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PubMed1996

Simultaneous assessment of isometric forces in fast- and slow-twitch muscles of single rat hindlimbs in situ.

An apparatus, consisting of a pair of small strain gauge transducers, was designed for the simultaneous assessment of isometric contractile function in two muscles, composed of a predominance of either fast- or slow-twitch fibers, and within a single rat hindlimb in situ. This facilitates assessment of mechanical performance of two separate muscles under identical conditions. In anesthetized rats (N = 10), the voltages and frequencies required to produce isometric twitch and tetanic forces from the soleus (SOL) and extensor digitorum longus (EDL) muscles were determined. The apparatus was then used to demonstrate the simultaneous assessment of forces produced by the SOL and EDL from the same hindlimbs (n = 5) during 30 min of fatigue and 30 min of recovery. With this apparatus, data collected were comparable to published data. The apparatus can be used for the simultaneous assessment of isometric contractile function and fatigue in both a fast- and a slow-twitch muscle of a single rat hindlimb in situ.

Animals↗

PubMed1996

Gender differences in isometric contractile properties and fatigability in elderly human muscle.

This study had two purposes: to compare the isometric contractile characteristics and fatigability in the elbow flexors (EF) and ankle dorsiflexors (DF) in older males and females (60-80 years), and to determine the effects of almost 2 years of twice weekly dynamic weight-lifting training on these properties One hundred nine male and female subjects completed the 22-month intervention, 54 in the exercise group and 55 in the control group. Isometric contractile properties and fatigue characteristics were assessed at baseline, 10 months, and 22 months. At baseline, males were significantly stronger (p < .05) than females in evoked twitch torque (Pt) and maximal voluntary strength (MVC) in both EF and DF. Females exhibited significantly less fatigue (expressed either absolutely or relatively) than males in both Pt and MVC during a 3-min protocol of intermittent MVCs (5 s contraction; 2 s rest). There were no significant effects of the 22 months of dynamic training on either the isometric contractile properties or the fatigue characteristics, suggesting a very strong and persistent specificity of training effect.

Aged↗

PubMed1989

The relative changes in isometric force and work during fatigue and recovery in isolated toad sartorius muscle.

Toad sartorius muscle was subjected to sinusoidal varying length changes at 2 Hz to measure work. Both isometric tetanic force and work per cycle were measured before, during, and after a 3-min fatigue. Both isometric tetanic force and positive work, the work done by the muscle during the shortening part of the cycle, rapidly decreased in parallel in the first 40 s of fatigue. Thereafter, force continued to decrease, but at a slower rate, to about 10% of prefatigue values, whereas positive work levelled off at about 30% of prefatigue values. Negative work, the work done on the muscle during the lengthening part of the cycle, increased during fatigue to the extent that net work became negative. This was due to a prolonged relaxation, which resulted in active force still being generated while the muscle was being stretched. Work and force recovered at about the same rate. Isometric force measurements alone do not give any clear indication that net work will be negative under a particular set of experimental conditions.

Animals↗

PubMed2002

Metabolic costs of isometric force generation and maintenance of human skeletal muscle.

During isometric contractions, no true work is performed, so the force-time integral (FTI) is often used to approximate isometric work. However, the relationship between FTI and metabolic cost is not as linear. We tested the hypothesis that this nonlinearity was due to the cost of attaining a given force being greater than that of maintaining it. The ATP consumed per contraction in the human medial gastrocnemius muscle (n = 6) was determined by use of (31)P-NMR spectroscopy during eight different electrical stimulation protocols. Each protocol consisted of 8 trains of a single frequency (20 or 80 Hz) and duration (300, 600, 1,200, or 1,800 ms) performed under ischemic conditions. The cost of force generation was determined from the ATP turnover during the short-duration trains that did not attain a steady force level. Estimates of the cost of force maintenance at each frequency were determined by subtracting the ATP turnover during the shorter-duration trains from the turnover during the long-duration trains. The force generation phase of an isometric contraction was indeed more metabolically costly than the force maintenance phase during both 20- and 80-Hz stimulation. Thus the mean rate of ATP hydrolysis appeared to decline as contraction duration increased. Interestingly, the metabolic costs of maintaining force during 20-Hz and 80-Hz stimulation were comparable, although different levels of force were produced.

Adenosine Triphosphate↗

PubMed1992

Neurally mediated renal vasoconstriction during isometric muscle contraction in cats.

