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PubMed1983

Thigh and calf blood flows after isometric contraction in untrained and trained subjects.

The present study was undertaken to examine whether or not there were any differences between untrained and trained subjects in the changes of blood flow in the ipsilateral and contralateral lower limbs after isometric exercise. Blood flow of the thigh and calf in both right and left legs were measured simultaneously before and after isometric contraction with mercury-in-silastic strain gauge venous occlusion plethysmography. In the present study, the main pattern of blood flow responses in the active and non-active limbs was strikingly similar in all subjects: a significant fall in blood flow immediately after isometric contraction at a force of about 50% of maximal muscle strength for 15 sec was observed in the non-active lower limbs. Peak blood flow of the exercised thigh in the trained group was significantly higher than that in the untrained ones. From these results, it was suggested that higher blood flow after isometric exercise in the trained subjects may be due to the improvement of degree of vasodilation in the lower limb as a result of physical training.

Adult↗
PubMed2003

Chronic hypobaric hypoxia does not affect electro-mechanical muscle activities during sustained maximal isometric contractions.

The aim of the study was to evaluate the influence of chronic hypobaric hypoxia [43 days at 5050 m above sea level, a.s.l.] on the electrical and mechanical activities of the elbow flexor and knee extensor muscles during sustained maximal isometric contractions. Seven subjects participated in this study. The maximal voluntary contraction (MVC) of the two muscle groups was assessed. The subjects were then asked to perform a sustained MVC for 1 min, during which surface EMG and MMG signals were recorded before (SL) and after 43 days at high altitude (HA). From the time and frequency domain analysis of the surface EMG and MMG, the root mean square (RMS) and the mean frequency (MF) were then calculated. After HA exposure, in both the investigated muscle groups the results showed that: (1) the MVC did not change; (2) the maximum force decay during the sustained MVC was similar; (3) surface EMG and MMG parameters did not show any statistical difference. These data suggest that exposure to HA did not affect the electrical and mechanical properties of the two investigated muscle groups. We conclude that exposure to chronic hypobaric hypoxia seems to have no effect on the central motor drive or on muscle performance during sustained maximal isometric contractions of the active muscles.

Acclimatization↗
PubMed2004

Comparison of surface electromyography and myotonometric measurements during voluntary isometric contractions.

OBJECTIVES: Muscle stiffness increases during muscle contraction. The purpose of this study was to determine the strength of the correlation between myotonometric measurements of muscle stiffness and surface electromyography (sEMG) measurements during various levels of voluntary isometric contractions of the biceps brachii muscle. SUBJECTS: Eight subjects (four female; four male), with mean age of 30.6 +/- 8.23, volunteered to participate in this study. METHODS: Myotonometer and sEMG measurements were taken simultaneously from the right biceps brachii muscle. Data were obtained: (1) at rest, (2) while the subject held a 15 lb (6.8 kg) weight isometrically and, (3) during a maximal voluntary isometric contraction. Myotonometer force-displacement curves (amount of tissue displacement to a given unit of force applied perpendicular to the muscle) were compared with sEMG measurements using Pearson's product-moment correlation coefficients. RESULTS: Myotonometer and sEMG measurement correlations ranged from -0.70 to -0.90. The strongest correlations to sEMG were from Myotonometer force measurements between 1.00 and 2.00 kg. CONCLUSIONS: Myotonometer and sEMG measurements were highly correlated. Tissue stiffness, as measured by the Myotonometer, appears capable of assessing changes in muscle activation levels.

Adult↗
PubMed2004

The relationship between isometric contraction durations during hold-relax stretching and improvement of hamstring flexibility.

