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PubMed1988

Coordination and inhomogeneous activation of human arm muscles during isometric torques.

1. In this study we have recorded the activity of motor units of the important muscles acting across the elbow joint during combinations of voluntary isometric torques in flexion/extension direction and supination/pronation direction at different angles of the elbow joint. 2. Most muscles are not activated homogeneously; instead the population of motor units of muscles can be subdivided into several subpopulations. Inhomogeneous activation of the population of motor units in a muscle is a general finding and is not restricted to some multifunctional muscles. 3. Muscles can be activated even if their mechanical action does not contribute directly to the external torque. For example, m. triceps is activated during supination torques and thus compensates for the flexion component of the m. biceps. On the other hand, motor units in muscles are not necessarily activated if their mechanical action contributes to a prescribed torque. For example, there are motor units in the m. biceps that are activated during flexion torques, but not during supination torques. 4. The relative activation of the muscles depends on the elbow angle. Changing the elbow angle affects the mechanical advantage of different muscles differently. In general, muscles with the larger mechanical advantage receive the larger input. 5. We have calculated the relative contributions of some muscles to isometric torques. These contributions depend on the combination of the torques exerted. 6. Existing theoretical models on muscle coordination do not incorporate subpopulations of motor units and therefore need to be amended.

Arm↗

PubMed1997

Cortical control of human motoneuron firing during isometric contraction.

We recorded whole scalp magnetoencephalographic (MEG) signals simultaneously with the surface electromyogram from upper and lower limb muscles of six healthy right-handed adults during voluntary isometric contraction. The 15- to 33-Hz MEG signals, originating from the anterior bank of the central sulcus, i.e., the primary motor cortex, were coherent with motor unit firing in all subjects and for all muscles. The coherent cortical rhythms originated in the hand motor area for upper limb muscles (1st dorsal interosseus, extensor indicis proprius, and biceps brachii) and close to the foot area for lower limb muscles (flexor hallucis brevis). The sites of origin corresponding to different upper limb muscles did not differ significantly. The cortical signals preceded motor unit firing by 12-53 ms. The lags were shortest for the biceps brachii and increased systematically with increasing corticomuscular distance. We suggest that the motor cortex drives the spinal motoneuronal pool during sustained contractions, with the observed cortical rhythmic activity influencing the timing of efferent commands. The cortical rhythms could be related to motor binding, but the rhythmic output may also serve to optimize motor cortex output during isometric contractions.

Adult↗

PubMedDate not supplied

Role of voltage-dependent and Ca(2+)-activated K(+) channels on the regulation of isometric force in porcine coronary artery.

We investigated the role of K(+) channels in the regulation of vascular tone in de-endothelialized porcine coronary artery. Isometric force and intracellular Ca(2+) ([Ca(2+)](i)) under resting conditions were increased by treatment with 4-aminopyridine (4-AP, 1 mM), an inhibitor of voltage-dependent K(+) (K(v)) channels, but not by tetraethylammonium chloride (TEA, 1 mM) or charybdotoxin (100 nM), both inhibitors of Ca(2+)-activated K(+) (K(Ca)) channels, or glibenclamide (10 microM), an inhibitor of ATP-sensitive K(+) channels. Under stimulated conditions with 9,11-dideoxy-11alpha, 9alpha-epoxymethano-prostaglandin F(2alpha) (U46619), 4-AP as well as TEA or charybdotoxin increased isometric force and [Ca(2+)](i), but not glibenclamide. 4-AP was the most potent in terms of depolarization of membrane potential compared with TEA or glibenclamide in the presence or absence of EGTA. In the presence of U46619, a high concentration of 4-AP (10 mM) caused a further contraction with oscillations. The force oscillations induced by 4-AP were inhibited by diltiazem (10 microM), an inhibitor of voltage-dependent Ca(2+) channels, or TEA (1 mM), but not by glibenclamide (10 microM). These force oscillations may be associated with the periodic activation of K(Ca) channels. These findings suggested that 4-AP-sensitive K(v) channels play an important role in the control of vascular tone in both resting and stimulated conditions. Moreover, under stimulated conditions, K(Ca) channels also have an important role in the regulation of vascular tone. Dysfunction of these channels induces abnormal vasoconstriction and may be implicated in vascular diseases such as hypertension and vasospasm.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗

PubMed1993

Left ventricular dynamics during strenuous isometric exercise in marathon runners, weight lifters and healthy sedentary men: comparative echocardiographic study.

