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PubMed1986

Temperature dependence of the crossbridge cycle during unloaded shortening and maximum isometric tetanus in frog skeletal muscle.

The primary objective of this study was to determine if the rate limiting step in the crossbridge cycle was the same during maximum rate of shortening and during maintenance of maximum tension in an isometric contraction. To this end the temperature dependence, Q10, of the crossbridge cycle was estimated during unloaded shortening and maximum isometric tetanus. Isolated semitendinosus muscles from the frog were studied at 0 and 10 degrees C. Crossbridge cycling during unloaded shortening was determined from the velocity of unloaded shortening estimated by the slack step technique. Crossbridge cycling during maintained isometric tetanus was determined from the steady rate of energy liberation during the tetanus after allowance for energy liberation due to Ca2+ cycling. The Q10 of the velocity of unloaded shortening was 2.5 and that of the steady rate of energy liberation was 4.6. After correction for the temperature dependence of energy liberation associated with Ca2+ cycling (5.7), the estimated Q10 of the steady rate of energy liberation became 3.9. These estimates of the Q10 of the crossbridge cycle are significantly different. These results support the conclusion that the rate limiting steps during unloaded shortening and maximum isometric force maintenance occur at different steps in the crossbridge cycle. Further the high Q10 of the energy liberation due to Ca2+ cycling may relate to the high concentration of parvalbumin in frog muscle. A second objective of this study was to document in the same muscle the variation of Q10s of mechanical and energetic properties of contraction. Over this temperature range the Q10s ranged from 1.1 to 5.7.

Animals↗

PubMed1993

Vascular and electromyographic responses evoked in forearm muscle by isometric contraction of the contralateral forearm.

There is controversy over whether isometric contraction of the forearm evokes vasoconstriction or vasodilatation in the muscles of the contralateral forearm. In the present study we have investigated in normal man, the effects of isometric contraction of one arm at 75, 50 and 25% maximum voluntary contraction (MVC) on arterial pressure, heart rate, blood flow and vascular resistance of the contralateral forearm and on electromyographic (EMG) activity recorded from that same arm with sensitive, surface electrodes. When EMG activity was not being recorded from the 'resting' arm, isometric contraction of the contralateral arm for 2 min evoked increases in arterial pressure and heart rate whose magnitudes were graded with % MVC and an increase in forearm blood flow and a decrease in forearm vascular resistance at 75, 50 and 25% MVC, indicating vasodilatation. Further experiments in which EMG activity was recorded from the 'resting' arm demonstrated that the decrease in forearm vascular resistance evoked by 75% MVC was associated with a substantial increase in EMG activity of the extensor and flexor muscles of that arm. By contrast, when forearm contraction was performed at 75% MVC whilst subjects viewed the EMG activity in the 'resting' arm on an oscilloscope and kept EMG activity minimal, vascular resistance increased in that arm, indicating vasoconstriction. Further, when subjects performed contraction at 25% MVC whilst showing minimal EMG activity in the contralateral arm, vascular resistance in that same arm increased (from 78 +/- 16 to 124 +/- 29 mmHg/ml/min/100 ml tissue). These results are discussed in relation to those of previous studies. We propose, that in normal man, isometric contraction of the forearm evokes primary vasoconstriction in the muscles of the contralateral forearm, but that this response may be overcome by muscle vasodilatation occurring secondary to unintended muscle contraction or as part of the alerting response to acute stress.

Adult↗

PubMed1996

Noradrenaline-induced depolarization is smaller in isobaric compared to isometric preparations of rat mesenteric small arteries.

The hypothesis was tested that wall tension can influence the membrane potential response to noradrenaline (NA) using isometric and isobaric vessel preparations of rat mesenteric small arteries. The resting membrane potential was significantly less negative in the isobaric (-49.7+/-0.5 mV, S.E.M., n=12 vessels) compared to the isometric preparation (-56.1+/-0.7 mV, n=10), although there was no difference in wall tension. The depolarization induced by 10(-5) M NA was 2.6-fold smaller in the isobaric preparation, where wall tension decreased, compared to the isometric preparation, where wall tension increased. Since wall tension decreases under isobaric conditions, but increases under isometric conditions, the latter finding can be explained by assuming that part of the NA-induced membrane potential change is wall tension dependent.

