Neuromuscular factors contributing to in vivo eccentric moment generation

Webber, Sandra, and Dean Kriellaars. Neuromuscularfactors contributing to in vivo eccentric moment generation.J. Appl. Physiol. 83(1): 40-45, 1997.Muscle series elasticity and its contribution to eccentricmoment generation was examined in humans. While subjects [male,n = 30; age 26.3 ± 4.8 (SD) yr; body mass 78.8 ± 13.1 kg] performed an isometric contractionof the knee extensors at 60° of knee flexion, a quick stretch was imposed with a 12°-step displacement at 100°/s. The test wasperformed at 10 isometric activation levels ranging from 1.7 to 95.2%of maximal voluntary contraction (MVC). A strong linear relationship was observed between the peak imposed eccentric moment derived fromquick stretch and the isometric activation level(y = 1.44x + 7.08; r = 0.99). This increase in theeccentric moment is consistent with an actomyosin-dependent elasticitylocated in series with the contractile element of muscle. Byextrapolating the linear relationship to 100% MVC, the predictedmaximum eccentric moment was found to be 151% MVC, consistent with invitro data. A maximal voluntary, knee extensor strength test was alsoperformed (5-95°, 3 repetitions, ±50, 100, 150, 200, and250°/s). The predicted maximum eccentric moment was 206% of theangle- and velocity-matched, maximal voluntary eccentric moments. Thiswas attributed to a potent neural regulatory mechanism that limits therecruitment and/or discharge of motor units during maximalvoluntary eccentric contractions.

     

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.

     

Lower limb skeletal muscle function after 6wk of bed rest

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

     

Maximal and submaximal isometric torque is elevated immediately following highly controlled active stretches of the hamstrings

Hamstring strain rehabilitation programs with an eccentric bias are effective but have a low adherence rate. Post-stretch isometric (PS-ISO) contractions which incorporate a highly controlled eccentric contraction followed by an isometric contraction resulting in elevated torque during following stretch, compared with isometric contractions at the same joint angle. This study measured torque, activation and musculotendinous unit behaviour of the hamstrings during PS-ISO contractions of maximal and submaximal levels using two stretch amplitudes. Ten male participants (24.6 years ± 2.22 years) completed maximal and submaximal baseline isometric contractions at 90°, 120° and 150° knee flexion and PS-ISO contractions of maximal and submaximal intensity initiated at 90° and 120° incorporating active stretch of 30° and 60° at 60°·s⁻¹. Torque and muscle activation of the knee flexors were simultaneously recorded. Musculotendinous unit behaviour of the biceps femoris long head was recorded via ultrasound during all PS-ISO contractions. Compared with baseline, torque was 8% and 39% greater in the maximal and submaximal PS-ISO conditions respectively with no change in muscle activation. The biceps femoris long head muscle lengthened during all PS-ISO contractions. PS-ISO contractions may be beneficial where the effects of highly controlled eccentric contractions and elevated isometric torque are desired, such as hamstring rehabilitation.     

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

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

     

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

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

     

The effect of leg flexion angle on the mechanomyographic responses to isometric muscle actions

The purpose of this investigation was to examine the effect of leg flexion angle on the relationship between mechanomyographic (MMG) amplitude and isometric torque production. Adult males (n = 9) performed isometric muscle actions of the leg extensors at 25, 50, 75, and 100 percent maximal voluntary contraction (%MVC) on a calibrated CYBEX 6000 dynamometer at 25, 50, and 75° below full extension. A piezoelectric MMG recording device was placed over the mid-portion of the rectus femoris. At 25° of leg flexion, the MMG amplitude increased to 100%MVC. At 50 and 75° of leg flexion, however, MMG amplitude increased to 75%MVC, and then did not change significantly (P > 0.05) between 75 and 100%MVC. These findings indicate that the MMG amplitude-isometric torque relationship is joint angle specific and may be the result of leg flexion angle differences in: (1) muscle stiffness, or (2) motor unit activation strategies. Accepted: 2 March 1998     

Motor unit firing patterns of lower leg muscles during isometric plantar flexion with flexed knee joint position