Global and segmental left ventricular dynamics during strenuous large-muscle isometric exercise were compared between endurance and static type professional athletes and sedentary men. Using a trunk dynamometer and supine echocardiography, only the weight lifters manifested increased (p < 0.01) wall thickness during both diastole and systole, resulting in decreased left ventricular volume at end systole. These group-related differences would suggest a causal relationship between chronic intensive weight-lifting training and efficient myocardial dynamics at the time of strenuous isometric exercise.

Adult↗

PubMed1980

Ventilatory and pressor response to isometric exercise in normal subjects.

We studied in 8 young, healthy males, the behaviour of heart rate (HR), systolic blood pressure at the upper (SBPUL) and lower (SBPLL) resting limbs, pulmonary ventilation (VE), oxygen uptake (VO2) and production of CO2 (VCO2) during and after two levels of isometric effort (handgrip at 50 and 100% maximum voluntary contraction; MVC). HR, SBPLL, VE, VO2, and VCO2 did not vary between the two efforts, but SBPUL is significantly different between them. The possible mechanism and predominant role of nervous reflexes upon the pressor and ventilatory response to isometric exercise are discussed.

Adult↗

PubMed1984

Reflex constriction of significant coronary stenosis as a mechanism contributing to ischemic left ventricular dysfunction during isometric exercise.

To study the mechanisms of myocardial ischemia during isometric exercise, handgrip was sustained, for 4.5 min at 25% of maximum by 11 patients with at least one significant coronary stenosis each, during cardiac catheterization. After recovery, the handgrip that was repeated with simultaneous infusion of nitroglycerin (50 micrograms over 4 min) directly into the diseased vessel. The cardiovascular response was assessed by hemodynamic and by computer-assisted measurements of stenosis. During the first handgrip test pulmonary capillary wedge pressure rose 56% (15 to 23 mm Hg; p less than .001), the heart rate-systolic pressure product rose 33% (p less than .01), and the diseased epicardial arteries constricted. Luminal area in the stenotic segment was reduced by 35% (p less than .01), resulting in a 243% increase in estimated stenotic flow resistance (30 to 103 mm Hg/ml/sec; p less than .001). During handgrip with intracoronary nitroglycerin, the pressure-rate product again increased 33%, but relative to resting control, capillary wedge pressure fell 4 mm Hg in association with a 32% increase in luminal area of the stenosis and a 28% reduction in flow resistance (all significantly different from the response to handgrip alone: p less than .001, .01, and .005, respectively). Thus, coronary vasoconstriction, not increased pressure-rate product, is the dominant mechanism for ischemic left ventricular dysfunction during isometric exercise in patients with significant coronary stenoses.

Adult↗

PubMed1985

The economy of isometric force development, myosin isoenzyme pattern and myofibrillar ATPase activity in normal and hypothyroid rat myocardium.

