integrates maximal voluntary static contraction(MVC) of knee extensor muscles with dynamic knee extension, we comparedprogressive muscle fatigue, i.e., rate of decline in force-generatingcapacity, in normoxia (758 Torr) and hypobaric hypoxia (464 Torr).Eight healthy men performed exhaustive constant work rate kneeextension (21 ± 3 W, 79 ± 2 and 87 ± 2% of 1-leg kneeextension O₂ peak uptake fornormoxia and hypobaria, respectively) from knee angles of90-150° at a rate of 1 Hz. MVC (90° knee angle) wasperformed before dynamic exercise and during 5-s pauses every 2 minof dynamic exercise. MVC force was 578 ± 29 N in normoxia and 569 ± 29 N in hypobaria before exercise and fell, at exhaustion, to similar levels (265 ± 10 and 284 ± 20 N for normoxia andhypobaria, respectively; P > 0.05)that were higher (P < 0.01) thanpeak force of constant work rate knee extension (98 ± 10 N, 18 ± 3% of MVC). Time to exhaustion was 56% shorter for hypobariathan for normoxia (19 ± 5 vs. 43 ± 7 min, respectively;P < 0.01), and rate of right leg MVC fall wasnearly twofold greater for hypobaria than for normoxia (mean slope = 22.3 vs. 11.9 N/min, respectively;P < 0.05). With increasing durationof dynamic exercise for normoxia and hypobaria, integratedelectromyographic activity during MVC fell progressively with MVCforce, implying attenuated maximal muscle excitation. Exhaustion, perse, was postulated to relate more closely to impaired shorteningvelocity than to failure of force-generating capacity.

     

Changes of calf muscle-tendon biomechanical properties induced by passive-stretching and active-movement training in children with cerebral palsy

Biomechanical properties of calf muscles and Achilles tendon may be altered considerably in children with cerebral palsy (CP), contributing to childhood disability. It is unclear how muscle fascicles and tendon respond to rehabilitation and contribute to improvement of ankle-joint properties. Biomechanical properties of the calf muscle fascicles of both gastrocnemius medialis (GM) and soleus (SOL), including the fascicle length and pennation angle in seven children with CP, were evaluated using ultrasonography combined with biomechanical measurements before and after a 6-wk treatment of passive-stretching and active-movement training. The passive force contributions from the GM and SOL muscles were separated using flexed and extended knee positions, and fascicular stiffness was calculated based on the fascicular force-length relation. Biomechanical properties of the Achilles tendon, including resting length, cross-sectional area, and stiffness, were also evaluated. The 6-wk training induced elongation of muscle fascicles (SOL: 8%, P = 0.018; GM: 3%, P = 0.018), reduced pennation angle (SOL: 10%, P = 0.028; GM: 5%, P = 0.028), reduced fascicular stiffness (SOL: 17%, P = 0.128; GM: 21%, P = 0.018), decreased tendon length (6%, P = 0.018), increased Achilles tendon stiffness (32%, P = 0.018), and increased Young's modulus (20%, P = 0.018). In vivo characterizations of calf muscles and Achilles tendon mechanical properties help us better understand treatment-induced changes of calf muscle-tendon and facilitate development of more effective treatments.     

Almitrine and doxapram decrease fatigue and increase subsequent recovery in isolated rat diaphragm

