Peak Isometric Force Research Articles

Transport and metabolism of exogenous fumarate and 3-phosphoglycerate in vascular smooth muscle.

The keto (linear) form of exogenous fructose 1,6-bisphosphate, a highly charged glycolytic intermediate, may utilize a dicarboxylate transporter to cross the cell membrane, support glycolysis, and produce ATP anaerobically. We tested the hypothesis that fumarate, a dicarboxylate, and 3-phosphoglycerate (3-PG), an intermediate structurally similar to a dicarboxylate, can support contraction in vascular smooth muscle during hypoxia. To assess ATP production during hypoxia we measured isometric force maintenance in hog carotid arteries during hypoxia in the presence or absence of 20 mM fumarate or 3-PG. 3-PG improved maintenance of force (p < 0.05) during the 30-80 min period of hypoxia. Fumarate decreased peak isometric force development by 9.5% (p = 0.008) but modestly improved maintenance of force (p < 0.05) throughout the first 80 min of hypoxia. 13C-NMR on tissue extracts and superfusates revealed 1,2,3,4-(13)C-fumarate (5 mM) metabolism to 1,2,3,4-(13)C-malate under oxygenated and hypoxic conditions suggesting uptake and metabolism of fumarate. In conclusion, exogenous fumarate and 3-PG readily enter vascular smooth muscle cells, presumably by a dicarboxylate transporter, and support energetically important pathways.

Changes in muscle morphology, electromyographic activity, and force production characteristics during progressive strength training in young and older men.

Effects of a 10-week progressive strength training program composed of a mixture of exercises for increasing muscle mass, maximal peak force, and explosive strength (rapid force production) were examined in 8 young (YM) (29+/-5 yrs) and 10 old (OM) (61+/-4 yrs) men. Electromyographic activity, maximal bilateral isometric peak force, and maximal rate of force development (RFD) of the knee extensors, muscle cross-sectional area (CSA) of the quadriceps femoris (QF), muscle fiber proportion, and fiber areas of types I, IIa, IIb, and IIab of the vastus lateralis were evaluated. Maximal and explosive strength values remained unaltered in both groups during a 3-week control period with no training preceding the strength training. After the 10-week training period, maximal isometric peak force increased from 1311+/-123 N by 15.6% (p <.05) in YM and from 976+/-168 N by 16.5% (p <.01) in OM. The pretraining RFD values of 4049+/-791 N*s(-1) in YM and 2526+/-1197 N*s(-1) in OM remained unaltered. Both groups showed significant increases (p < .05) in the averaged maximum IEMGs of the vastus muscles. The CSA of the QF increased from 90.3+/-7.9 cm2 in YM by 12.2% (p <.05) and from 74.7+/-7.8 cm2 in OM by 8.5% (p <.001). No changes occurred in the muscle fiber distribution of type I during the training, whereas the proportion of subtype IIab increased from 2% to 6% (p < .05) in YM and that of type IIb decreased in both YM from 25% to 16% (p < .01) and in OM from 15% to 6% (p < .05). The mean fiber area of type I increased after the 10-week training in YM (p < .001) and OM (p < .05) as well as that of type IIa in both YM (p < .01) and OM (p < .01). The individual percentage values for type I fibers were inversely correlated with the individual changes recorded during the training in the muscle CSA of the QF (r=-.56, p < .05). The present results suggest that both neural adaptations and the capacity of the skeletal muscle to undergo training-induced hypertrophy even in older people explain the gains observed in maximal force in older men, while rapid force production capacity recorded during the isometric knee extension action remained unaltered during the present mixed strength training program.