Hypothyroidism was induced in Wistar-Kyoto rats by adding propylthiouracil to the drinking water (0.8 mg/ml). Initial heat, total activity-related heat, and resting heat rate were measured in left ventricular papillary muscle preparations of propylthiouracil-treated and control rats contracting isometrically at 12 beats/min (21 degrees C), using Hill type, planar vacuum-deposited bismuth and antimony thermopiles. In the propylthiouracil preparations, relative to control, time-to-peak tension increased from 288 +/- 27 (mean +/- SD) to 411 +/- 25 msec (P less than 0.001), dp/dtmax decreased from 38.3 +/- 9.5 to 20.4 +/- 3.5 g X mm-2/sec (P less than 0.001), and peak developed tension decreased from 6.11 +/- 1.75 to 4.64 +/- 0.89 g X mm-2 (P less than 0.05). In the propylthiouracil preparations, initial heat was significantly (P less than 0.001) reduced by 27 or 43% when normalized to peak twitch tension or tension-time integral, respectively. In experiments where the papillary muscles were tetanized, the slope of the linear function of total activity-related heat versus tension-time integral was decreased by 43% (P less than 0.001) in the propylthiouracil preparations, indicating an improved economy of isometric tension maintenance. The predominant myosin isoenzyme of the left ventricular wall, as well as the papillary muscle myocardium, was the V3 variety in the propylthiouracil animals, in contrast to V1 in the controls. Myofibrillar actomyosin calcium-magnesium-stimulated adenosine triphosphatase activity was significantly (P less than 0.02) decreased from 55 +/- 18 (control) to 31 +/- 8 nmol inorganic phosphate ion/mg X min (propylthiouracil).(ABSTRACT TRUNCATED AT 250 WORDS)

Actomyosin↗

PubMed1994

Effects of pHi on isometric force and [Ca2+]i in porcine coronary artery smooth muscle.

During ischemia or hypoxia, alterations in pHi may play a significant role in alteration of vessel wall function. We studied the effects of altering pHi on isometric force and [Ca2+]i in porcine coronary artery. pHi was altered at constant pHo by use of NH4Cl and measured with the fluorescent dye BCECF. [Ca2+]i was monitored with fura 2 and ratiometric fluorescence measurements. Addition of NH4Cl elicited a concentration-dependent (2 to 30 mmol/L) sustained increase in isometric force in unstimulated tissues. In tissues stimulated with KCl (29 mmol/L) or U46619 (1 mumol/L), addition of NH4Cl elicited a rapid but transient decrease followed by a sustained increase in force above the initial stimulated levels. Removal of NH4Cl was associated with a transient decrease and increase followed by a prolonged depression of force and slow recovery to initial levels. Addition of NH4Cl elicited a rapid monotonic increase in pHi and then a slow recovery toward initial levels; washout of NH4Cl led to a rapid acidification followed by recovery. In contrast to the steady state effects of NH4Cl on force, its effects on [Ca2+i were in the opposite direction. During the sustained increase in force elicited by NH4Cl alkalinization, [Ca2+]i was substantially decreased, whereas when force was depressed during the acidification elicited by NH4Cl washout, [Ca2+i was increased to values observed before addition of NH4Cl. The initial transients in force elicited by NH4Cl addition or washout were also associated with opposite changes in [Ca2+]i. Thus, the effects on force of the NH4Cl-induced changes in pHi are associated with changes in the Ca2+ sensitivity of the contractile apparatus rather than mediated through changes in [Ca2+]i.

Ammonium Chloride↗

PubMedDate not supplied

Factors affecting the region of most isometric femoral attachments. Part I: The posterior cruciate ligament.

We measured how the distance between selected tibial and femoral attachments of the PCL changes with knee flexion in six intact cadaver knee. The femoral location was the primary determinant of whether the distance increased, decreased, or remained nearly constant. The proximal-distal location of a fiber's femoral attachment had a stronger effect than had the anterior-posterior location. The tibial location had only a small statistically significant effect. These results suggest that the function of fibers within the PCL is determined primarily by their femoral attachment location. We determined all femoral attachments whose tibio-femoral distance changed 0, 2, 4, 6, 8, and 10 mm during flexion from 0 degrees to 90 degrees. No absolutely isometric point existed. Attachments whose separation distance changed less than 2 mm formed a bullet-shaped region whose base was against the roof of the intercondylar notch and whose nose pointed posteriorly and slightly distally. The axis of the "bullet" was near the proximal edge of the femoral insertion of the PCL. Along the axis, anterior attachments, located near the roof of the intercondylar notch, were more isometric than were posterior attachments, located near the cartilage. Attachments located distal to the axis moved away from the tibial insertion of the PCL when the knee was flexed. The more distal the femoral attachment, the larger the increase in tibiofemoral distance that occurred with flexion. The opposite was true of attachments proximal to the 2 mm region.