The aim of this study was to determine if the reflex increase in renal sympathetic nerve activity (RSNA) during static (isometric) muscle contraction evokes renal vasoconstriction and decreases renal blood flow. RSNA, renal blood flow velocity, and arterial pressure were measured simultaneously during isometric contraction of the hindlimb triceps surae muscle in eight chloralose-anesthetized cats. A 1-min contraction was evoked by stimulating the peripheral ends of the cut L7 and S1 ventral roots. RSNA and mean arterial pressure (MAP) increased 41 +/- 14% (SE) and 50 +/- 10 mmHg during static contraction, whereas mean renal blood flow velocity (MRBV) decreased 14 +/- 5%. Calculated renal vascular resistance increased 73 +/- 20% during the contraction. The increase in RSNA preceded the decrease in MRBV by 20 s. Passive mechanical stretch of the muscle increased RSNA 21 +/- 12% but did not alter MRBV. Renal denervation abolished the decrease in MRBV during isometric contraction but only attenuated the rise in MAP. Cutting the L4-S1 dorsal roots or muscle paralysis abolished the MRBV and MAP responses. Thus reflex stimulation of RSNA from the contracting muscle can induce renal vasoconstriction and decrease renal blood flow.

Animals↗

PubMed1992

Agonist-induced myosin phosphorylation during isometric contraction and unloaded shortening in airway smooth muscle.

We measured myosin phosphorylation during isometric contraction at optimal length (Lo) and unloaded shortening induced by K(+)-depolarization, electrical stimulation, carbachol, histamine, and phorbol dibutyrate (PDB) in bovine trachealis. Peak myosin phosphorylation during unloaded shortening was lower than that during isometric contraction in response to all stimuli. The lower peak myosin phosphorylation during unloaded shortening appeared to be a stretch-sensitive response because myosin phosphorylation was either equally low or further reduced during the second unloaded shortening of preshortened tissues. Similar to peak myosin phosphorylation, steady-state myosin phosphorylation was also lower during unloaded shortening in carbachol-induced contractions. However, steady-state phosphorylation during unloaded shortening and isometric contraction were not significantly different in histamine- and PDB-induced contractions. Since the coupling between Ca2+ and myosin phosphorylation was not stretch sensitive, these results suggest the coexistence of stretch-sensitive and stretch-insensitive signal transduction mechanisms in the airway smooth muscle cell membrane, and the stretch-insensitive signal transduction mechanism might involve protein phosphorylation by protein kinase C.

Animals↗

PubMed1977

Insulin and glucose responses during bed rest with isotonic and isometric exercise.

The effects of daily intensive isotonic (68% maximum oxygen uptake (VO2 max)) and isometric (21% maximum extension force) leg exercise on plasma insulin and glucose responses to an oral glucose tolerance test (OGTT) during 14-day bed-rest (BR) periods were investigated in seven young healthy men. The OGTT was given during ambulatory control and on day 10 of the no-exercise, isotonic, and isometric exercise BR periods during the 15-wk study. The subjects were placed on a controlled diet (mean +/- SD = 344 +/- 34 g CHO/day and 3,073 +/- 155 (SD) kcal/day) starting 10 days before each BR period. During BR, basal plasma glucose concentration remained unchanged with no exercise, but increased (P less than 0.05) to 87-89 mg/100 ml with both exercise regimens on day 2, and then fell slightly below control levels on day 13. The fall of glucose content (-11 to -15%) during BR was independent of the exercise regimen and was an adjustment for the loss of plasma vol. The intensity of the response of insulin and glucose to the OGTT (integrated area under the curves) was inversely proportional to the total daily energy expenditure during BR; i.e., the largest response with no exercise, then isometric, isotonic, and ambulatory exercise. It was estimated that at least 1,020 kcal/day must be provided by supplemental exercise to restore the hyperinsulinemia to control levels.

Adult↗

PubMed1978

Influence of posture on isometric fatigue.

The isometric strength of four trained subjects was unaltered by changes in posture. But the endurance of an isometric contraction held to fatigue at 25 and 40% of the maximum voluntary contraction (MVC) was 20% greater in the sitting than in the recumbent posture. This difference was abolished when the exercise was performed with the arm's circulation arrested. At rest, the blood flow through the forearm was greater when the subjects were in the recumbent than in the sitting position but the reverse was true during isometric contractions. In these two postures, there was no difference in the right atrial pressure during the contraction, suggesting that the low-pressure baroreceptors are not responsible for the differences in blood flow during exercise. To date no mechanism is available to explain these observations.