Sport-Seasonal Changes in Body Composition, Strength, and Power of College Wrestlers

Changes in body composition, muscular strength, and anaerobic power of 25 college-age wrestlers were studied throughout the season. Wrestlers were divided into cyclers (n=14) and noncyclers (n=11) based on their reported weight-cutting and dieting history. Isometric strength measures were evaluated from a midthigh clean pull using a special isometric rack with the subject standing on a 61.0 ± 121.9-cm AMTI force plate. Body density was measured via hydrostatic weighing and residual volume was determined via nitrogen washout. For fat-free mass (FFM) and body weight (BW), there was a significant difference in the pattern of change (p < 0.001), highlighted by a decrease in FFM and BW at midseason compared to preseason for the cyclers. A significant increase in FFM occurred postseason. No significant interaction effects were seen for % body fat, fat mass (FM), anaerobic power, or any isometric strength measure. Significant time main effects were found for % fat, BW, FM, and FFM. FFM correlated to isometric peak force and isometric rate of force development at midseason. Taken together, the data indicate that college wrestlers who fluctuate their weight throughout the competitive season lose a significant amount of FFM without concomitant decreases in isometric strength or power.

SILDENAFIL, A TYPE-5 CGMP PHOSPHODIESTERASE INHIBITOR, SPECIFICALLY AMPLIFIES ENDOGENOUS cGMP-DEPENDENT RELAXATION IN RABBIT CORPUS CAVERNOSUM SMOOTH MUSCLE IN VITRO

Purpose The primary mechanism for relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide (NO)-induced elevation of myoplasmic cyclic guanosine monophosphate (cGMP). Agents that enhance the NO-cGMP signal transduction pathway may prove beneficial in treating erectile dysfunction. Sildenafil, a selective type-5 cGMP phosphodiesterase inhibitor, was investigated to determine the specific mechanism(s) involved in the therapeutic use of this compound to treat impotence. Materials and Methods Isolated strips of rabbit corpus cavernosum were stimulated isometrically with phenylephrine. Graded relaxations were induced using various concentrations of sodium nitroprusside (SNP) alone and in combination with sildenafil. At fixed times, the tissues were rapidly frozen and processed for myosin light chain (MLC) phosphorylation using isoelectric focusing with Western blot analysis, and cGMP content using radioimmunoassay techniques. Results Sildenafil alone reduced spontaneous tone in unstimulated CCSM, but had little effect on phenylephrine-induced isometric tension in the absence of a NO donor (SNP). Sildenafil sensitized the tissue to SNP for relaxation, but the relationship between relaxation and [cGMP] was unchanged by sildenafil. Relaxation from peak isometric force was correlated with [cGMP] but not MLC phosphorylation. Conclusions Sildenafil relaxes CCSM by amplifying the effects of the normal, endogenous cGMP dependent relaxation mechanisms.

SPORT-SEASONAL CHANGES IN BODY COMPOSITION, STRENGTH AND POWER OF COLLEGIATE WRESTLERS

616 Changes in body composition, muscular strength, and anaerobic power of collegeaged wrestlers (N = 25, 19.3 ± 0.3 yrs) were studied pre-, mid-, and post-season. Wrestlers were divided into two groups “cyclers”(N=14) and “noncyclers” (N=11), based upon body weight fluctuation and self-reported dietary practices. Isometric strength measures were evaluated from a mid-thigh clean pull using a specially constructed isometric rack while standing on a 61.0 × 121.9 cm AMTI force plate. Body density was measured via hydrostatic weighing with residual volume determined via the nitrogen washout technique. For fat-free mass (FFM), a significant difference in the pattern of change was found [F(2, 46) = 9.52, p < 0.001], highlighted by a decrease in FFM (-2.9%, -2.1 Kg, -4.6 Lb.) at mid-season compared to pre-season for the cyclers. A significant increase (1.2 Kg, p = 0.025) in FFM occurred at post-season. No significant interaction effects were seen for percent body fat (%fat), fat mass (FM), anaerobic power (AP), or any of the isometric strength measures. Significant time main effects were found for% fat, body weight (BW), FM, and FFM. FFM correlated to isometric peak force (IPF) ( r = 0.53, p < 0.05) and isometric rate of force development (IRFD) ( r = 0.41, p < 0.05) at mid-season. Taken together, the data indicate that collegiate wrestlers who fluctuate their body weight throughout the competitive season lose a significant amount of FFM without concomitant decreases in isometric strength or power.