Adult↗

PubMedDate not supplied

Age-related changes in normal isometric shoulder strength.

Normative data are useful for assessing isometric shoulder strength in patients with bilateral shoulder abnormalities. The purpose of this study was to develop a normative database for shoulder strength in the clinically relevant positions and movements. Twenty combinations of exertions and postures-including flexion, extension, abduction, adduction, internal rotation, and external rotation-were tested for both the dominant and nondominant sides. Strength was measured isometrically using a modified Cybex II dynamometer. The cross-sectional study design included 120 subjects (60 women and 60 men) ranging in age from 20 to 78 years from southern Minnesota. Tables of normative strength data were constructed. Multivariate analyses were performed to assess the effect of age, sex, and weight on strength. Age was negatively associated with all strength measures, and weight was positively associated with them. Men were stronger than women when controlling for age and weight. Statistically significant differences between dominant and nondominant shoulders were found for only some of the strength measures taken. These normal data will be useful to the clinician, as they permit a standard against which to compare shoulder strength.

Adult↗

PubMed2006

Motor recovery as assessed with isometric finger movements and perfusion magnetic resonance imaging after acute ischemic stroke.

OBJECTIVE: Recovery from hemiparetic stroke is variable. An important goal for clinicians and clinical researchers is to identify predictors of recovery. The initial phase after acute ischemic stroke is considered to be of major importance for neurological outcome. The authors sought to determine in patients with acute ischemic stroke whether early motor recovery, as measured by repetitive isometric index-thumb oppositions, is correlated with ischemic lesion volume. METHODS: Thirty-six acute hemiparetic stroke patients with residual hand function were investigated. The European Stroke Scale (ESS) score was determined on admission and at discharge. Performance of repetitive index finger-thumb pinch movements was measured daily during the 1st 8 days after stroke onset. Brain ischemia volume was determined digitally in time-to-peak magnetic resonance images of per-fusion. RESULTS: The recovery of patients with (P = 0.002) and without (P < 0.001) thrombolysis as assessed with the ESS was paralleled by an increase in isometric grip force and movement rate (P < 0.05). Recovery was predicted by the area of moderately impaired perfusion indicated by the per-fusion mismatch volume (r = 0.578, P < 0.001). CONCLUSIONS: In acute stroke, recovery of hand function is predicted by the volume of salvageable ischemic tissue, as determined by the perfusion mismatch.

Aged↗

PubMed2002

Does four weeks of TENS and/or isometric exercise produce cumulative reduction of osteoarthritic knee pain?

OBJECTIVE: To evaluate the cumulative effect of repeated transcutaneous electrical nerve stimulation (TENS) on chronic osteoarthritic (OA) knee pain over a four-week treatment period, comparing it to that of placebo stimulation and exercise training given alone or in combination with TENS. DESIGN: Sixty-two patients, aged 50-75, were stratified according to age, gender and body mass ratio before being randomly assigned to four groups. INTERVENTIONS: Patients received either (1) 60 minutes of TENS, (2) 60 minutes of placebo stimulation, (3) isometric exercise training, or (4) TENS and exercise (TENS & Ex) five days a week for four weeks. MAIN OUTCOME MEASURES: Visual analogue scale (VAS) was used to measure knee pain intensity before and after each treatment session over a four-week period, and at the four-week follow-up session. RESULTS: Repeated measures ANOVA showed a significant cumulative reduction in the VAS scores across the four treatment sessions (session 1, 10, 20 and the follow-up) in the TENS group (45.9% by session 20, p < 0.001) and the placebo group (43.3% by session 20, p = 0.034). However, linear regression of the daily recordings of the VAS indicated that the slope in the TENS group (slope = -2.415, r = 0.943) was similar to the exercise group (slope = -2.625, r = 0.935), which were steeper than the other two groups. Note that the reduction of OA knee pain was maintained in the TENS group and the TENS & Ex group at the four-week follow-up session, but not in the other two groups. CONCLUSIONS: The four treatment protocols did not show significant between-group difference over the study period. It was interesting to note that isometric exercise training of the quadriceps alone also reduced knee pain towards the end of the treatment period.