AIM: Previous research suggests proprioceptive neuromuscular facilitation (PNF) stretching techniques produce greater increases in range of motion than static or ballistic. The purpose of this study was to determine the isometric contraction hold time that best produces gains in hip range of motion after a 3-s (3-HR), 6-second (6-HR), and 10-s (10-HR) hold-relax PNF stretch. METHODS: The experimental design was a within subjects repeated measures 2 factor (baseline and treatment) by 3 factor (3-HR, 6-HR, 10-HR) ANOVA. The dependent variable was hamstring flexibility as measured in degrees of passive hip flexion with the knee extended. Measurements were taken in a university athletic therapy clinic. Sixty active individuals without history of knee or hip injury volunteered for this study. Each subject was randomly assigned to a 3-HR, 6-HR, or 10-HR group for the hold-relax PNF stretch. All subjects were passively taken to the end range of motion 3 times and a goniometric measurement was taken. Each subject then performed the hold-relax PNF stretch 3 times with either a 3-HR, 6-HR, or 10-HR isometric contraction hold time and another goniometric measurement was taken. An average of each of these 3 trials was then calculated. RESULTS: The data analysis revealed a significant main effect difference (F(1,5 7)+/-633.97, p<.001, eta2=0.918) between the baseline and treatment measurements. There were no significant differences for the between groups (3-HR, 6-HR, 10-HR) main effect or for the interaction term. CONCLUSIONS: All 3 hold-time conditions produced significant gains in range of motion compared to baseline measurements. The application of the findings suggest that clinicians could choose any of the hold-times and produce the same result to patient hip joint flexibility.

Adolescent↗
PubMedDate not supplied

Brain macropotentials associated with distinct phases of voluntary sustained isometric contraction in man.

Brain potentials recorded from the scalp during voluntary sustained isometric contraction have been consistently found to accompany both the beginning and the termination of the contraction. This study attempts to evaluate the dependence of the potentials related to the voluntary termination of a sustained effort on the physical parameters of the motor task and also to further investigate the relationship between potentials related to the initiation and to the termination of action. Brain potentials from healthy male volunteers performing hand-grip squeeze were time-locked to (1) beginning of contraction; (2) execution of an additional effort; and (3) the moment of voluntary relaxation, and then averaged. The waveshape and amplitude of the entire potential curve preceding and following the decision to act were evaluated with best-fit mathematical approximation procedures. Few correlations between the various descriptive parameters of the three types of potentials were found. The brain potentials accompanying beginning of the contraction from state of rest differed significantly from those accompanying execution of an additional effort and both potentials preceding initiation of voluntary effort differed from potentials preceding decision to terminate the action. It is hypothesised that brain macropotentials are linked to separate underlying commands for initiation and termination of voluntary action.

Adult↗
PubMed1999

A comparison of electrical activity in the triceps surae at maximum isometric contraction with the knee and ankle at various angles.

The purpose of this study was to test the endurance of the soleus muscle, and to examine the joint position at which it is most active, while simultaneously suppressing the activity of the gastrocnemius. Ten young males performed maximum isometric contraction of the triceps surae for 100 s, and the endurance and plantar flexion torque of this muscle were measured at various angles of the knee and ankle joints. The electromyogram was measured simultaneously and subsequently converted into integrated electromyogram (IEMG) values. With the knee flexed at 130 degrees, the rate of change in IEMG values for the soleus (0.454% x s(-1)) with the ankle in a neutral position was significantly higher than that for the medial and lateral gastrocnemius. Both with the ankle dorsiflexed at 10 degrees and in the neutral position, the rate of change in IEMG for the soleus was significantly higher with the knee flexed at 90 degrees and 130 degrees than with the knee fully extended. With the knee flexed at 90 degrees and 130 degrees, the IEMG activity of the soleus during the initial (5-10 s) and final 5 s tended to be higher than those for the medial and lateral gastrocnemius, regardless of the ankle joint position. We conclude that the position in which the soleus acts most selectively during a sustained maximum isometric contraction of the triceps surae is with the ankle in a neutral position and the knee flexed at 130 degrees.

Adult↗
PubMed2005

Sex differences in glycolysis during brief, intense isometric contractions.