Cross-bridge dynamics in the contracting heart.

The mechanical characteristics of the myosin motor is one of the key determinants of ventricular function. In small mammals there are two myosin isoforms, V1 and V3, with profoundly different performance characteristics. We used myothermal and mechanical analysis of intact papillary muscles from thryoxine (V1) and popylthiouracil (V3) treated rabbit hearts to assess the mechanical attributes of the myosin cross-bridge cycle. The average cross-bridge force time integral for V1 papillary muscles is 0.15 +/- 0.02 pNs for the entire isometric twitch and 0.19 +/- 0.03 pNs for the portion of the isometric twitch between 0.9 peak isometric force for the rising and declining portions of the twitch. The ratio of V1/V3 for the cross-bridge force time integral for the entire twitch and at the peak of the twitch is 0.5 (p < 0.05) and 0.4 (p < 0.05), respectively. Since the peak of the twitch measurements minimize internal shortening only these will be presented below. The average unitary force and attachment time during the peak of the twitch for V1 hearts was 1.55 +/- 0.37 pN and 140 +/- 20 msec, respectively. The ratios of V1/V3 for these parameters were 0.6 (p < 0.05) and 0.8 (ns). The cycling rate and duty cycle for V1 were 4.37 +/- 0.81 cycles per head-second and 0.66 +/- 0.22. The ratios of V1/V3 for cycling rate and duty cycle were 2.8 (p < 0.05) and 2.7 (ns). These measurements are consistent with and help explain the energetic and mechanical function of the intact heart.

Effect of 17 days of bed rest on peak isometric force and unloaded shortening velocity of human soleus fibers.

The purpose of this study was to examine the effect of prolonged bed rest (BR) on the peak isometric force (P0) and unloaded shortening velocity (V0) of single Ca(2+)-activated muscle fibers. Soleus muscle biopsies were obtained from eight adult males before and after 17 days of 6 degrees head-down BR. Chemically permeabilized single fiber segments were mounted between a force transducer and position motor, activated with saturating levels of Ca2+, and subjected to slack length steps. V0 was determined by plotting the time for force redevelopment vs. the slack step distance. Gel electrophoresis revealed that 96% of the pre- and 87% of the post-BR fibers studied expressed only the slow type I myosin heavy chain isoform. Fibers with diameter > 100 microns made up only 14% of this post-BR type I population compared with 33% of the pre-BR type I population. Consequently, the post-BR type I fibers (n = 147) were, on average, 5% smaller in diameter than the pre-BR type I fibers (n = 218) and produced 13% less absolute P0. BR had no overall effect on P0 per fiber cross-sectional area (P0/CSA), even though half of the subjects displayed a decline of 9-12% in P0/CSA after BR. Type I fiber V0 increased by an average of 34% with BR. Although the ratio of myosin light chain 3 to myosin light chain 2 also rose with BR, there was no correlation between this ratio and V0 for either the pre- or post-BR fibers. In separate fibers obtained from the original biopsies, quantitative electron microscopy revealed a 20-24% decrease in thin filament density, with no change in thick filament density. These results raise the possibility that alterations in the geometric relationships between thin and thick filaments may be at least partially responsible for the elevated V0 of the post-BR type I fibers.

Contractile Properties of Obliquely Striated Muscle from the Mantle of Squid (Alloteuthis Subulata) and Cuttlefish (Sepia Officinalis)