Aged↗

PubMed1982

Influence of sex and obesity on plasma catecholamine response to isometric exercise.

The effects of obesity and gender on plasma catecholamine levels were studied in 10 lean men, 10 lean women, and 10 obese women. Measurements (mean +/- SE) were obtained sequentially after 30 min of bedrest, 10 min of standing, and 5 min of sustained handgrip, and following 15 min of rest while standing. The supine plasma norepinephrine (NE) levels were comparable among the 3 groups, whereas plasma epinephrine (E) was lowest in obese women (14 +/- 2 pg/ml), intermediate in lean women (19 +/- 2 pg/ml), and highest in lean men (29 +/- 5 pg/ml). Peak plasma levels were reached during handgrip. E values in men (139 +/- 27 pg/ml) exceeded those in lean women (71 +/- 9 pg/ml; P less than 0.05), and both were higher than the E response in obese women (38 +/- 7 pg/ml; P less than 0.01). While peak plasma NE levels in lean women (654 +/- 62 pg/ml) and obese women (524 +/- 46 pg/ml) were comparable, both were significantly lower than the NE response in lean men (1014 +/- 114 pg/ml; P less than 0.02). Increments in plasma FFA during handgrip were 21% and 28% above values during standing (P less than 0.005) in lean men and lean women, respectively, whereas no net increment occurred in obese women. The excursion of plasma beta-hydroxybutyrate in recovery was 59% above the standing concentration for lean women (P less than 0.05), but no net increments were observed in obese women or lean men. We conclude that obesity in women is attended by suppressed plasma E responses to isometric exercise, which may explain subnormal excursions of plasma FFA and beta-hydroxybutyrate during handgrip and recovery, respectively. Lean men have greater plasma elevations of E and NE during isometric exertion than women, suggesting an influence of sex on plasma catecholamine regulation.

3-Hydroxybutyric Acid↗

PubMed2006

Isometric contractile properties of sexually dimorphic forelimb muscles in the marine toad Bufo marinus Linnaeus 1758: functional analysis and implications for amplexus.

It has been shown in the bullfrog, Rana catesbeiana Shaw 1802, that certain forelimb muscles in males have different contractile properties when compared with females, which may result from adaptation for amplexus. We extended this study to a distantly related species, Bufo marinus Linnaeus 1758, by testing the isometric contractile properties of three muscles, abductor indicus longus (AIL), and flexor carpi radialis (FCR) (both dimorphic muscles), and extensor carpi ulnaris (ECU) (non-dimorphic control). In males the dimorphic muscles had greater wet mass and cross-sectional area than in the females, and also produced significantly greater isometric force. As in bullfrogs, however, the maximum tetanic force per cm(2) of muscle cross-section did not differ between the sexes. In spite of this similarity in maximum force, the two dimorphic muscles were much less fatigable in the males than in the females. Lower fatigability in males correlated with exceptionally elongated relaxation times that maintained high levels of force between stimulus trains. This sustained force was negligible in the females, suggesting that this feature may allow males to maintain amplexus for prolonged periods. The same sustained force response was observed in the earlier study of Rana catesbeiana. Because this response is similar in Bufo and Rana, muscular properties correlated with amplexus may be shared across anurans by inheritance of this response from a common ancestor.

Analysis of Variance↗

PubMed2000

Sexual dimorphism in forelimb muscles of the bullfrog, Rana catesbeiana: a functional analysis of isometric contractile properties.

