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PubMed2000

Perceived exertion during isometric quadriceps contraction. A comparison between men and women.

BACKGROUND: 1) To examine the validity and accuracy of the CR-10 scale for evaluating perceived exertion, and 2) to assess gender differences in perceived exertion across different levels of contraction intensity. METHODS EXPERIMENTAL DESIGN: cross-sectional, comparative design. SETTING: Human Performance and Fatigue Laboratory, Eastern Washington University. SUBJECTS: 30 healthy, college age volunteers (15 males, 15 females). MEASURES: All subjects were assessed for isometric torque and perceived exertion of the quadriceps femoris muscles, via the CR-10 scale. One low anchor was applied under resting conditions with the knee flexed to 60 degrees, and a high anchor was applied during a maximal voluntary muscle contraction (MVC). SUBJECTS performed five-second isometric contractions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of their MVC, in a random order, and were assessed for perceived exertion by visually observing the CR-10 scale. One sample "t"-tests and 95% confidence intervals were calculated for perceived exertion at each relative torque level. A single factor ANOVA with repeated measures was performed across al levels of exercise intensity. Linearity for perceived exertion was assessed via regression analysis. RESULTS: Perceived exertion at each exercise intensity were as follows: 10%: 1.87+/-1.14, 20%: 2.43+/-1.19, 30%: 3.5+/-1.36, 40%: 3.97+/-1.52, 50%: 4.73+/-1.28, 60%: 5.53+/-1.28, 70%: 6.73+/-1.62, 80%: 7.57+/-1.72, and 90%: 8.6+/-1.52. The increase in perceived exertion across the intensity spectrum was found to fit both linear and quadratic trends. There were no gender differences in perceived exertion across all levels of exercise intensity. CONCLUSIONS: The findings demonstrate that the CR-10 scale closely approximates perceived exertion of the quadriceps femoris muscles during sub-maximal, static contractions, and is not gender specific.

Cross-Sectional Studies↗

PubMed2001

Muscle fatigue assessment during sustained high isometric contractions.

The myoelectrical fatigue manifestation was assessed during sustained isometric maximal and submaximal (75% of maximal (MVC)) isometric contractions by means of a level-trigger averaging of the surface electromyogram (EMG) recorded from m. biceps brachii by branched bipolar electrodes. The conduction velocity (CV) of excitation was calculated from the time shift of the negative peaks of the averaged potentials (APs). The amplitude and the duration of the negative phase of APs were also measured. The conduction velocity was the most appropriate EMG-derived parameter for muscle fatigue assessment. The duration of the endurance time and the time pattern of CV changes during submaximal sustained contractions were found to resemble very closely those during maximal contraction efforts sustained to 30 percent reduction of the initial torque. Thus the recruitment of additional motor units (MUs) during sustained high contractions does not affect essentially the calculated values of CV.

Action Potentials↗

PubMed2002

Differences in the oxygenation of the forearm muscle during isometric contraction in trained and untrained subjects.

The aim of this study was to test a hypothesis that a longer duration of isometric contraction is related to an increased oxygenation status of the muscle. A group of trained rock climbers and untrained subjects performed 15 kp sustained isometric contraction until fatigue set in. The oxygenation status was assessed using near infrared spectroscopy (ISS, USA). The results support the hypothesis. The concentration of relative oxygenated hemoglobin was higher in the rock climbers than in the untrained subjects. The relative total hemoglobin did not differentiate the two groups enough to show that blood volume also strongly influences contraction time.

Adult↗

PubMedDate not supplied

Isometric cervical extension force and dimensions of semispinalis capitis muscle.