McGuire, Michelle, Michael F. Carey, and John J. O'Connor.Almitrine and doxapram decrease fatigue and increase subsequent recovery in isolated rat diaphragm. J. Appl.Physiol. 83(1): 52-58, 1997.The effects ofalmitrine bimesylate and doxapram HCl on isometric force produced by invitro rat diaphragm were studied during direct muscle activation at37°C. Doxapram and almitrine ameliorate respiratory failureclinically by indirectly increasing phrenic nerve activity. This studywas carried out to investigate possible direct actions of these agentson the diaphragm before and after fatigue of the fibers. Two age groupsof animals were chosen [6-14 wk (group1) and 50-55 wk (group2)] because it is known that increasing agedecreases a muscle fiber's resistance to fatigue. Muscle strips wereisolated from both group 1 and group 2 and directly stimulated (2-mspulse duration, 5-15 V) to produce twitch tensions of 1.3 and 2.1 N/cm², respectively. At lowconcentrations, doxapram (20 µg/ml) and almitrine (12 µg/ml)had no effect on twitch contraction or 100-Hz tetanic tension. However,40 µg/ml doxapram and 30 µg/ml almitrine increased twitch tensionby 9.0 ± 1.4 and 11.6 ± 1.9%, respectively, in animals ofgroup 2 (n = 5). A fatigue protocol consistingof low-frequency stimulation (30-Hz trains, 250-ms duration every 2 sfor 5 min) caused a reduction of twitch tension in animals ofgroup 1 (48 ± 4% ofcontrol) and group 2 (28 ± 4% ofcontrol). At 90 min postfatigue, the twitch tension recovered to 72 ± 3 and 42 ± 2% of control values ingroup 1 and group2, respectively. In the presence of doxapram (20 µg/ml), there was a significant increase in the recovery of twitchtension at 90 min in group 1 andgroup 2 (84.5 ± 3.2 and 80.1 ± 2.8%, respectively) compared with controls at 90 min postfatigue. Inthe presence of almitrine (12 µg/ml), there was a full recovery fromfatigue in group 1 animals (100% ofcontrol) and a recovery to 95.6 ± 2.1% of control ingroup 2 animals at 90 min. Theseresults demonstrate a significant improvement in the rapidity andmagnitude of recovery from fatigue in the rat diaphragm muscle in thepresence of both doxapram and, especially, almitrine. These effects maybe due to changes in intracellular calcium, ADP/ATP ratios, or oxygenfree radical scavenging.

     

Negative interstitial pressure in the peritendinous region during exercise

In the presentstudy, tissue pressure in the peritendinous area ventral to the humanAchilles tendon was determined. The pressure was measured during restand intermittent isometric calf muscle exercise at three torques (56, 112, and 168 Nm) 20, 40 and 50 mm proximal to the insertion of thetendon in 11 healthy, young individuals. In allexperiments a linear significant decrease in pressure was obtained withincreasing torque [e.g., at 40 mm: 0.4 ± 0.3 mmHg(rest) to 135 ± 12 mmHg (168 Nm)]. No significant differences were obtained among the three areas measured. On the basisof these observations, microdialysis was performed in the peritendinousregion with a colloid osmotic active substance (Dextran 70, 0.1 g/ml)added to the perfusate with the aim of counteracting the negativetissue pressure. Dialysate volume was found to be fully restored (100 ± 4%) during exercise. It is concluded that a marked negativetissue pressure is generated in the peritendinous space around theAchilles tendon during exercise in humans. Negative tissue pressurecould lead to fluid shift and could be involved in the increase inblood flow previously noted in the peritendinous tissue during exercise(H. Langberg, J. Bülow, and M. Kjær. Acta Physiol. Scand. 163: 149-153, 1998; H. Langberg,J. Bülow, and M. Kjær. Clin.Physiol. 19: 89-93, 1999).

     

Repeated contractions alter the geometry of human skeletal muscle.