Adult↗

PubMed1982

Effect of aerobic conditioning on cardiovascular response to isometric exercise.

The response to isometric handgrip exercise (IHE) and isometric quadriceps exercise (IQE) (30% maximum voluntary contraction held 3 min) was studied before and after 5 wk of aerobic training. Training exercises involved only the lower extremities. Seventeen healthy unconditioned males aged 21--35 yr were subjects. During training nine subjects received propranolol in doses that provided a high degree of beta-adrenergic blockade; eight received a placebo. All subjects were tested before training or drug and after training, 3--5 days off drug. With IGE after training, the placebo group had lower maximum heart rate (91 +/- 4 to 79 +/- 5 beats/min, P less than 0.05), systolic blood pressure (151 +/- 5 to 139 +/- 4 mmHg, P less than 0.05), and double product (heart rate x systolic blood pressure) (138 +/- 10 x 10(2) to 110 +/- 7 x 10(2), P less than 0.05). The response to IQE in the propranolol group was unchanged after training. Heart rate, systolic blood pressure, and double product with IHE was unchanged in either group after training. Aerobic conditioning can modify the hemodynamic response to isometric exercise. This effect is specific for the trained muscle group and is prevented by beta-adrenergic blockade.

Adult↗

PubMed1989

Influence of muscle mass on sympathetic neural activation during isometric exercise.

The primary purpose of this study was to determine whether the sympathetic neural activation induced by isometric exercise is influenced by the size of the contracting muscle mass. To address this, in nine healthy subjects (aged 19-27 yr) we measured heart rate, systolic arterial blood pressure, and muscle sympathetic nerve activity in the leg (MSNA; peroneal nerve) before (control) and during 2.5 min of isometric handgrip exercise (30% of maximal voluntary force). Exercise was performed with the right and left arms separately and with both arms simultaneously (random order). During exercise, heart rate, systolic pressure, and MSNA increased above control under all conditions (P less than 0.05). For each variable, the magnitudes of the increases from control to the end of exercise were significantly greater when exercise was performed with two arms compared with either arm alone (P less than 0.05). In general, the increases in heart rate, systolic pressure, and MSNA elicited during two-arm exercise were significantly less than the simple sums of the responses evoked during exercise of each arm separately. These findings indicate that the magnitude of the sympathetic neural activation evoked during isometric exercise in humans is determined in part by the size of the active muscle mass. In addition, our results suggest that the sympathetic cardiovascular adjustments elicited during exercise of separate limbs are not simply additive but instead exhibit an inhibitory interaction (i.e., neural occlusion).

Adult↗

PubMed1994

Stimulus-response relationships for isotonic shortening and isometric tension generation in rabbit trachealis.

Nonspecific bronchial hyperresponsiveness in asthma is characterized by increased maximal airway narrowing (reactivity) and increased sensitivity of the airways. A decreased load on airway smooth muscle (ASM) has been suggested as a mechanism of increased reactivity. We hypothesized that decreased ASM load can also cause a leftward shift in the dose-response curve and explain increased sensitivity. We tested this hypothesis using rabbit tracheal smooth muscle strips in vitro by measuring isotonic shortening and isometric force during electrical field stimulation (1-100 Hz) at the length at which maximal active tension developed (Lmax), 90% Lmax, and 110% Lmax The frequency-response relationships expressed as frequency vs. percent maximal shortening or tension were not different at Lmax or 110% Lmax, but at 90% Lmax the frequency vs. shortening relationship was significantly shifted leftward relative to the frequency vs. tension relationship (P < 0.05). The electrical field stimulation frequencies that produced 50% maximal response for isometric tension and for isotonic shortening, respectively, were 6.7 +/- 1.9 and 3.9 +/- 0.7 Hz at 90% Lmax, 9.2 +/- 2.1 and 7.5 +/- 1.9 Hz at 100% Lmax, and 2.8 +/- 1.0 and 1.2 +/- 0.5 Hz at 110% Lmax. We conclude that, at lengths below Lmax, isotonic shortening is facilitated compared with isometric tension and therefore decreased ASM load in vivo may result in increased sensitivity.