Animals↗

PubMed1996

The increase of duration of isometric contraction may not relate to change of relative oxygenation of forearm muscle.

The aim of this study is to ascertain, how an increase in duration of isometric contraction influences tissue oxygenation of forearm muscles, undergoing physical training. Four subjects underwent 6 weeks of training of the left forearm muscles by performing isometric contractions. Subjects repeated 10 s contractions, 10-15 times in the first, and 20 s contractions in the second 3 week period. Relative oxygenation of forearm muscles was measured by using. Near Infrared Spectroscopy (CWS2000, NIM Incorporated, Philadelphia). The training increased the duration of isometric contraction at 20 kp of experimental forearm muscles by 41 +/- 25 s, which was more (P < 0.05) than in control forearm (8 +/- 4 s). This increase was not reflected in changes of maximum relative deoxygenation of experimental muscles, which decreased by only -6.9 +/- 14.4%. The results show that an increase in duration of isometric contraction may not depend on the oxygenation of muscle tissue at fixed force of 20 kp.

Exercise↗

PubMed1997

Influence of afferent feedback on isometric fine force resolution in humans.

The influence of afferent feedback on isometric fine force resolution was studied in humans. Subjects performed the smallest possible isometric flexion force increments with the index finger while visual, cutaneous, and muscle spindle feedback conditions were varied. In the control conditions with visual feedback, isometric force resolution was finest and independent of cutaneous or muscle spindle feedback. In the absence of visual cues, force resolution was significantly coarser. When agonist muscle spindles were vibrated (100 Hz and 150 Hz), fine force resolution capabilities declined further. Diminution of cutaneous feedback per se did not affect fine force resolution. However, the effect of agonist vibration was attenuated when full cutaneous feedback was available. We conclude that in voluntary isometric contractions the degree of fine force resolution depends on the type of afferent feedback available for calibrating central motor commands. Visual feedback is more powerful than spindle feedback, which is more efficient than cutaneous feedback. The extent to which the central motor command itself contributes to the sensation of force is indirectly implied by reproducible, yet coarser force resolution levels when peripheral information is minimized.

Adult↗

PubMed2003

Long-term spinal cord injury increases susceptibility to isometric contraction-induced muscle injury.

Complete spinal cord injury (SCI) results in inactivation and unloading of affected skeletal muscles. Unloading causes an increased susceptibility of muscle to contraction-induced injury. This study used magnetic resonance imaging (MRI) to test the hypothesis that isometric contractions would evoke greater muscle damage to the quadriceps femoris muscle (mQF) of SCI subjects than that of able-bodied (AB) controls. MR images were taken of the mQF prior to, immediately post, and 3 days post electromyostimulation (EMS). EMS consisted of five sets of ten isometric contractions (2 s on/6 s off, 1 min between sets) followed by another three sets of ten isometric contractions (1 s on/1 s off, 30 s between sets). Average muscle cross-sectional area (CSA) and the relative areas of stimulated and injured muscle were obtained from MR images by quantifying the number of pixels with an elevated T2 signal. SCI subjects had significantly greater relative area [90 (2)% versus 66 (4)%, P<0.05; mean (SE)] but a lesser absolute area [16 (3) cm(2) versus 44 (6) cm(2), P<0.05] of mQF stimulated than AB controls. During EMS, peak torque was reduced by 66% and 37% for SCI and control subjects, respectively. Three days post EMS, there was a greater relative area of stimulated mQF injured for the SCI subjects [25 (6)% versus 2 (1)%, P<0.05]. Peak torque remained decreased by 22% on day 3 in the SCI group only. These results indicate that affected muscle years after SCI is more susceptible to contraction-induced muscle damage, as determined by MRI, compared to AB controls. They also support the contention that electrically elicited isometric contractions are sufficient to cause muscle damage after a prolonged period of inactivity.