The aim of this study was to investigate the effect of repeated contractions on the geometry of human skeletal muscle. Six men performed two sets (sets A and B) of 10 repeated isometric plantarflexion contractions at 80% of the moment generated during plantarflexion maximal voluntary contraction (MVC), with a rest interval of 15 min between sets. By use of ultrasound, the geometry of the medial gastrocnemius (MG) muscle was measured in the contractions of set A and the displacement of the MG tendon origin in the myotendinous junction was measured in the contractions of set B. In the transition from the 1st to the 10th contractions, the fascicular length at 80% of MVC decreased from 34 +/- 4 (means +/- SD) to 30 +/- 3 mm (P < 0.001), the pennation angle increased from 35 +/- 3 to 42 +/- 3 degrees (P < 0.001), the myotendinous junction displacement increased from 5 +/- 3 to 10 +/- 3 mm (P < 0.001), and the average fascicular curvature remained constant (P > 0.05) at approximately 4.3 m(-1). No changes (P > 0.05) were found in fascicular length, pennation angle, and myotendinous junction displacement after the fifth contraction. Electrogoniometry showed that the ankle rotated by approximately 6.5 degrees during contraction, but no differences (P > 0.05) were obtained between contractions. The present results show that repeated contractions induce tendon creep, which substantially affects the geometry of the in-series contracting muscles, thus altering their potential for force and joint moment generation.     

Alternate activity in the synergistic muscles during prolonged low-level contractions

The purpose ofthis study was to investigate the functional interrelationship betweensynergistic muscle activities during low-level fatiguing contractions.Six human subjects performed static and dynamic contractions at anankle joint angle of 110° plantar flexion and within the range of90-110° (anatomic position = 90°) under constant load(10% maximal voluntary contraction) for 210 min. Surfaceelectromyogram records from lateral gastrocnemius (LG), medialgastrocnemius (MG), and soleus (Sol) muscles showed high and silentactivities alternately in the three muscles and a complementary andalternate activity between muscles in the time course. In the secondhalf of all exercise times, the number of changes in activity increasedsignificantly (P < 0.05) in each muscle. The ratios of active to silent periods of electromyogram activity were significantly higher (P < 0.05) in MG (4.5 ± 2.2) and Sol (4.3 ± 2.8) than in the LG(0.4 ± 0.1), but no significant differences were observed betweenMG and Sol. These results suggest that the relativeactivation of synergistic motor pools are not constant during alow-level fatiguing task.

     

Time course of sympathovagal imbalance and left ventricular dysfunction in conscious dogs with heart failure

To elucidate thetime course of sympathovagal balance and its relationship to leftventricular function in heart failure, we serially evaluated leftventricular contractility and relaxation and autonomic tone in 11 conscious dogs with tachycardia-induced heart failure. We determined adynamic map of sympathetic and parasympathetic modulation by powerspectral analysis of heart rate variability. The left ventricular peak+dP/dt substantially fell from 3,364 ± 338 to 1,959 ± 318 mmHg/s (P < 0.05) on the third day and declined gradually to 1,783 ± 312 mmHg/s at 2 wk of rapid ventricular pacing. In contrast, the timeconstant of left ventricular pressure decay and end-diastolic pressureincreased gradually from 25 ± 4 to 47 ± 5 ms(P < 0.05) and from 10 ± 2 to21 ± 3 mmHg (P < 0.05), respectively, at 2 wk of pacing. The high-frequency component(0.15-1.0 Hz), a marker of parasympathetic modulation, decreasedfrom 1,928 ± 1,914 to 62 ± 68 × 10³ms²(P < 0.05) on the third day andfurther to 9 ± 12 × 10³ms²(P < 0.05) at 2 wk. Similar to thetime course of left ventricular diastolic dysfunction, plasmanorepinephrine levels and the ratio of low (0.05- to 0.15-Hz)- tohigh-frequency component increased progressively from 135 ± 50 to 532 ± 186 pg/ml (P < 0.05) and from 0.06 ± 0.06 to 1.12 ± 1.01 (P < 0.05), respectively, at 2 wk ofpacing. These cardiac and autonomic dysfunctions recovered graduallytoward the normal values at 2 wk after cessation of pacing. Thus aparallel decline in left ventricular contractility with parasympatheticinfluence and a parallel progression in left ventricular diastolicdysfunction with sympathoexcitation suggest a close relationshipbetween cardiac dysfunction and autonomic dysregulation duringdevelopment of heart failure.