The anatomy of the human cervical region has a special complexity. Ultrasonography is a scanning technique, which has been successfully used to identify and measure the cervical muscle dimensions. The purpose of this study was to detect with the use of real-time ultrasonography the dimensional changes of the human semispinalis capitis muscle (SECM) during isometric cervical extension. Six junior ice hockey players took part in the study. We simultaneously measured the values of estimates, such as breadth or anteroposterior dimension (APD) and width or lateral dimension (LD), of the SECM cross-sectional area during isometric cervical extension at different maximum voluntary contraction (MVC) percent levels (0%, 20%, 40%, 60%, 80%, and 100%). The result of multiplication of the linear dimensions (APD x LD), considered as the size of the SECM, increased (p < 0.05) with increasing the level of the cervical extension force. The correlation between muscle size and extension force of 100% MVC was r = 0.79 (p = 0.05). The ultrasonography method may be useful in evaluating the function of an individual cervical muscle.

Adolescent↗

PubMed2002

[Maximal isometric bite force and sports. Preliminary study].

INTRODUCTION: The evaluation of the bite forces coupled with EMG activity of masseter muscles allows to point out temporo-mandibular joint disorders. The intense practice of sports induces stress which may affect the mandibular statics, due to an hyperfunction of the elevator masticatory muscles, especially the masseter muscles. This concept has led us to compare the maximum isometric bite forces of sedentary and physically trained subjects. MATERIAL AND METHODS: The aim of this experimentation is to study the maximum isometric bite forces of the premolars, with a force transducer, for two groups of physically trained and sedentary volunteers (25 subjects), with distinction between male and female subjects. Subjects with normal denture and no temporo-mandibular joint disorder were retained. EMG and force recording were synchronized and recorded during 10 seconds of maximal contraction. RESULTS: The male subjects developed a maximal bite-force significantly higher (p < 0.05) compared to the female subjects. There is no significant difference between subjects practising a sport (at least 6 hours a week) and sedentary ones. The evolution of force during the contraction is different between sexes. EMG activity allowed to control that subjects developed a maximal force and an increase in muscular fatigability of physically active females compared to sedentary ones was noticed. DISCUSSION: The difference of maximal force between men and women is similar to the one found for the locomotor muscles. The analysis of the evolution of the force according to the contraction duration, might be linked to a different distribution of muscular fibers according to the sex. The practice of a sport might increase the fatigability of the masseter muscles and might be a factor inducing a muscular imbalance of the mandibular posture. But the relatively low number of subjects and the absence of well defined distinctions between different kinds of sport limit our conclusions.

Adult↗

PubMed1992

Isometric and isokinetic knee joint performance in Japanese alpine ski racers.

Isometric and isokinetic (concentric and eccentric, strength of alpine skiers with different performance levels were measured. Nine national (elite, EG) and 10 collegiate (trained, TG) female alpine skiers (16 to 23 years of age) performed maximal voluntary knee extension and flexion. Peak torque was measured at an angular velocity of 30 deg.s-1. The cross-sectional area (CSA) of thigh muscles (quadriceps and hamstring muscles) was determined by an ultrasonic method. No significant differences in anthropometric variables and CSA were observed between EG and TG. EG had significantly greater (p < 0.01 for extensors and p < 0.05 for flexors) eccentric knee extensor and flexor strength than that of TG whereas no significant differences were noted in isometric and concentric strength. Eccentric strength/CSA ratio was also higher for EG than for TG. It was suggested that knee extension and flexion strength during eccentric muscle action might be related to the performance level of alpine skiers.

Adolescent↗

PubMedDate not supplied

Activity in three parts of the quadriceps recorded isometrically at two different knee angles and during a functional exercise.

The purpose of this study was to evaluate individual differences in three parts of the quadriceps activated isometrically at 60 degrees and 90 degrees of knee flexion, and during a functional activity involving both concentric and eccentric muscle work. Surface EMG amplitudes were therefore recorded from oblique parts of vastus medialis (VMO) and vastus lateralis (VLO) and from rectus femoris (RF). VMO and VLO showed less activation at 60 degrees than at 90 degrees, but in RF there was no difference between the two angles. In the second experiment, where 11 subjects stepped on and off a stool; these amplitudes were compared with those from a maximal isometric voluntary contraction at 90 degrees of knee flexion. For VMO & VLO the normalised peak amplitude in stepping up was 1.41 +/- 0.12, & 1.46 +/- 0.15 respectively, showing that higher activity is necessary during concentric contractions. These two results suggest that the motor control of VMO/VLO may be different from the bulk of quadriceps. Our findings have implications for patellofemoral function.