The mechanical properties of obliquely striated muscle fibres were investigated using thin slices of mantle from squid Alloteuthis subulata and cuttlefish Sepia officinalis. Brief tetani or twitch stimuli were used as this pattern is likely to occur during jetting of the intact animal. The length&shy;active force relationship for twitches and tetani (0.1s, 50Hz) was similar to that of vertebrate cross-striated fibres. Passive force at the length giving maximum tetanic force was 0.13&plusmn;0.05P0 (mean &plusmn; s.e.m., N=6, where P0 is maximum isometric tetanus force) and increased steeply at longer lengths. Peak force in a brief isometric tetanus (0.2s, 100&shy;150Hz) was 262&plusmn;16mNmm-2 cross-sectional area of wet tissue (N=6) for squid, and 226&plusmn;19mNmm-2 (N=7) for cuttlefish. The force&shy;velocity relationship for isotonic shortening during twitches of squid mantle slices was a 'double hyperbolic' relationship as described for cross-striated fibres by Edman. Fitting Edman's equation to the results gave: P*=1.18&plusmn;0.07, Vmax=2.43&plusmn;0.11Ltws-1 and 1/G=0.69&plusmn;0.13 (N=8), where P* is the intercept on the force axis expressed relative to Ptw, peak isometric twitch force, Vmax is the intercept on the velocity axis, Ltw is the length at which Ptw is produced and G is the constant expressing curvature. The large values of 1/G indicate that the force&shy;velocity relationship is not very curved. Maximum power was produced during shortening at 0.45&plusmn;0.03Ptw (N=8). Maximum power during twitch contraction was 18.3&plusmn;1.7mWg-1wetmass or, expressed in relative units, (V/Vmax)(P/Ptw), where V is the velocity during shortening and P is the force during shortening, was 0.16&plusmn;0.01 (N=8), which is higher than that of many cross-striated locomotor muscles.

THE EFFECT OF COMBINED VERSUS ECCENTRIC TRAINING ON MARKERS OF OVERSTRESS298

The main purpose of the study was to determine if the eccentric phase of a squat would induce muscle trauma and subsequent performance decrements in IRM comparable to combined concentric-eccentric training. 28 weight trained subjects were assigned to an eccentric (ECC, n=11) or combined (COM, n=9) training group. ECC lowered the weight from a standing to a parallel position. COM performed both phases of the squat. All performed 13 training sessions (10 sets; 1 rep/set, 100% IRM 2min rest between sets). IRM was assessed on a daily basis. Peak isometric and isokinetic forces were assessed before, mid-, and after training. Muscle biopsies to assess glycogen levels and muscle damage, were performed before and after the training. Profile of Mood States (POMS) were completed every 3d. A repeated measures ANOVA (p<.05) revealed no significant group difference in: IRM squat, glycogen depletion, and POMS. There was a significant decrease in isometric force for COM, and a significant increase for ECC. Z-line streaming, indicative of muscle damage, was seen in ECC. Conclusion: similar physiological and psychological responses were observed between the two training groups. Surprisingly, even though damage was evident in ECC, they demonstrated a significant increase in isometric strength, suggesting that muscle trauma might not be a limiting factor in an increase in isometric force.

Acidosis has no effect on the ATP cost of contraction in cat fast- and slow-twitch skeletal muscles.

Studies of skinned fibers suggest that the rate of ATP turnover in skeletal muscle is depressed by acidosis. To examine whether this occurs in intact muscles, the ATP cost of isometric contractions was measured in ex vivo, arterially perfused cat biceps (predominantly fast-twitch) and soleus (slow-twitch) muscles under normocapnic (5% CO2) and hypercapnic (70% CO2) conditions. Hypercapnia decreased extracellular pH from 7.4 to 6.7 and intracellular pH from 7.1 to 6.5 (soleus) or 6.6 (biceps) but had no significant effect on the phosphocreatine (PCr)-to-ATP ratio in muscles at rest. The ATP cost of contraction was estimated from PCr changes, measured by gating the acquisition of 31P-nuclear magnetic resonance spectra to times before and after brief tetani (1 s at 100 Hz and 2 s at 25 Hz for biceps and soleus, respectively) or 10-s trains of twitches (2 and 1 Hz, respectively). Peak isometric force and the ATP cost of tetanic contraction (PCr/force x time integral) were not significantly different under hypercapnic compared with normocapnic conditions in either muscle (mean: 7.97 and 2.44 micromol x kg(-1) x s(-1) for biceps and soleus, respectively). Twitch force and the ATP cost per twitch decreased by nearly 50% during hypercapnic perfusion in both muscle types. The results indicate that hypercapnic acidosis has no significant effect on the ATPase rate per active myosin head in intact mammalian skeletal muscle.