Adult↗

PubMed2005

Power output, isometric strength and steadiness in the leg muscles of pre- and postmenopausal women; the effects of hormone replacement therapy.

There are conflicting reports of the effects of hormone replacement therapy (HRT) on strength preservation in postmenopausal women, while any effect on power output has received little attention. Decreased steadiness of force generation has been reported in older muscles and may be related to the hormonal changes associated with the menopause, but the effect of HRT has not been investigated. We have studied the effect of HRT on strength, power output and isometric force steadiness in healthy women. Sixteen young (aged 27.4 +/- 1.4 years, mean +/- SEM) and 29 postmenopausal women were studied. Fifteen of the latter were taking HRT (68.1 +/- 1.4 years, HRT+) and 14 (70.5 +/- 1.5 years, HRT-) had never done so. During isometric quadriceps contractions the force steadiness (coefficient of variation of force) was measured at 10, 25, 50 and 100% maximum voluntary contraction (MVC). The average power generated by an explosive leg extension was recorded. The HRT- group generated less power (110.2 +/- 7.2 W) than both the HRT+ (136.5 +/- 10.9 W, P = 0.027) and young (136.2 +/- 5.8 W, P = 0.027) subjects. Power output was similar in the HRT+ and younger subjects. The HRT- subjects were weaker than the younger ones (241.3 +/- 14.0 N vs. 297.6 +/- 13 N, P = 0.006). The strength of the HRT+ group (255.5 +/- 14 N) was not significantly different to the other two groups. There was no difference in steadiness between the three groups at any of the force levels. HRT appears to maintain power output to a greater extent than isometric strength in postmenopausal women. There was no evidence for an effect of either age or HRT on isometric steadiness in the quadriceps.

Adult↗

PubMed2005

Effects of isometric squat training on the tendon stiffness and jump performance.

The present study aimed to investigate the effect of isometric squat training on human tendon stiffness and jump performances. Eight subjects completed 12 weeks (4 days/week) of isometric squat training, which consisted of bilateral leg extension at 70% of maximum voluntary contraction (MVC) for 15 s per set (10 sets/day). Before and after training, the elongations of the tendon-aponeurosis complex in the vastus lateralis muscle and patella tendon were directly measured using ultrasonography while the subjects performed ramp isometric knee extension up to MVC. The relationship between the estimated muscle force and tendon elongation was fitted to a linear regression, the slope of which was defined as stiffness. In addition, performances in two kinds of maximal vertical jumps, i.e. squatting (SJ) and counter-movement jumps (CMJ), were measured. The training significantly increased the volume (P < 0.01) and MVC torque (P < 0.01) of the quadriceps femoris muscle. The stiffness of the tendon-aponeurosis complex increased significantly from 51 +/- 22 (mean +/- SD) to 59 +/- 24 N/mm (P = 0.04), although that of the patella tendon did not change (P = 0.48). The SJ height increased significantly after training (P = 0.03), although the CMJ height did not (P = 0.45). In addition, the relative difference in jump height between SJ and CMJ decreased significantly after training (P = 0.02). These results suggest that isometric squat training changes the stiffness of human tendon-aponeurosis complex in knee extensors to act negatively on the effects of pre-stretch during stretch-shortening cycle exercises.

Adult↗

PubMed2000

Cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise.