Animals↗

PubMed1996

Metabolic heat production during fatigue from voluntary repetitive isometric contractions in humans.

The effect of repetitive isometric knee extensions on the energy cost of contraction was examined. The rate of temperature rise (dT/dt) was determined in test contractions at 30 and 50% of maximal voluntary contraction (MVC) force before and during 30% MVC repetitive isometric exercise (RIE) to exhaustion and regularly in a 30-min postexercise recovery period (n = 9). Pulmonary O2 uptake and muscle temperature (Tmus) were determined at regular intervals. During the 30% MVC test contractions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing linearly by 68% during exercise. In the 50% MVC test contractions, dT/dt rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test contractions at both force levels did not decrease significantly throughout the 30-min postexercise recovery period. The rise in dT/dt was paralleled by 76% increased in O2 uptake. In contrast, Tmus rose initially and then leveled off. The present data indicate that RIE induced a gradual rise in the rate of energy turnover associated with isometric force production. Neither increased Tmus nor recruitment of less economic type II fibers can fully explain the increased energy cost. We suggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occurring during fatigue from submaximal voluntary RIE.

Adult↗

PubMed1997

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

The purpose of this study was to estimate the efficiency of ATP utilization for concentric, eccentric, and isometric muscle action in the human tibialis anterior and extensor digitorum longus in vivo. A dynamometer was used to quantitate muscle work, or tension, while simultaneous 31P-nuclear magnetic resonance data were collected to monitor ATP, phosphocreatine, inorganic phosphate, and pH. The relative efficiency of the actions was estimated in two ways: steady-state effects on high-energy phosphates and a direct comparison of ATP synthesis rates with work. In the steady state, the cytosolic free energy dropped to the lowest value with concentric activity, followed by eccentric and isometric action for comparative muscle tensions. Estimates of ATP synthesis rates revealed a mechanochemical efficiency [i.e., ATP production rate/work (both in J/s)] of 15.0 +/- 1.3% in concentric and 34.7 +/- 6.1% in eccentric activity. The estimated maximum ATP production rate was highest in concentric action, suggesting an activation of energy metabolism under these conditions. By using direct measures of metabolic strain and ATP turnover, these data demonstrate a decreasing metabolic efficiency in human muscle action from isometric, to eccentric, to concentric action.

Adenosine Diphosphate↗

PubMed1998

Absence of myofibrillar creatine kinase and diaphragm isometric function during repetitive activation.

Creatine kinase (CK) provides ATP buffering in skeletal muscle and is expressed as 1) cytosolic myofibrillar CK (M-CK) and 2) sarcomeric mitochondrial CK (ScCKmit) isoforms that differ in their subcellular localization. We compared the isometric contractile and fatigue properties of 1) control CK-sufficient (Ctl), 2) M-CK-deficient (M-CK[-/-]), and 3) combined M-CK/ScCKmit-deficient null mutant (CK[-/-]) diaphragm (Dia) to determine the effect of the absence of M-CK activity on Dia performance in vitro. Baseline contractile properties were comparable across groups except for specific force, which was approximately 16% lower in CK[-/-] Dia compared with M-CK[-/-] and Ctl Dia. During repetitive activation (40 Hz, (1)/(3) duty cycle), force declined in all three groups. This decline was significantly greater in CK[-/-] Dia compared with Ctl and M-CK[-/-] Dia. The pattern of force decline did not differ between M-CK[-/-] and Ctl Dia. We conclude that Dia isometric muscle function is not absolutely dependent on the presence of M-CK, whereas the complete absence of CK acutely impairs isometric force generation during repetitive activation.

Adenylate Kinase↗

PubMed2001

Caffeine increases endurance and attenuates force sensation during submaximal isometric contractions.

Caffeine has known ergogenic effects, some of which have been observed during submaximal isometric contractions. We used 15 subjects in a randomized, double-blind, repeated-measures experiment to determine caffeine's ergogenic effects on neuromuscular variables that would contribute to increased endurance capacity. Subjects performed repeated submaximal (50% maximal voluntary contraction) isometric contractions of the right quadriceps to the limit of endurance (T(lim)) 1 h after oral caffeine administration (6 mg/kg). Time to reach T(lim) increased by 17 +/- 5.25% (P < 0.02) after caffeine administration compared with the placebo trial. The changes in contractile properties, motor unit activation, and M-wave amplitude that occurred as the quadriceps reached T(lim) could not account for the prolonged performance after caffeine ingestion. In a separate experiment with the same subjects, we used a constant-sensation technique to determine whether caffeine influenced force sensation during 100 s of an isometric contraction of the quadriceps. The results of this experiment showed that caffeine reduced force sensation during the first 10-20 s of the contraction. The rapidity of this effect suggests that caffeine exerts its effects neurally. Based on these data, the caffeine-induced increase in T(lim) may have been caused by a willingness to maintain near-maximal activation longer because of alterations in muscle sensory processes.