The ankle flexor and extensor muscles are essential for pedal movements associated with car driving. Neuromuscular activation of lower leg muscles is influenced by the posture during a given task, such as the flexed knee joint angle during car driving. This study aimed to investigate the influence of flexion of the knee joint on recruitment threshold-dependent motor unit activity in lower leg muscles during isometric contraction. Twenty healthy participants performed plantar flexor and dorsiflexor isometric ramp contractions at 30 % of the maximal voluntary contraction (MVC) with extended (0°) and flexed (130°) knee joint angles. High-density surface electromyograms were recorded from medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA) muscles and decomposed to extract individual motor units. The torque-dependent change (Δpps /Δ%MVC) of the motor unit activity of MG (recruited at 15 %MVC) and SOL (recruited at 5 %MVC) muscles was higher with a flexed compared with an extended knee joint (p < 0.05). The torque-dependent change of TA MU did not different between the knee joint angles. The motor units within certain limited recruitment thresholds recruited to exert plantar flexion torque can be excited to compensate for the loss of MG muscle torque output with a flexed knee joint.     

Augmentation of exercise-induced muscle sympathetic nerve activity during muscle heating

Ray, Chester A., and Kathryn H. Gracey. Augmentation ofexercise-induced muscle sympathetic nerve activity during muscle heating. J. Appl. Physiol. 82(6):1719-1725, 1997.The muscle metabo- and mechanoreflexes have beenshown to increase muscle sympathetic nerve activity (MSNA) duringexercise. Group III and IV muscle afferents, which are believed tomediate this response, have been shown to be thermosensitive inanimals. The purpose of the present study was to evaluate the effect ofmuscle temperature on MSNA responses during exercise. Eleven subjectsperformed ischemic isometric handgrip at 30% of maximal voluntarycontraction to fatigue, followed by 2 min of postexercise muscleischemia (PEMI), with and without local heating of the forearm. Localheating of the forearm increased forearm muscle temperature from 34.4 ± 0.2 to 38.9 ± 0.3°C(P = 0.001). Diastolic andmean arterial pressures were augmented during exercise in the heat.MSNA responses were greater during ischemic handgrip with local heatingcompared with control (no heating) after the first 30 s. MSNA responsesat fatigue were greater during local heating. MSNA increased by 16 ± 2 and 20 ± 2 bursts per 30 s for control and heating,respectively (P = 0.03). Whenexpressed as a percent change in total activity (total burstamplitude), MSNA increased 531 ± 159 and 941 ± 237% forcontrol and heating, respectively (P = 0.001). However, MSNA was not different during PEMI between trials.This finding suggests that the augmentation of MSNA during exercisewith heat was due to the stimulation of mechanically sensitive muscleafferents. These results suggest that heat sensitizes skeletal muscleafferents during muscle contraction in humans and may play a role inthe regulation of MSNA during exercise.

     

Changes in agonist-antagonist EMG, muscle CSA, and force during strength training in middle-aged and older people

Effects of 6 mo of heavy-resistance trainingcombined with explosive exercises on neural activation of the agonistand antagonist leg extensors, muscle cross-sectional area (CSA) of thequadriceps femoris, as well as maximal and explosive strength wereexamined in 10 middle-aged men (M40; 42 ± 2 yr), 11 middle-agedwomen (W40; 39 ± 3 yr), 11 elderly men (M70; 72 ± 3 yr) and 10 elderly women (W70; 67 ± 3 yr). Maximal andexplosive strength remained unaltered during a 1-mo control period withno strength training. After the 6 mo of training, maximal isometric anddynamic leg-extension strength increased by 36 ± 4 and 22 ± 2%(P < 0.001) in M40, by 36 ± 3 and 21 ± 3% (P < 0.001) in M70,by 66 ± 9 and 34 ± 4% (P < 0.001) in W40, and by 57 ± 10 and 30 ± 3%(P < 0.001) in W70, respectively.All groups showed large increases (P < 0.05-0.001) in the maximum integrated EMGs (iEMGs) of theagonist vastus lateralis and medialis. Significant(P < 0.05-0.001) increasesoccurred in the maximal rate of isometric force productionand in a squat jump that were accompanied with increased(P < 0.05-0.01) iEMGs of theleg extensors. The iEMG of the antagonist biceps femoris muscle duringthe maximal isometric leg extension decreased in both M70 (from 24 ± 6 to 21 ± 6%; P < 0.05)and in W70 (from 31 ± 9 to 24 ± 4%;P < 0.05) to the same level asrecorded for M40 and W40. The CSA of the quadriceps femoris increasedin M40 by 5% (P < 0.05), in W40 by9% (P < 0.01), in W70 by 6%(P < 0.05), and in M70 by 2% (notsignificant). Great training-induced gains in maximal and explosivestrength in both middle-aged and elderly subjects were accompanied bylarge increases in the voluntary activation of the agonists, withsignificant reductions in the antagonist coactivation in the elderlysubjects. Because the enlargements in the muscle CSAs in bothmiddle-aged and elderly subjects were much smaller in magnitude, neuraladaptations seem to play a greater role in explaining strength andpower gains during the present strength-training protocol.