In occupational work, continuous repetitive and isometric actions performed with the upper extremity primarily cause local muscle strain and musculoskeletal disorders. They may also have some adverse effects on the cardiorespiratory system, particularly, through the elevation of blood pressure. The aim of the present study was to compare peak cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise. The subjects were 21 untrained healthy men aged 24-45 years. The dynamic hand-grip exercise (DHGE) was performed using the left hand-grip muscles at the 57 (SD 4)% level of each individual's maximal voluntary contraction (MVC) with a frequency of 51 (SD 4) grips x min(-1). The isometric hand-grip exercise (IHGE) was done using the right hand at 46 (SD 3)% of the MVC. The endurance time, ventilatory gas exchange, heart rate (HR) and blood pressure were measured during both kinds of exercise. The mean endurance times for DHGE and IHGE were different, 170 (SD 62) and 99 (SD 27) s, respectively (P < 0.001). During DHGE the mean peak values of the breathing frequency [20 (SD 6) breaths x min(-1)] and tidal volume [0.89 (SD 0.34) l] differed significantly (P < 0.01) from peak values obtained during IHGE [15 (SD 5) breaths x min(-1) and 1.14 (SD 0.32) l, respectively]. The corresponding peak oxygen consumptions, pulmonary ventilations, HR and systolic blood pressures did not differ, and were 0.51 (SD 0.06) and 0.46 (SD 0.11) l x min(-1), 17.1 (SD 3.0) and 16.7 (SD 4.7) l x min(-1), 103 (SD 18) and 102 (SD 17) beats x min(-1), and 156 (SD 17) and 161 (SD 17) mmHg, respectively. The endurance times of both DHGE and IHGE were short (< 240 s). The results indicate that the peak responses for the ventilatory gas exchange, HR and blood pressure were similar during fatiguing DHGE and IHGE, whereas the breathing patterns differed significantly between the two types of exercise. The present findings emphasize the importance of following ergonomic design principles in occupational settings which aim to reduce the output of force, particularly in tasks requiring isometric and/or one-sided repetitive muscle actions.

Adult↗

PubMed1998

Changes in reflex excitability following isometric contraction in humans.

Enhancement of muscle stretch following isometric contraction has been thought to occur as a result of inhibitory reflex mechanisms. Experiments with electrical stimulation (H-reflex) have demonstrated maximal H-reflex suppression during force relaxation followed by gradual recovery over the following 20 s. There has been considerable speculation as to whether electrical and mechanical stimulation elicit similar response behaviour. The present study examined postisometric reflex modulation following both stimulation modalities. In ten subjects dorsiflexion stimuli varying in speed and amplitude were applied after 30% and 60% maximal voluntary contraction (MVC). Modulation of the mechanically and electrically evoked responses following isometric plantarflexion was investigated. Reflex responses following both stimulation modalities were depressed during the course of force relaxation. A rather fast recovery was observed in mechanical stimulation. Postisometric response modulation was neither altered by the amount of isometric plantarflexion, nor by the amplitude of the applied stretch stimulus. With increasing velocity of the applied dorsiflexion, however, the shape of the reflex modulation persisted, but the magnitude of the responses was significantly enhanced. In electrical stimulation, however, recovery was delayed. It is suggested that postisometric reflex modulation is due to presynaptic inhibition. Moreover, possible peripheral mechanisms resulting from alpha-gamma-coactivation may also affect the stretch receptor itself because of inherent stiffness properties. The latter possibility particularly would explain the differences between mechanical and electrical stimulus modalities. With respect to practical implications, the very fast recovery (< 400 ms) of the stretch responses to control values strongly contradicts the interpretation that after isometric precontraction, suppression of reflex activity might be used for more efficient stretching of the tendomuscle system.

Adult↗

PubMed1983

Effects of nitroglycerin on isometric exercise.

Eight patients with ischemic heart disease performed isometric handgrip of five minutes' duration at 30% of their maximum voluntary contraction, before and after administration of 0.4 mg sublingual nitroglycerin (NTG). Although isometric exercise resulted in similar rise of left ventricular systolic pressure (LVSP) before and after NTG, the level of LVSP during the post NTG effort was lower. Heart rate, cardiac index, stroke index, left ventricular stroke work index, and systemic resistance were not different during the pre- and post NTG exercise. Left ventricular end-diastolic pressure rose to a significantly lower level (18.3 +/- 14.4 mm Hg) during the post NTG handgrip than during the pre NTG effort (31.4 +/- 17.6 mm Hg, P less than 0.005). It is concluded that NTG reduces preload and afterload both at rest and during isometric exercise and improves left ventricular performance during isometric exercise.

Blood Pressure↗

PubMed1983

Response to isometric exercise in children and young adults with aortic regurgitation.