Adult↗

PubMed2002

Human neuromuscular fatigue is associated with altered Na+-K+-ATPase activity following isometric exercise.

The purpose of this study was to investigate the hypothesis that reductions in Na+-K+- ATPase activity are associated with neuromuscular fatigue following isometric exercise. In control (Con) and exercised (Ex) legs, force and electromyogram were measured in 14 volunteers [age, 23.4 +/- 0.7 (SE) yr] before and immediately after (PST0), 1 h after (PST1), and 4 h after (PST4) isometric, single-leg extension exercise at ~60% of maximal voluntary contraction for 30 min using a 0.5 duty cycle (5-s contraction, 5-s rest). Tissue was obtained from vastus lateralis muscle before exercise in Con and after exercise in both the Con (PST0) and Ex legs (PST0, PST1, PST4), for the measurements of Na+-K+-ATPase activity, as determined by the 3-O-methylfluorescein phosphatase (3-O-MFPase) assay. Voluntary (maximal voluntary contraction) and elicited (10, 20, 50, 100 Hz) force was reduced 30-55% (P < 0.05) at PST0 and did not recover by PST4. Muscle action potential (M-wave) amplitude and area (measured in the vastus medialis) and 3-O-MFPase activity at PST0-Ex were less than that at PST0-Con (P < 0.05) by 37, 25, and 38%, respectively. M-wave area at PST1-Ex was also less than that at PST1-Con (P < 0.05). Changes in 3-O-MFPase activity correlated to changes in M-wave area across all time points (r = 0.38, P < 0.05, n = 45). These results demonstrate that Na+-K+- ATPase activity is reduced by sustained isometric exercise in humans from that in a matched Con leg and that this reduction in Na+-K+-ATPase activity is associated with loss of excitability as indicated by M-wave alterations.

Action Potentials↗

PubMed2004

Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance.

We investigated the role of central activation in muscle length-dependent endurance. Central activation ratio (CAR) and rectified surface electromyogram (EMG) were studied during fatigue of isometric contractions of the knee extensors at 30 and 90 degrees knee angles (full extension = 0 degree). Subjects (n = 8) were tested on a custom-built ergometer. Maximal voluntary isometric knee extension with supramaximal superimposed burst stimulation (three 100-mus pulses; 300 Hz) was performed to assess CAR and maximal torque capacity (MTC). Surface EMG signals were obtained from vastus lateralis and rectus femoris muscles. At each angle, intermittent (15 s on 6 s off) isometric exercise at 50% MTC with superimposed stimulation was performed to exhaustion. During the fatigue task, a sphygmomanometer cuff around the upper thigh ensured full occlusion (400 mmHg) of the blood supply to the knee extensors. At least 2 days separated fatigue tests. MTC was not different between knee angles (30 degrees : 229.6 +/- 39.3 N.m vs. 90 degrees: 215.7 +/- 13.2 N.m). Endurance times, however, were significantly longer (P < 0.05) at 30 vs. 90 degrees (87.8 +/- 18.7 vs. 54.9 +/- 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30 degrees: 0.95 +/- 0.05 vs. 90 degrees: 0.96 +/- 0.03) and full occlusion of blood supply to the knee extensors. Furthermore, rectified surface EMG values of the vastus lateralis (normalized to prefatigue maximum) were also similar at torque failure (30 degrees : 56.5 +/- 12.5% vs. 90 degrees : 58.3 +/- 15.2%), whereas rectus femoris EMG activity was lower at 30 degrees (44.3 +/- 12.4%) vs. 90 degrees (69.5 +/- 25.3%). We conclude that differences in endurance at different knee angles do not find their origin in differences in central activation and blood flow but may be a consequence of muscle length-related differences in metabolic cost.

Adult↗

PubMed2005

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures.

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures (r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps (r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.

Adult↗