     

Contractile function and low-intensity exercise effects of old dystrophic (mdx) mice

Oldmdx mice display a severe myopathyalmost identical to Duchenne's muscular dystrophy. This study examinedthe contractile properties of old mdxmuscles and investigated any effects of low-intensity exercise.Isometric contractile properties of the extensor digitorum longus (EDL)and soleus muscles were tested in adult (8-10 mo) and old (24 mo,split into sedentary and exercised groups)mdx mice. The EDL and soleus from oldmdx mice exhibited decreased absolutetwitch and tetanic forces, and the soleus exhibited a >50% decreasein relative forces (13.4 ± 0.4 vs. 6.0 ± 0.9 N/cm²) compared with adult mice.Old mdx muscles also showed longer contraction times and a higher percentage of type I fibers. Normal andmdx mice completed 10 wk of swimming,but mdx mice spent significantly lesstime swimming than normal animals (7.8 ± 0.4 vs. 15.8 ± 1.1 min, respectively). However, despite their severe dystrophy,mdx muscles responded positively tothe low-intensity exercise. Relative tetanic tensions were increased(~25% and ~45% for the EDL and soleus, respectively) after theswimming, although absolute forces were unaffected. Thus these resultsindicate that, even with a dystrophin-deficient myopathy,mdx muscles can still respond to low-intensity exercise. This study shows that the contractile functionof muscles of old mdx mice displaysmany similarities to that of human dystrophic patients and providesfurther evidence that the use of non-weight-bearing, low-intensityexercises, such as swimming, has no detrimental effect on dystrophicmuscle and could be a useful therapeutic aid for sufferers of musculardystrophy.

     

Effects of emphysema on diaphragm blood flow during exercise

Chronichyperinflation of the lung in emphysema displaces the diaphragmcaudally, thereby placing it in a mechanically disadvantageous positionand contributing to the increased work of breathing. We tested thehypothesis that total and regional diaphragm blood flows are increasedin emphysema, presumably reflecting an increased diaphragm energeticdemand. Male Syrian Golden hamsters were randomly divided intoemphysema (E; intratracheal elastase 25 units/100 g body wt) andcontrol (C; saline) groups, and experiments were performed 16-20wk later. The regional distribution of blood flow withinthe diaphragm was determined by using radiolabeled microspheres inhamsters at rest and during treadmill exercise (walking at 20 feet/min,20% grade). Consistent with pronounced emphysema, lung volume per unitbody weight was greater in E hamsters (C, 59.3 ± 1.8; E, 84.5 ± 5.0 ml/kg; P < 0.001) and arterialPO₂ was lower both at rest (C, 74 ± 3; E, 59 ± 2 Torr; P < 0.001) and during exercise (C, 93 ± 3; E, 69 ± 4 Torr; P < 0.001). At rest, total diaphragm blood flow was not different between C and Ehamsters (C, 47 ± 4; E, 38 ± 4 ml · min¹ · 100 g¹;P = 0.18). In both C and E hamsters,blood flow at rest was lower in the ventral costal region of thediaphragm than in the dorsal and medial costal regions and the cruraldiaphragm. During exercise in both C and E hamsters, blood flowsincreased more in the dorsal and medial costal regions and in thecrural diaphragm than in the ventral costal region. Total diaphragmblood flow was greater in E hamsters during exercise (C, 58 ± 7; E,90 ± 14 ml · min¹ · 100 g¹;P = 0.03), as a consequence ofsignificantly higher blood flows in the medial and ventral costalregions and crural diaphragm. In addition, exercise-induced increasesin intercostal (P < 0.005) andabdominal (P < 0.05) muscle bloodflows were greater in E hamsters. The finding that diaphragm blood flowwas greater in E hamsters during exercise supports the contention thatemphysema increases the energetic requirements of the diaphragm.