Human heart failure: determinants of ventricular dysfunction.

Thin muscle strips were obtained from non-failing (NF) and failing (dilated cardiomyopathy (DCM)) hearts, using a new harvesting and dissection technique. The strips were used to carry out a myothermal and mechanical analysis so that contractile and excitation coupling phenomena in the NF and failing (DCM-F) preparations can be compared. Peak isometric force and rate of relaxation in DCM-F were reduced 46% (p < 0.02) while time to peak tension was increased 14% (p < 0.03). Initial, tension dependent, tension independent and the rate of tension independent heat liberation were reduced 62-70% in DCM-F (p < 0.03). The crossbridge force-time integral (FTIXBr) was calculated from these measurements and was shown to increase 40% while the amount and rate of calcium cycled per beat was reduced 70%. As a result of these changes in the contractile and excitation-contraction coupling systems in DCM-F, the force-frequency relationship was significantly blunted while the power output was markedly reduced. These fundamental alterations account for the substantial ventricular dysfunction found in the dilated cardiomyopathic failing heart.

Effects of asymmetric dynamic and isometric liftings on strength/force and rating of perceived exertion

A laboratory study was undertaken to determine the postural and physical characteristics and subjective stress during dynamic lifting of a usual load (10 kg) compared with during isometric lifting. The authors also aimed to clarify the effects of asymmetric lifting on these parameters. The subjects were thirteen male college students. They were asked to lift a box weighing 10 kg. They performed sixteen different lifting tasks from the floor to a height of 71 cm, involving a combination of three independent factors: two lifting modes (isometric lifting and dynamic lifting), four lifting angles in relation to the sagittal plane (sagittal plane, right 45 degree, right 90 degree and left 90 degree planes) and two lifting postures (squat and stoop). For each lifting task, strengths or forces and ground reaction forces were measured. At the end of each task, the authors asked the subjects to rate their perceived exertion (RPE) during lifting at ten sites of the body. Angle factor had a significant effect on isometric strengths and dynamic peak forces. Isometric strengths during the maximum 3 s were highest in lifting in the right 45 degree plane, followed by that in the sagittal plane, while those in the right 90 degree and left 90 degree planes were the lowest. However, peak forces in dynamic lifting were the highest in the lifting in the sagittal plane, followed by that in the right 45 degree plane, while those in the right 90 degree and left 90 degree planes were the lowest. Postural factor had a significant effect on height at peak force, which is higher in squat lifting than in stoop lifting. RPEs for the left arm, the backs and the right whole body in isometric lifting were significantly higher than in dynamic lifting of 10 kg. There were remarkably high RPEs for the ipsilateral thigh to the box in right 90 degree and left 90 degree planes during both isometric and dynamic liftings. Locations of the resultant force consisting of three component forces on the force plate were closer to the foot on the same side as the box in asymmetric lifting. Thus, some similarities and differences were found between isometric lifting and dynamic liftings regarding the indexes of strength used in this experiment. The authors consider that the subjects used the foot nearer to the box as a fulcrum during asymmetric lifting. Dynamic measurement using the 10 kg weight is less stressful than the conventional isometric measurement. It was possible to obtain the height data at peak force and time-based changes in the force and the box location during lifting only through dynamic lifting measurement. The results provide new knowledge about the biomechanical features of dynamic lifting tasks.

Soleus fiber force and maximal shortening velocity after non-weight bearing with intermittent activity.