Adult↗

PubMed2003

Mechanism of cross-bridge detachment in isometric force relaxation of skeletal and cardiac myofibrils.

Skeletal and cardiac muscle relaxation is governed by the interplay between two macromolecular systems: (i) membrane bound Ca2+ transport proteins and (ii) sarcomeric proteins. Photolysis experiments in skinned muscle preparations and fast solution switching studies in single myofibrils offer means for isolating sarcomeric mechanisms of relaxation from those related to myoplasmic Ca2+ removal. Single myofibril experiments have recently shown that cross-bridge mechanics and detachment kinetics are the major determinants of the time course of relaxation. Full force decay in myofibrils occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere 'give' and dominated by inter-sarcomere dynamics while the former occurs under nearly isometric conditions. Strong evidence has been found that the slow rate of force decay in myofibril relaxation reflects the rate at which cross-bridges leave force-generating states under isometric conditions. Dissection of chemo-mechanical transduction process in myofibrils indicates that both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

Animals↗

PubMed1992

[Control of isometric muscle contraction in muscle hypotonia of central origin: EMG mapping analysis].

Electromyographic and mechanographic investigations in patients with muscular hypotonia, which is, for instance, a side-effect after stereotactic treatment of tremor syndromes, permit the presumption that in this sensomotor open-loop situation the decreased muscular resistance to stretching during isometric contraction (initial stiffness) is caused by changes of muscular innervation pattern. Probably, the innervation pattern during tonic activity is changed by a shift of a more tonic motoneurone behaviour to motoneurone activities with predominantly phasic characteristics. In 17 controls and 4 patients with muscular hypotonia caused by stereotactic lesions of VIM area (treatment of tremor syndromes) the EMG of right and left side forearm flexors (especially the activity of the M. biceps brachii) was investigated by a sophisticated, topographically oriented 16-channel-surface-EMG-technique ("EMG-Mapping") during slight isometric contraction. EMG-Maps of forearm flexors (especially of M. biceps brachii) in patients with centrally evoked muscular hypotonia demonstrate that in these open-loop conditions the motor control is changed. For this the reason could be a shift of the activated motor units from a predominantly static to a more phasic functional behaviour. The latest results on muscular activation processes in cats support this presumption.

Adult↗

PubMed2004

Modulation of stress induced by isometric handgrip test in hypertensive patients following yogic relaxation training.

13 essential hypertensive patients aged 41 to 60 years were given yoga training for 60 min daily, Monday through Saturday, for a total duration of 4 weeks. Blood pressure and heart rate (HR) were measured with non-invasive semi-automatic blood pressure monitor. Measurements were recorded before the training and at weekly intervals during the 4 week training period. Results of our study show a significant (P<0.001) reduction in resting HR and rate-pressure-product (RPP) after 2 weeks of yoga training. Systolic pressure (SP), diastolic pressure (DP) (P<0.001) and mean pressure (MP) (P<0.05) showed a significant reduction at 3 weeks of training period. After 4 weeks of training, there was further fall in SP, DP, pulse pressure (PP) (P<0.05), MP (P<0.001), HR and RPP. Isometric handgrip test before yoga training produced a significant rise in SP and MP and insignificant rise in DP, HR and RPP. After yoga training, there was a significant rise in all these parameters. Our results show that yoga training optimises the sympathetic response to stressful stimuli like isometric handgrip test and restores the autonomic regulatory reflex mechanisms in hypertensive patients.

Adult↗

PubMed2004

Muscle control associated with isometric contraction in different joint positions.