     

Skeletal muscle fiber quality in older men and women

Wholemuscle strength and cross-sectional area (WMCSA), andcontractile properties of chemically skinned segments from single fibers of the quadriceps were studied in 7 young men (YM, 36.5 ± 3.0 yr), 12 older men (OM, 74.4 ± 5.9 yr), and 12 olderwomen (OW, 72.1 ± 4.3 yr). WMCSA was smaller in OMcompared with YM (56.1 ± 10.1 vs. 79.7 ± 13.1 cm²; P = 0.031) and in OW (44.9 ± 7.5; P < 0.003) compared with OM. Age-related, but notsex-related, differences in strength were eliminated after adjustingfor WMCSA. Maximal force was measured in 552 type I and 230 type IIAfibers. Fibers from YM (type I = 725 ± 221; type IIA = 792 ± 271 µN) were stronger (P < 0.001) thanfibers from OM (I = 505 ± 179; IIA = 577 ± 262 µN) even after correcting for size. Type IIA fibers were stronger(P < 0.005) than type I fibers in YM and OM but not inOW (I = 472 ± 154; IIA = 422 ± 97 µN).Sex-related differences in type I and IIA fibers were dependent onfiber size. In conclusion, differences in WMCSA explain age-relateddifferences in strength. An intrinsic defect in contractile proteinscould explain weakness in single fibers from OM. Sex-relateddifferences exist at the whole muscle and single fiber levels.

     

A method for isolating adult and neonatal airway smooth muscle cells and measuring shortening velocity

Methods are described for isolating smooth muscle cells from thetracheae of adult and neonatal sheep and measuring the single-cell shortening velocity. Isolated cells were elongated,Ca²⁺ tolerant, and contractedrapidly and substantially when exposed to cholinergic agonists, KCl,serotonin, or caffeine. Adult cells were longer and widerthan preterm cells. Mean cell length in 1.6 mMCaCl₂ was 194 ± 57 (SD) µm(n = 66) for adult cells and 93 ± 32 µm (n = 20) for preterm cells(P < 0.05). Mean cell width at thewidest point of the adult cells was 8.2 ± 1.8 µm(n = 66) and 5.2 ± 1.5 µm(n = 20) for preterm cells(P < 0.05). Cells were loaded into aperfusion dish maintained at 35°C and exposed to agonists, andcontractions were videotaped. Cell lengths were measured from 30 videoframes and plotted as a function of time. Nonlinear fitting of celllength to an exponential model gave shortening velocities faster thanmost of those reported for airway smooth muscle tissues. For a sampleof 10 adult and 10 preterm cells stimulated with 100 µM carbachol,mean (± SD) shortening velocity of the preterm cells was notdifferent from that of the adult cells (0.64 ± 0.30 vs. 0.54 ± 0.27 s¹, respectively), butpreterm cells shortened more than adult cells (68 ± 12 vs. 55 ± 11% of starting length, respectively;P < 0.05). The preparative andanalytic methods described here are widely applicable to other smoothmuscles and will allow contraction to be studied quantitatively at thesingle-cell level.

     

Leg intramuscular pressures during locomotion in humans

To assess the usefulness of intramuscularpressure (IMP) measurement for studying muscle function during gait,IMP was recorded in the soleus and tibialis anterior muscles of 10 volunteers during treadmill walking and running by usingtransducer-tipped catheters. Soleus IMP exhibited single peaks duringlate-stance phase of walking [181 ± 69 (SE) mmHg] andrunning (269 ± 95 mmHg). Tibialis anterior IMP showed a biphasicresponse, with the largest peak (90 ± 15 mmHg during walking and151 ± 25 mmHg during running) occurring shortly after heel strike.IMP magnitude increased with gait speed in both muscles. Linearregression of soleus IMP against ankle joint torque obtained by adynamometer produced linear relationships (n = 2, r = 0.97 for both). Application ofthese relationships to IMP data yielded estimated peak soleus momentcontributions of 0.95-1.65 N · m/kgduring walking, and 1.43-2.70 N · m/kg during running. Phasic elevations of IMP during exercise are probably generated by local muscle tissue deformations due to muscle force development. Thus profiles of IMP provide a direct, reproducible indexof muscle function during locomotion in humans.