This study examined the effectiveness of intermittent weight bearing (IWB) as a countermeasure to non-weight-bearing (NWB)-induced alterations in soleus type I fiber force (in mN), tension (Po; force per fiber cross-sectional area in kN/m-2), and maximal unloaded shortening velocity (Vo, in fiber lengths/s). Adult rats were assigned to one of the following groups: normal weight bearing (WB), 14 days of hindlimb NWB (NWB group), and 14 days of hindlimb NWB with IWB treatments (IWB group). The IWB treatment consisted of four 10-min periods of standing WB each day. Single, chemically permeabilized soleus fiber segments were mounted between a force transducer and position motor and were studied at maximal Ca2+ activation, after which type I fiber myosin heavy-chain composition was confirmed by sodium dodecyl sufate-polyacrylamide gel electrophoresis. NWB resulted in a loss in relative soleus mass (-45%), with type I fibers displaying reductions in diameter (-28%) and peak isometric force (-55%) and an increase in Vo (+33%). In addition, NWB induced a 16% reduction in type I fiber Po, a 41% reduction in type I fiber peak elastic modulus [Eo, defined as (delta force/delta length) x (fiber length/fiber cross-sectional area] and a significant increase in the Po/Eo ratio. In contrast to NWB, IWB reduced the loss of relative soleus mass (by 22%) and attenuated alterations in type I fiber diameter (by 36%), peak force (by 29%), and Vo (by 48%) but had no significant effect on Po, Eo, or Po/Eo. These results indicate that a modest restoration of WB activity during 14 days of NWB is sufficient to attenuate type I fiber atrophy and to partially restore type I peak isometric force and Vo to WB levels. However, the NWB-induced reductions in Po and Eo, which we hypothesize to be due to a decline in the number and stiffness of cross bridges, respectively, are considerably less responsive to this countermeasure treatment.

Fatigue and heat production in repeated contractions of mouse skeletal muscle.

1. This study tested the hypothesis that moderate fatigue of skeletal muscle arises from a mismatch between energy demand and energy supply. Fatigue was defined as the decline in isometric force. Energy supply and demand were assessed from measurements of muscle heat production. 2. Experiments were performed in vitro (21 degrees C) with bundles of muscle fibres from mouse fast-twitch extensor digitorum longus muscle and slow-twitch soleus muscle. Fibre bundles were fatigued using a series of thirty isometric tetani. Cycle duration (time between successive tetani) was 5 s. The amount of fatigue that occurred during a series of tetani was varied by varying contraction duty cycle (tetanus duration/cycle duration) by varying tetanus duration. 3. Peak isometric force and total heat production in each cycle were measured. For each cycle, the amounts of initial heat (H(i)) and recovery heat (Hr) produced were calculated and used as indices of energy use and supply, respectively. H(i) and Hr were used to estimate the net initial chemical breakdown (in energy units) in each cycle (H(i,net)). 4. The magnitude of H(i,net) was greatest in the early stages of the contraction protocol when Hr was still increasing towards a steady value. The magnitude of decline in force between successive tetani was proportional to H(i,net) for both muscles. 5. The results are consistent with the idea that the development of moderate levels of fatigue at the start of a series of contractions is due to the rate of energy supply being inadequate to match the rate of energy use.

Comparison of twitch force and calcium handling in papillary muscles from right ventricular pressure overload hypertrophy in weanling and juvenile ferrets

The aim was to compare differences in peak twitch force occurring despite similar degrees of right ventricular hypertrophy in ferrets with pulmonary artery banding at either weanling or juvenile age. After inducing pressure overload hypertrophy by banding the pulmonary artery of weanling and juvenile age ferrets, mechanical function (that is, isometric twitch force and passive stiffness), intracellular [Ca2+] using the calcium indicator aequorin, markers of myocardial energy supply, and quantified connective tissue content were studied. It was previously found that there was a reduced peak isometric twitch force despite normal [Ca2+]i in juvenile banded ferrets age 10-12 weeks with right ventricular pressure overload hypertrophy (POHj). In the present study we report findings in banded weanling ferrets (POHw) age 7 weeks. POHw animals showed a similar degree of hypertrophy to that found in the POHj. However, there was a greater peak twitch force in hypertrophied muscles at higher [Ca2+]o. There was no difference in peak [Ca2+]i: -3.1(SEM 0.1) v -3.1(0.3) (log fractional luminescence) at 16 mM [Ca2+]o for control and POHw, respectively. Connective tissue content for control animals was 10(1)% versus 10(2)% in POHw. Despite a lack of quantitative change in connective tissue content or resting [Ca2+]i in POHw, passive stiffness in papillary muscles was increased. Retrospective analysis of tissue from POHj revealed a connective tissue content of 24(6.8)% (P << 0.001). Thus the decreased peak twitch force reported in POHj might in part be due to an increase in fibrous connective tissue. In this study, lactate dehydrogenase was significantly higher (38%) in POHw animals. In contradistinction, total creatine kinase activity and total creatine content were significantly less (22%) in hearts from POHj animals, indicating differences in myocyte remodelling despite similar degrees and durations of hypertrophy. Comparison of POHw and POHj showed that, when there is restructuring of the extracellular space in terms of increased fibrosis, there is also molecular remodelling in the myocyte, as demonstrated by a decrease in the creatine kinase system.