We have previously observed less muscle fatigue in women than men under conditions of intact circulation, but similar fatigue across the sexes during local ischemia. Thus, we hypothesized that women utilize their aerobic metabolic pathways to a greater extent than do men. To test this hypothesis, we examined the extent to which different pathways of intramuscular adenosine triphosphate (ATP) production were utilized by men and women during maximal voluntary isometric contractions. Force production during 15-s and 60-s contractions were recorded in parallel sessions. In one session, central activation was assessed with electrical stimulation. In the other, phosphorus magnetic resonance spectroscopy was used to quantify muscle oxidative capacity, and the contributions of glycolysis and oxidative phosphorylation to ATP synthesis during the 60-s contraction. Fatigue and central activation were similar in men and women during both the 15-s and 60-s contractions. The rate constants of phosphocreatine recovery following the 15-s contraction were similar in men and women, indicating similar oxidative capacities. Men exhibited greater acidosis and peak glycolytic rates compared with women during the 60-s contraction, with no differences observed in creatine kinase flux or the percent of oxidative capacity utilized. We conclude that men exhibit greater in vivo glycolysis during brief, intense isometric contractions. Although this metabolic difference did not contribute to any observable differences in fatigue in the present study, these results highlight a potentially important mechanism to explain sex-related differences in muscle function.

Adenosine Triphosphate↗
PubMed2006

Characterization of isometric contractions of rat skeletal muscle in vivo: duty cycle effects.

Many work related injuries stem from the exertion of skeletal muscle forces over an extended period of time. Musculoskeletal injury can be caused by muscle's inability to maintain force during occupational exposure. The goal of the present study is to test how various rest times (duty cycles) between long isometric contractions will affect decrements in force, and develop a model that characterizes force decrements due to skeletal muscle fatigue. All tests were performed in vivo on the tibialis anterior muscle of anesthetized Sprague-Dawley rats. Animals were randomly assigned to either a 10 second (N=8), 1 minute (N=8), or 5 minute (N=8) duty cycle group. All animals were then subjected to 7 isometric contractions (duration of 2.8 seconds). A model was constructed to characterize forces changes over the duration of a contraction and over multiple contractions. The model consisted of a power law and an exponential component; these two components were combined by using an exponential weighting function. Overall, the combination of a power law and exponential model with a weighting function satisfactorily characterized the changes in isometric force for the 10 second duty cycle, but a simpler exponential model could be used where longer duty cycles are performed.

Animals↗
PubMed1975

Derivation of some parameters of myoelectric signals recorded during sustained constant force isometric contractions.

Mathematical expressions are derived for some parameters of the myoelectric (ME) signal recorded during a constant force isometric contraction. The expressions are developed from a stochastic model for the motor-unit action-potential trains obtained from empirical results. The following parameters: (a) the mean rectified value, (b) the mean integrated rectified value, (c) the root-mean-square value, and (d) the power density spectrum are described as functions of contraction time and constant force of an isometric muscle contraction. The calculated parameters are compared to their corresponding empirically obtained measurements which have been reported in the literature. A discussion on the behavior of the parameters during increasing contraction time is presented. Synchronization of the motor-unit action-potential trains is shown to have a pronounced effect on the parameters of the myoelectric signal. This result should be considered when analyzing long records of myoelectric signals.

Action Potentials↗
PubMed2001

Cross-bridge cooperativity during isometric contraction and unloaded shortening of skeletal muscle.

Whether the two heads of skeletal muscle myosin work independently or cooperatively remains an open question in muscle biophysics. While individual myosin heads are sufficient for ATPase activity (Reisler (1980) J Mol Biol 138: 93-107) and force production (Harada et al. (1987) Nature 326: 805-808), it has also been reported that in situ, the two heads of a myosin molecule work cooperatively (Chaen et al. (1986) J Biol Chem 261(29): 13,632-13,636). To examine the role of cross-bridge cooperativity on isometric contraction and unloaded shortening we progressively inactivated myosin cross-bridges via titration with para-phenylenedimaleimide. The resting fiber ATPase was measured to provide an estimate of the fraction of active cross-bridges remaining during the titration. Isometric force and unloaded shortening velocity decline more rapidly than the resting ATPase as the titration proceeds. This is inconsistent with models for independent force generation and suggests cooperative action of myosin cross-bridges when muscle is isometrically contracting or shortening under zero load. However the degree of cooperativity depends on the type of muscle activity. While isometric force declines in a manner consistent with pair-wise cooperative action of myosin heads, unloaded shortening velocity declines more rapidly (greater cooperativity). Therefore, myosin cross-bridges in situ may be capable of at least two types of cooperative interactions, pair-wise cooperativity (when isometric) and another form of cooperativity that is sensitive to longer range interactions transmitted from other cross-bridges in the ensemble (during unloaded shortening).