     

Effect of hip flexion angle on stiffness of the adductor longus muscle during isometric hip flexion

This study examined the effect of hip flexion angle on the stiffness of the adductor longus (AL) muscle during isometric hip flexion. Seventeen men were recruited. Ten participants performed submaximal voluntary contraction at 0%, 25%, 50%, and 75% of maximal voluntary contraction (MVC) during isometric hip flexion after performing MVC at 0°, 40°, and 80° of hip flexion. Seven participants performed submaximal voluntary tasks during isometric hip extension in addition to hip flexion task. The shear modulus of the AL muscle was used as the index of muscle stiffness, and was measured using ultrasound shear-wave elastography during the tasks at each contraction intensity for each hip flexion angle. During hip flexion, the shear modulus of the AL muscle was higher at 0° than at 40° and 80° of hip flexion at each contraction intensity (p < 0.016). Conversely, a significant effect was not found among hip flexion angle during hip extension at 75% of MVC (p = 0.867). These results suggest that mechanical stress of the AL muscle may be higher at 0° of hip flexion during isometric hip flexion.     

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

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

     

Greater initial adaptations to submaximal muscle lengthening than maximal shortening

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

     

Stretch-induced enhancement of mechanical work production in frog single fibers and human muscle

Takarada, Yudai, Hiroyuki Iwamoto, Haruo Sugi, YuichiHirano, and Naokata Ishii. Stretch-induced enhancement ofmechanical work production in frog single fibers and human muscle.J. Appl. Physiol. 83(5):1741-1748, 1997.The relations between the velocity of prestretchand the mechanical energy liberated during the subsequent isovelocityrelease were studied in contractions of frog single fibers and humanmuscles. During isometric contractions of frog single fibers, a rampstretch of varied velocity (amplitude, 0.02 fiber length; velocity,0.08-1.0 fiber length/s) followed by a release (amplitude, 0.02 fiber length; velocity, 1.0 fiber length/s) was given, and the amountof work liberated during the release was measured. For human muscles,elbow flexions were performed with a prestretch of variedvelocity (range, 40°; velocity, 30-180°/s) followed by anisokinetic shortening (velocity, 90°/s). In both frog single fibersand human muscles, the work production increased with both the velocityof stretch and the peak of force attained before the release up to acertain level; thereafter it declined with the further increases ofthese variables. In human muscles, the enhancement of work productionwas not associated with a significant increase in integratedelectromyogram. This suggests that changes in intrinsic mechanicalproperties of muscle fibers play an important role in thestretch-induced enhancement of work production.

     

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.

     

Neuromuscular drive and force production are not altered during bilateral contractions

Jakobi, J. M., and E. Cafarelli. Neuromuscular driveand force production are not altered during bilateral contractions. J. Appl. Physiol. 84(1): 200-206, 1998.Several investigators have studied the deficit in maximalvoluntary force that is said to occur when bilateral muscle groupscontract simultaneously. A true bilateral deficit (BLD) would suggest asignificant limitation of neuromuscular control; however, some of thedata from studies in the literature are equivocal. Our purpose was todetermine whether there is a BLD in the knee extensors of untrainedyoung male subjects during isometric contractions and whether thisdeficit is associated with a decreased activation of the quadriceps,increased activation of the antagonist muscle, or an alteration inmotor unit firing rates. Twenty subjects performed unilateral (UL) and bilateral (BL) isometric knee extensions at 25, 50, 75, and 100% maximal voluntary contraction. Total UL and BL force (3%) and maximal rate of force generation (2.5%) were not significantly different. Total UL and BL maximal vastus lateralis electromyographic activity (EMG; 2.7 ± 0.28 vs. 2.6 ± 0.24 mV) andcoactivation (0.17 ± 0.02 vs. 0.20 ± 0.02 mV) were also notdifferent. Similarly, the ratio of force to EMG during submaximal ULand BL contractions was not different. Analysis of force production byeach leg in UL and BL conditions showed no differences in force, rateof force generation, EMG, motor unit firing rates, and coactivation.Finally, assessment of quadriceps activity with the twitchinterpolation technique indicated no differences in the degree ofvoluntary muscle activation (UL: 93.6 ± 2.51 Hz, BL: 90.1 ± 2.43 Hz). These results provide no evidence of a significant limitationin neuromuscular control between BL and UL isometric contractions ofthe knee extensor muscles in young male subjects.