Contraction characteristics and myosin heavy chain composition of rabbit masseter motor units

1. We studied isometric twitch peak force (TPF) and twitch contraction time (TCT) of 249 motor units of the masseter muscle in 41 rabbits after extracellular electrical stimulation of single trigeminal motoneurons in the brain stem. In 41 of these units we determined the amount of tension decrease during a partially fused tetanus (sag) and the ratio between peak tetanic force after 2 min of intermittent tetanic stimulation and initial tension (fatigue index). Muscle fibers of 24 motor units were identified by the glycogen depletion method and characterized in serial sections with monoclonal antibodies against type IID, IIA, "cardiac" alpha, and I isoforms of myosin heavy chain (MHC). 2. The motor units had TCTs ranging from 13 to 32 ms. The majority of the units showed forces < 35 mN. The TPFs were larger and varied more for motor units with short and intermediate TCTs than for units with long TCTs. There is a small but statistically significant negative correlation between the motor unit TPF and the TCT. 3. All units exhibited "sag" and, with the exeption of one, had fatigue indexes > 0.75. The studied rabbit masseter motor units can therefore be classified as fast, fatigue-resistant, except for one that belonged to the FF (fast, fatigable) category. No slow units were represented in the sample pool. Significant correlations were not found either between TCT and the amount of sag or between TCT and the fatigue index. 4. Immunohistochemical analysis showed that the FF unit had fibers containing only IID-MHC. Five other units were found with a single MHC--three with IIA-MHC and two with alpha-MHC. In three other units all fibers showed one combination of two MHCs (1 IIA/IID, 1 IIA/alpha, and 1 alpha/I). The remaining 15 units contained two MHCs spread unevenly over the constituting fibers. Large variations in myosin composition of fibers within one motor unit cast doubts on the presumed dominant neuronal influence on myosin expression in the adult animal. 5. We found a close, statistically significant correlation between the TCT and the estimated MHC content of the units: the TCT was 13 ms for the IID unit, 18 ms for the pure IIA units, and 28 ms for the pure alpha units. Units with two MHCs had intermediate TCTs; units with alpha/I-MHC mixtures had TCTs of 29-30 ms. No pure MHC-I units were identified.(ABSTRACT TRUNCATED AT 400 WORDS)

Assessment of Quadriceps Muscle Performance by a Hand-Held Dynamometer and an Isokinetic Dynamometer

Assessment of muscle performance is an integral component of patient evaluation. The primary purposes of this study were to examine the relationship between isokinetic dynamometer isokinetic peak force values and hand-held dynamometer isometric peak force values when assessing quadriceps muscle performance in subjects with orthopaedic knee dysfunctions (N = 21) and to test for differences in muscle performance between injured and noninjured lower extremities using a hand-held dynamometer and an isokinetic dynamometer. An analysis of variance was used to compare injured and noninjured extremities for both testing devices. A Pearson product moment correlation was used to examine the relationship between the hand-held dynamometer and isokinetic dynamometer values. No significant difference between extremities at 0 degrees (p = 0.1224) or 60 degrees (p = 0.8267) was revealed when testing with the hand-held dynamometer. The isokinetic dynamometer, however, revealed a significant difference between extremities at 60 degrees/sec (p = 0.0041). Correlations between the two devices were significant, ranging from r = 0.57 to 0.80. We concluded that these correlations were misleading because more force was generated by the quadriceps muscles tested than a tester of average strength could resist. Further research is needed to validate the use of the hand-held dynamometer by a tester of average strength.