Adenosine Triphosphatases↗
PubMed1981

The effects of local anaesthetics on the isometric contraction of the isolated hemidiaphragm of the rat.

Local anaesthetics (benzocaine, tetracacine, lidocaine, cocaine, procaine) were found to depress both Td and dT/dt max of the isometric contraction of the isolated hemidiaphragm of the rat during direct electrical stimulation. The same substances were found to antagonize the action of aminophylline on Td and dT/dt max of the isolated hemidiaphragm during direct stimulation. The effect of higher concentrations of aminophylline was even reversed by cocaine, procaine and lidocaine. Increasing of calcium concentrations did not reverse the antagonistic action of aminophylline on Td and dT/dt max of the isometric contraction of the isolated hemidiaphragm. It is suggested that a change in the muscle membrane is responsible for the observed action of local anaesthetics on Td and dT/dt max, as well as for their antagonistic action towards aminophylline.

Aminophylline↗
PubMed1996

Distribution of muscle fiber conduction velocity of M. biceps brachii during voluntary isometric contraction with use of surface array electrodes.

Surface electromyogram (EMG) was recorded in m. biceps brachii during the contractions of 20, 40 and 60% of maximum voluntary isometric contraction (MVC) in twelve healthy male subjects, using surface array electrodes. The distribution of muscle fiber conduction velocity (MFCV) was found directly using the averaging technique and the cross-correlation function technique. MFCVs in the region of 20-40 mm measured from end-plate denoted constant value of about 4 m/s in 20% MVC, while MFCVs in the region around end-plate and tendons showed about 10 m/s in 20% MVC. The values of MFCV depended on the contraction levels of muscle. The model for the generation of MFCV which considered the ensemble of muscle fibers with the shape of a cone was proposed. The theoretical values of MFCV by the muscle fiber ensemble model (MFE model) proposed in the paper showed in good agreement with the experimental results.

Action Potentials↗
PubMed1990

Organizing principles for single-joint movements. IV. Implications for isometric contractions.

1. Normal human subjects made isometric pulse and step contractions about the elbow to visually defined target torques of different amplitudes and at different rates. We measured joint torque and electromyograms (EMG) from two agonist and two antagonist muscles. 2. When the task specification requires that the subject explicitly alter the rate at which torque is increased, the rates of rise of the agonist and antagonist EMG bursts covary with the rate of rise of the torque. For pulses of torque the duration of motoneuron excitation varies with the duration of the task-defined contractile event. 3. When a subject is asked to generate torques of different amplitudes without specifying a time interval, torque amplitude is positively correlated with how long, and therefore how high, the EMG rose. Subjects usually proportionately covary the strength of the agonist and antagonist contractions but are not constrained to do so. Some subjects use a strategy of varying the antagonist inversely with the agonist contraction. 4. We extend the organizing principles for the control of movement about a single joint to the control of isometric torque. These rules state that control of torque about a single joint is exercised by one of two strategies: the speed-sensitive strategy modulates the rate at which contraction rises by varying the intensity of motoneuron-pool excitation. The speed-insensitive strategy varies the duration over which contraction rises but does not change the rate. These two respective patterns of torque emerge from pulse-height and pulse-width modulation of motoneuron-pool excitation. 5. The rules defining speed-sensitive and speed-insensitive strategies for movements are broadened for isometric contractions because of the wider range of torque patterns that we observe under these conditions. We propose a step-excitation component for prolonged isometric step contractions and slowly rising ramp patterns of excitation for contractions that develop over several hundreds of milliseconds. 6. The choice of strategies is based on task-specific torque requirements. The same two strategies that control torque to produce movement apply to the control of isometric torque. Unlike movements, however, isometric tasks are more often controlled by a blending of the two patterns. Possible reasons for this are discussed.