     

Effect of ingested sodium bicarbonate on muscle force, fatigue, and recovery

Verbitsky, O., J. Mizrahi, M. Levin, and E. Isakov.Effect of ingested sodium bicarbonate on muscle force, fatigue, and recovery. J. Appl. Physiol. 83(2):333-337, 1997.The influence of acute ingestion ofNaHCO₃ on fatigue and recovery ofthe quadriceps femoris muscle after exercise was studied in six healthymale subjects. A bicycle ergometer was used for exercising under three loading conditions: test A, loadcorresponding to maximal oxygen consumption; testB, load in test A + 17%; test C, load intest B but performed 1 h after acuteingestion of NaHCO₃.Functional electrical stimulation (FES) was applied to provokeisometric contraction of the quadriceps femoris. The resulting kneetorque was monitored during fatigue (2-min chronic FES) and recovery (10-s FES every 10 min, for 40 min). Quadriceps torques were higher inthe presence of NaHCO₃(P < 0.05): withNaHCO₃ the peak, residual, andrecovery (after 40 min) normalized torques were, respectively, 0.68 ± 0.05 (SD), 0.58 ± 0.05, and 0.73 ± 0.05; withoutNaHCO₃ the values were 0.45 ± 0.04, 0.30 ± 0.06, and 0.63 ± 0.06. The increasedtorques obtained after acute ingestion ofNaHCO₃ indicate the possibleexistence of improved nonoxidative glycolysis in isometric contraction,resulting in reduced fatigue and enhanced recovery.

     

Instantaneous force-velocity-length relationship in diaphragmatic sarcomere

Coirault, Catherine, Denis Chemla, Jean-Claude Pourny,Francine Lambert, and Yves Lecarpentier. Instantaneousforce-velocity-length relationship in diaphragmatic sarcomere.J. Appl. Physiol. 82(2): 404-412, 1997.The simultaneous analysis of muscle force, length, velocity, andtime has been shown to precisely characterize the mechanicalperformance of isolated striated muscle. We tested the hypothesis thatthe three-dimensional force-velocity-length relationship reflectsmechanical properties of sarcomeres. In hamster diaphragm strips,instantaneous sarcomere length (SL) and muscle length were simultaneously measured during afterloaded twitches. SL was measured by means of laser diffraction. Wealso studied the influence of initialSL, abrupt changes in total load, and2 × 10⁷ M dantrolene.Baseline resting SL at the apex of thelength-active tension curve was 2.2 ± 0.1 µm, whereasSL at peak shortening was 1.6 ± 0.1 µm in the preloaded twitch and 2.1 ± 0.1 µm in the "isometric" twitch. Over the whole load continuum and at anygiven level of isotonic load, there was a unique relationship between instantaneous sarcomere velocity and instantaneousSL. Part of this relationship was timeindependent and initial SL independent and was markedly downshifted after dantrolene. When five different muscle regions were considered, there were no significant variations ofSL and sarcomere kinetics along themuscle. These results indicate that the time- and initiallength-independent part of the instantaneous force-velocity-lengthrelationship previously described in muscle strips reflects intrinsicsarcomere mechanical properties.

     

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

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

     

Nonisometric behavior of fascicles during isometric contractions of a human muscle

Fascicle length, pennation angle, and tendonelongation of the human tibialis anterior were measured in vivo byultrasonography. Subjects (n = 9) wererequested to develop isometric dorsiflexion torque gradually up tomaximal at the ankle joint angle of 20° plantarflexion from theanatomic position. Fascicle length shortened from 90 ± 7 to 76 ± 7 (SE) mm, pennation angle increased from 10 ± 1 to 12 ± 1°, and tendon elongation increased up to 15 ± 2 mm with gradedforce development up to maximum. The tendon stiffness increased withincreasing tendon force from 10 N/mm at 0-20 N to 32 N/mm at240-260 N. Young's modulus increased from 157 MPa at 0-20 Nto 530 MPa at 240-260 N. It can be concluded that, in isometriccontractions of a human muscle, mechanical work, some of which isabsorbed by the tendinous tissue, is generated by the shortening ofmuscle fibers and that ultrasonography can be used to determine thestiffness and Young's modulus for human tendons.