We studied the hemodynamic response to isometric exercise in 28 children and young adults with aortic regurgitation (AR). Results were compared with those of 18 control subjects without heart disease. Sustained isometric exercise at 33% maximal handgrip capability produced similar increase in heart rate in the two groups. Systolic blood pressure rose to a greater extent in patients with AR than in controls (mean rise 33 mm Hg vs 13 mm Hg). Thirteen of 28 patients with AR had elevated left ventricular end-diastolic diameters at rest. During isometric exercise, left ventricular end-diastolic diameter remained constant in both groups. However, end-systolic diameter increased in patients with AR, resulting in a fall in shortening fraction. The mean left ventricular shortening fraction of the patients with AR fell from 36.0 +/- 1.0% to 32.8 +/- 1.2% (p less than 0.001), but did not change in controls. The fall in shortening fraction was most marked in patients with severe AR. We conclude that patients with AR have an abnormal cardiovascular response to isometric exercise consisting of greater increase in blood pressure and a decrease in left ventricular fractional shortening.

Adolescent↗

PubMed1983

Left ventricular response to isometric exercise and its value in predicting the change in ventricular function after mitral valve replacement for mitral regurgitation.

Reduced left ventricular (LV) afterload and its effect on the resting ejection fraction may lead to overestimation of LV function in mitral regurgitation (MR). To evaluate LV function during increased afterload of the heart, an isometric handgrip test was performed during cardiac catheterization in 15 patients with mitral regurgitation (MR group) and in 9 normal subjects (normal group). Twelve months after successful mitral valve replacement (MVR) the patients were recatheterized, and the value of preoperative stress testing in predicting the change in resting ventricular function after surgery was estimated. Isometric exercise caused an increase in endsystolic wall stress, a measure of ventricular afterload, in both the MR group and the control group (p less than 0.001). The ejection fraction remained unchanged in the control group, but decreased from 0.58 +/- 0.08 to 0.53 +/- 0.08 in the MR group (p less than 0.001). After MVR, end-systolic wall stress increased significant (p less than 0.001) and the ejection fraction decreased from 0.58 +/- 0.05 to 0.51 +/- 0.1 (p less than 0.05). A positive correlation existed between the change in the ejection fraction during preoperative stress testing and the change in the resting ejection fraction after MVR (r = 0.65, p less than 0.01). In 8 patients whose resting ejection fraction was within normal limits (greater than 0.55) preoperatively, the ejection fraction was depressed (less than 0.55) 1 year after surgery. In all but 1 of these patients the isometric exercise revealed the reduced ventricular response to afterload stress preoperatively (decrease of the ejection fraction greater than 0.03 during exercise). Therefore, the isometric exercise-induced change in LV function appears to predict the influence of MVR on LV function.

Adult↗

PubMed1985

Effects of training on systolic time intervals at rest and during isometric exercise in men and women 61 to 64 years old.

To determine whether regular exercise improves left ventricular (LV) contractile function in persons 60 years and older, systolic time intervals (STIs) were measured in 10 healthy men and women (mean age 62 +/- 1 year [+/- standard deviation]) before and after 6 months of intense endurance training. STIs, systolic and diastolic blood pressure (BP) and heart rate (HR) were determined at rest and in response to isometric handgrip exercise. Systolic BP, diastolic BP and HR increased acutely from rest in response to handgrip (p less than 0.002). The indexes of total electromechanical systole and LV ejection time (ET) index increased (p less than 0.01), preejection period (PEP) index increased (p less than 0.05) or remained unchanged and PEP/LVET did not change from values at rest in response to handgrip. Training resulted in an 18% increase in maximal oxygen uptake (p less than 0.01). After training, systolic and diastolic BP were reduced at rest (p less than 0.002) and, along with HR, were lower in response to handgrip (p less than 0.002). However, training did not alter STIs at rest or during handgrip. These findings indicate that healthy persons in their 60s have a normal LV response to isometric exercise. Prolonged, intense endurance training does not alter LV contractile function at rest or in response to isometric exercise. However, training can significantly reduce BP at rest, and markedly lower the HR-systolic BP product attained during acute isometric stress, even in normotensive older subjects.