Mechanical and electrophysiological effects of milrinone on the force‐frequency relationship in mammalian myocardium

Isometric force, action potential and current-voltage relation were studied in guinea-pig and ferret papillary muscles. Milrinone (1 microM) increased peak twitch force by 40 +/- 4%, reduced time to peak tension (TPT) by 12.1 +/- 3% (n = 6, P < 0.01) and reduced time to half relaxation by 17.3 +/- 4.1% (n = 6, P < 0.01). The effect of milrinone was potentiated by rolipram, a RI-PDE inhibitor which in itself had no inotropic effect. After the addition of rolipram peak isometric force was increased by 104 +/- 8% (n = 6, P < 0.001), TPT was further reduced whereas time to half relaxation was slightly increased after the addition of rolipram. Action potential duration at 75% repolarization was decreased by 11 +/- 5 ms (n = 6, P < 0.05). Milrinone also potentiated the second inward current (Isi) by 21 +/- 3.2% (n = 6, P < 0.01). Peak twitch force in response to a test stimulus after an interval, i.e. mechanical restitution was increased at all intervals. The onset of restitution was faster and time to full restitution also shortened. Maximum postextrasystolic potentiation was greater in the presence of milrinone, whereas relative potentiation was smaller in presence of milrinone (46 +/- 7%) than in control (74 +/- 7%). The recirculation fraction of activator calcium was enhanced by milrinone from 0.35 +/- 0.04 to 0.48 +/- 0.07. The results support the view that the positive inotropic effect of milrinone is due to a greater inflow of calcium during the action potential and a more efficient intracellular calcium handling.(ABSTRACT TRUNCATED AT 250 WORDS)

Threshold for force potentiation associated with skeletal myosin phosphorylation

Phosphate incorporation by the phosphorylatable light chains (P-LC) of myosin is associated with isometric twitch force potentiation in intact fast-twitch muscle. The purpose of this study was to examine the association between myosin P-LC phosphorylation and force potentiation at higher stimulation frequencies (1-150 Hz) using mouse extensor digitorum longus (EDL) muscles at 25 degrees C. Peak isometric force and the peak rate of isometric force development (+dF/dtmax) were measured at selected test frequencies before and after the application of a 5-Hz 20-s conditioning stimulation known to increase P-LC phosphate content. Associated with a ninefold elevation in myosin P-LC phosphate content (to 0.72 mol phosphate/mol P-LC), +dF/dtmax was increased at all test frequencies (mean 27%, range 20-37%). After the conditioning stimulus, peak isometric force was increased by approximately 15% for frequencies 1-15 Hz. However, at 20-150 Hz, the increase in +dF/dtmax was not associated with force potentiation, since peak force was diminished by 5-40%. These data reveal that the stimulation frequency limit for the potentiation of peak force production associated with myosin P-LC phosphorylation is < 20 Hz in mouse EDL at 25 degrees C. Furthermore, the data suggest that increases in the rate constant describing the rate of cross-bridge transition from a non-force-generating to a force-generating state mediated by myosin P-LC phosphorylation may be responsible for the general increase in +dF/dtmax and for the force potentiation at 1-15 Hz.

Isokinetic Trunk Strength and Lifting Strength Measures

Fifty-eight back pain patients and 21 entry-level Postal Service workers without low-back pain were evaluated using a variety of lumbar function measures. Isolated trunk strength and full lifting strength were gauged with isokinetic and isometric methods. Lumbar range-of-motion was computed using toe-touch and goniometers. Conventional clinical techniques such as toe touch and straight leg raise were effective in distinguishing back-injured from normal subjects. Isometric and isokinetic peak force and torque tests failed to show significant differences between low-back pain and job applicant groups. When compared with published norms, our job applicant group was significantly deconditioned. Our data suggest that asymptomatic, deconditioned subjects could be mistaken for back-impaired patients or symptom magnifiers.