Electromyography↗
PubMed2005

Caffeine increases time to fatigue by maintaining force and not by altering firing rates during submaximal isometric contractions.

Caffeine increases time to fatigue [limit of endurance (T(lim))] during submaximal isometric contractions without altering whole muscle activation or neuromuscular junction transmission. We used 10 male volunteers in a randomized, double-blind, repeated-measures experiment to examine single motor unit firing rates during intermittent submaximal contractions and to determine whether administering caffeine increased T(lim) by maintaining higher firing rates. On 2 separate days, subjects performed intermittent 50% maximal voluntary contractions of the quadriceps to T(lim), 1 h after ingesting a caffeine (6 mg/kg) or placebo capsule. Average motor unit firing rates recorded with tungsten microelectrodes were constant for the duration of contractions. Caffeine increased average T(lim) by 20.5 +/- 8.1% (P < 0.05) compared with placebo conditions. This increase was due to seven subjects, termed responders, who increased T(lim) significantly. Two other subjects showed no response, and a third had a shorter T(lim). Neither the increased T(lim) nor the responders' performance could be explained by alterations in firing rates or other neuromuscular variables. However, the amplitude of the evoked twitch and its maximal instantaneous rate of relaxation did not decline to the same degree in the caffeine trial of the responders; this resulted in values 20 and 30% higher at the time point matching the end of the placebo trial (P < 0.05). The amplitude of the evoked twitch and the maximal instantaneous rate of relaxation were linearly correlated (caffeine r = 0.72, placebo r = 0.80, both P < 0.001), suggesting that the increase in T(lim) may be partially explained by caffeine's effects on calcium reuptake and twitch force.

Action Potentials↗
PubMed1999

Relation between intrinsic viscoelasticity and activation level of the human finger muscle during voluntary isometric contraction.

The purpose of the present study was to isolate the length perturbation-evoked force attributed to intrinsic muscle viscoelasticity, and to investigate the relation between muscle viscoelasticity and the level of muscle activation during isometric contraction in five healthy male subjects. A small length perturbation (stretching or contraction) was applied to the flexor pollicis longus muscle while the subject maintained constant isometric force; the time courses of the length perturbation was found to be almost identical in all the experiments. The force (Fv) induced by the muscle viscoelasticity was calculated using the equation Fv = F - Fc - Fp over an interval of 35 ms after the onset of perturbation, where F is the measured force, Fc is the tonic isometric force before the onset of perturbation and Fp is the force at rest obtained from the same length perturbation. The force response attributed to the stretch reflex is not included during this interval. These experiments were repeated at varying levels of isometric force. An almost linear relationship was obtained between the muscle viscoelasticity-induced force and the tonic isometric force during both the stretching and contraction of the muscle, i.e. the intrinsic muscle viscoelasticity varied almost linearly with the level of isometric contraction.

Adult↗
PubMed2002

Finite element model of intramuscular pressure during isometric contraction of skeletal muscle.

The measurement of in vivo intramuscular pressure (IMP) has recently become practical and IMP appears well correlated with muscle tension. A numerical model of skeletal muscle was developed to examine the mechanisms producing IMP. Unipennate muscle is modelled as a two-dimensional material continuum that is incompressible and nonlinearly anisotropic. The finite element technique is used to calculate IMP and muscle stress during passive stretch and during isometric contraction. A novel element models the contractile portion of muscle, incorporating sarcomere length-force and velocity-force relations. A range of unipennate muscle geometries can be modelled. The model was configured to simulate the rabbit tibialis anterior muscle over a range of lengths. Simulated IMP and stress results were validated against animal experimentation data. The simulation agreed well with the experimental data over the range of 0.8-1.1 of the optimal length. Severe pressure gradients were produced near the musculo-tendinous junctions while IMP was more uniform in the central muscle belly. IMP and muscle stress in relaxed (unstimulated) muscle increased nonlinearly with muscle length. IMP and stress in isometrically contracting muscle showed a local maximum at optimal length and were reduced at shorter lengths. At muscle lengths longer than optimal, stress and IMP increased predominately due to tension in the passive elastic structures.