In many species of frog, the forelimb muscles important in amplexus are known to be much larger in males than in females. We studied this dimorphism in three forelimb muscles in the bullfrog [abductor indicus longus (AIL), flexor carpi radialis (FCR) and extensor carpi radialis (ECR)] by testing their isometric contractile properties. One muscle that is not dimorphic, the extensor carpi ulnaris (ECU), was also studied as a control. In addition to being greater in wet mass and in cross-sectional area in the males, our data show that the dimorphic muscles also produce significantly larger isometric forces in males than in females. The tetanic force per cm(2) of muscle cross-sectional area did not differ between the sexes, so that force within a muscle varies directly with muscle size. However, a number of the contractile variables we measured show that male muscles differ functionally from those of females. The male twitch contraction times were significantly longer in the AIL, and the male half-relaxation times were longer in both the AIL and FCR. These two dimorphic muscles were also significantly less fatiguable in males than were the corresponding female muscles. Their higher endurance resulted from the maintenance of high levels of unrelaxed force sustained between trains of stimuli during the fatigue test. This sustained force is much less pronounced in the female muscles, suggesting that high levels of sustained force may be a key functional feature that enables males to maintain amplexus economically for prolonged periods.

Animals↗

PubMed2006

Sex differences in mechanomyographic responses to voluntary isometric contractions.

PURPOSE: This study was designed to compare mechanomyography (MMG) and the force relationship during isometric ramp contractions of biceps brachii muscles in females and males to identify sex differences in the MMG responses. METHODS: Subjects (10 females and 9 males; age range, 20-26 yr) were asked to exert an isometric elbow flexion torque from 5 to 80% maximal voluntary contraction (MVC) at a constant rate of 10% MVC per second. The MMG signal was normalized to muscle cross-sectional area (CSA) as measured by ultrasound imaging. RESULTS: MVC and CSA were significantly different between the two sex groups (males>females); however, there was no sex difference in the MVC relative to muscle CSA (MVC/CSA). The root mean squared amplitude of the MMG (RMSMMG) was significantly greater in the male group than the female group. The RMSMMG relative to muscle CSA was also different between the two sex groups (males>females). The sex difference in the RMSMMG/CSA was more pronounced with increasing torque. The torque levels at which the inflection points in the MMG amplitude were located were different between the two sex groups. The mean power frequency (MPF) of the MMG in the female group increased monotonously, which was different from that in the male group. CONCLUSION: Our results suggest that the sex differences in MMG responses and motor unit (MU) activation strategy result from the predominant activity of the MU with slow-twitch fibers and an effective fused tetanus in females. In addition, the sex-related differences in muscle morphology, subcutaneous adipose tissue, and muscle stiffness appear to be insufficiently reflected in the present MMG responses, particularly relative to muscle CSA.

Adult↗

PubMed1989

Maximal isometric patellofemoral contact force in patients with anterior knee pain.

We assessed patellofemoral joint function by combining the measurement of maximal isometric extensor torque at the knee with clinical and radiological measurements in order to calculate the patellofemoral contact force. Eighteen volunteers established the normal ranges of results and the reliability of the system. Of the 39 patients with a variety of knee problems, 29 had anterior knee pain, and all had a subsequent arthroscopy. Patients with anterior knee pain and lesions in the patellar cartilage had significantly reduced isometric contact forces, but those with normal patellofemoral cartilage had normal contact forces. Our method may be useful in providing an objective assessment of anterior knee pain and a quantitative means of monitoring its treatment.

Adolescent↗

PubMed1980

Hemodynamic response to isometric handgrip in acute myocardial infarction.

Thirteen patients underwent right cardiac catheterization during the first 24 hours after the onset of symptoms of acute myocardial infarction. All had normal (less than 12 mm Hg) pulmonary arterial diastolic pressure or normal mean pulmonary wedge pressure (7.6 +/- 0.6 mm Hg). The patients did isometric forearm contraction to a measured level of 100 mm Hg. All patients had elevation of systemic systolic blood pressure, with a mean rise of 14.2 +/- 2.9 mm Hg; heart rate rose by 12 +/- 2 beats per minute. When compared to a control group, patients with acute myocardial infarction had a significant (P less than 0.005) elevation of 5.4 +/- 1.3 mm Hg in the pulmonary arterial diastolic or pulmonary wedge pressure. This may be a result of either increased left ventricular stiffness or decreased myocardial functional reserve. In the setting of acute myocardial infarction, patients with normal left ventricular filling pressures have abnormal ventricular performance. Isometric effort is poorly tolerated and should be avoided.

Adult↗