Aged↗

PubMed1985

Electromyographic study of human jaw-closing muscle endurance, fatigue and recovery at various isometric force levels.

The effect of a sustained isometric clenching, at various force levels or maximum voluntary bite-force levels, was evaluated on normal jaw-function subjects. Surface electromyographic (EMG) recordings of the masseter and temporalis were taken and bite force was measured using an intra-oral force transducer placed unilaterally in the 2nd premolar-1st molar region. Subjects sustained isometric force at the 25, 50, 75 and 100 per cent force level. Maximal bite force and EMG contraction levels were measured before, during and after these sustained isometric tasks. Subjects showed no change in their brief maximal contraction or force levels during or after various fatigue-inducing isometric tasks. The findings suggested a lack of contractile failure in the jaw-closing muscles. Pain intolerance rather than neuromuscular fatigue is the limiting factor of a sustained submaximal or even maximal clenching effort.

Adult↗

PubMed1994

Clinical use of neck isometric strength measurement in rehabilitation.

The purpose of this study was to evaluate for diagnostic purposes the isometric strength of the neck flexor and extensor muscles in 56 patients with neck and shoulder pain and to quantify the effect of a specific rehabilitation program. An isometric measurement protocol was performed at the beginning and at the end of a 3-week rehabilitation program that included physiotherapy, stretching, aerobic exercises, and circuit training to improve arm, shoulder, and neck muscle strength. In the beginning, mean neck flexor muscle isometric strength was 83N (+/- 48 SD) and extensor muscle strength 158N (+/- 76 SD), whereas after rehabilitation the strength measurements were 117N (+/- 43 SD) and 207N (+/- 84 SD) respectively. There was a significant increase in the ability to push forward and backward, which correlated with the lessening of neck pain and disability found at the end of the program (p < .05). Isometric strength measurement is a useful and practical method of objectively showing a functional improvement in response to rehabilitation.

Adult↗

PubMed1995

The effect of tendon on muscle force in dynamic isometric contractions: a simulation study.

Recently, Baratta and Solomonow J. Biomechanics 24, 109-116 (1991) studied the effect of tendon on muscle-tendon complex behavior in cat tibialis anterior (TA) muscle. This was done by determining the relation between neural stimulation and muscle force in a dynamic isometric experiment, both before and after the removal of the distal tendon. From their results, Baratta and Solomonow concluded that in isometric and concentric contractions at mid-range force levels, tendon behaves as a rigid force conductor. This conclusion is in conflict with literature in which several functions are attributed to the elastic behavior of the series elastic element (SEE), of which tendon is the major part. The present study investigates the expected generalizability of their findings, by simulating the experiments using a straightforward Hill-type muscle model. First, model predictions are shown to be in line with the experimental results on cat TA: in dynamic isometric experiments at mid-range force levels, the effect of SEE removal is indeed negligible. Second, the effect of SEE removal is predicted to vary largely among muscles. Third, the most important determinants of the effect of SEE removal in dynamic isometric contractions are shown to be maximum fiber shortening velocity and the ratio of SEE slack length to fibre optimum length.

Animals↗

PubMed1994

Somatosensory evoked potentials modification related to isometric voluntary contraction.

Somatosensory evoked potentials were elicited by applying an electrical stimulus to the median nerve while the subjects performed a handgrip isometric contraction. Somatosensory evoked potentials (SEPs) were recorded from scalp positions C3 + 2 and C3-2. SEPs during the increase phase and decrease phase of force, and during the hold phase (period of maintenance of steady force) of isometric contraction were compared with potentials during rest. SEPs during the three phases of isometric contraction were also compared. Changes in SEP latencies were not observed. The increase or decrease phase of force and the hold phase caused suppression of the relatively late N55-P100 component when compared to rest. There was no difference in the amplitude of SEPs during the increase or decrease phase. During the hold phase the early N20-P30 component was increased when compared to the other two phases. The selective changes of SEPs during the three phases of isometric-anisotonic contraction suggested task dependence of the gating process.

Adult↗