Animals↗
PubMed1979

The control of blood flow through human forearm muscles following brief isometric contractions.

1. The blood flow through the forearm was measured 2 sec after single, brief isometric hand-grip contractions. The tension and duration of those contractions varied from 10 to 100% of the maximal voluntary contraction (m.v.c.) and from 2 to 12 sec, respectively. 2. The blood flow increased linearly with tension up to about 60% m.v.c. but further increases in tension, up to 100% m.v.c., did not elicit higher blood flows than were found at 60% m.v.c. The same relationship between tension and the resultant blood flow held for all durations of contractions, from 2 to 12 sec. The blood flow immediately after (2 sec) contractions at a given tension increased linearly with the duration of the contraction, from 2 to 12 sec. Maximal exercise blood flow was approached only in response to the longest contractions (12 sec) at tensions of 60% m.v.c. or higher. 3. Brief alterations (2--5 sec) of transmural pressure across blood vessels did not result in a significant change of blood flow, either in the resting forearm or when the vessels were dilated by brief, isometric contractions. When the tension was applied or released either rapidly or gradually ('ramp' contractions) there was no correlation between the rate of change of stretch on arterial vessels and the resultant blood flow. However, there was a direct relationship between a force--time integral (duration of contraction x peak tension) and blood flow. All these results make it clear that changes in blood flow in the forearm elicited by brief isometric contractions are not the result of a myogenic reflex but are metabolically induced. 4. Successive contractions exerted at 60% m.v.c. for 4 sec induced a blood flow of 21.2 +/- 1.6 ml.min-1.100 ml.-1 when a rest interval of 8 sec was allowed between the contractions. Blood flows remained constant at this submaximal level, even when muscular fatigue was induced, and when there was an accompanying large increase in blood pressure. 5. Isometric muscular activity by the contralateral arm which resulted in fatigue, associated with a large increase in mean blood pressure, did not alter the level of vasodilation that was induced by brief, isometric contractions in the 'test' arm. 6. It is suggested that the vasodilatation in response to intermittent isometric contractions is the result of metabolic vasodilatation of distal segments and continued sympathetic vasoconstriction of the proximal segments of the forearm vascular bed.

Blood Pressure↗
PubMed2002

Changes in muscle activation can prolong the endurance time of a submaximal isometric contraction in humans.

Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximal voluntary contraction (MVC) force on three occasions. Endurance time for session 3 [1,718 +/- 1,189 (SD) s] was longer than for session 1 (1,225 +/- 683 s) and session 2 (1,410 +/- 977 s). Five men and four women increased endurance time between session 1 and 3 by 60 +/- 28% (responders), whereas two men and three women did not (-3 +/- 11%; nonresponders). The MVC force was similar for the responders and nonresponders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the electromyogram (EMG) amplitude and number of bursts during the fatiguing contractions. The responders achieved a similar level of EMG at exhaustion but a reduced rate of increase in the EMG across sessions. The rate of increase in EMG across sessions declined for the nonresponders, but it remained greater than that of the responders. The increase in burst rate during the contractions declined across sessions with a negative relation between burst rate and endurance time (r = -0.42). Normalized force fluctuations increased during the fatiguing contractions, and there was a positive relation (r = 0.60) between the force fluctuations and burst rate. Changes in mean arterial pressure and heart rate during the fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who increased the endurance time of a submaximal contraction across three sessions did so by altering the level and pattern of muscle activation.

Adult↗