The purpose of this study was to investigate the dependence of the surface EMG and varied internal effort due to different joint positions, and its muscle control strategies. Ten healthy subjects performed a fixed level (40% MVC measured in the neutral position) of isometric dorsiflexion and plantarflexion contraction in the ankle neutral, dorsiflexion, and plantarflexion positions. Quantitative EMG analyses with feature extraction in the time, frequency domain, as well as time-varying spectrum were employed to estimate recruitment pattern, code rating, and recruitment stationarity of activated motor units. Both RMS and median frequency of the tibial anterior (TA) and gastrocnemius (GS) were strongly dependent on foot position (p < .001). Dominant firing rate of the TA muscle during the effort related isometric contraction was significant different with respect to ankle position (p < .05). Irrespective of ankle position, the regression slope of median frequency across time was not different from zero for both TA and GS muscles (p > .05). Identical torque exertion in different ankle positions called for considerable changes of control strategies of motor units. Possible mechanisms could be augmented excitatory central afferents and release of inhibition from Golgi tendon organs in compensation for biomechanical disadvantage in shorter muscle length.

Adult↗

PubMed2004

H-reflex and reciprocal Ia inhibition after fatiguing isometric voluntary contraction in soleus muscle.

This study investigated the H-reflex and reciprocal Ia inhibition during fatigue in the human soleus muscle. Ten healthy subjects participated in this study, and performed intermittent isometric voluntary contraction of the ankle plantarflexion at 50% MVC as the fatiguing task. Reciprocal Ia inhibition was evaluated by the degree of H-reflex amplitude depression in the soleus muscle by the test stimulus following conditioning stimulus to the common peroneal nerve. The difference in H-reflex amplitude between before and after fatiguing task was also checked. There was no significant difference in the degree of H-reflex amplitude depression, although the H-reflex amplitude significantly decreased after the fatiguing task (p < 0.01). From the results of this study, it was considered that the decrease in H-reflex amplitude was caused by descending inhibitory input from the supraspinal to alpha-motoneuron, and the excitability of the Ia inhibitory interneuron was not involved. It was suggested that the function of reciprocal Ia inhibition was difficult to modulate during fatigue caused by isometric voluntary contraction in this study.

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↗

PubMed2005

Isometric force production in high Gz: mechanical effects, proprioception, and central motor commands.

BACKGROUND: We have shown in the past that human subjects produce exaggerated isometric responses when exposed to high +Gz. The present study investigated the role played by different factors in this phenomenon. METHODS: There were 12 healthy non-pilot volunteers who were seated in the gondola of a centrifuge and were exposed to a +1, +1.5, and +3 Gz environment. During each exposure, they produced forces of prescribed magnitudes and directions in their frontal plane using an isometric joystick. From the registered data, we determined initial (100 ms after onset), peak, and end (70 ms before trial end) force. RESULTS: For a given target magnitude, initial, peak, and end force were each smallest in 1 G, higher in 1.5 G, and higher still in 3 G. This G-related exaggeration was most pronounced for initial, and less for peak and end force. We further found that responses in high G were biased downwards for end force, but not for initial and peak force. CONCLUSIONS: The G-related excesses of produced force are probably due to less appropriate central commands, since it manifests before proprioception becomes effective. The amelioration of this deficit for peak and end force is probably achieved by partial corrections based on proprioceptive feedback, or by direct mechanical effects. Our findings are relevant for the safe operation of high-performance aircraft during high-G maneuvers.

Adult↗

PubMed2004

[Relationship between surface electromyographic signal (sEMG) changes and subjective assessment of muscle fatigue during isometric contractions].

OBJECTIVE: To investigate the numerical relationship between the changes of surface electromyographic (sEMG) signal indexes and subjective assessment of muscle fatigue during isometric contractions. METHOD: Eleven healthy subjects participated in the experiment. The sEMG signals of biceps were recorded during their sustained isometric contractions at 50% maximum voluntary contraction (MVC). Fatigue was subjectively rated using a Borg CR-10 scale. Correlations between Borg ratings and a series of fatigue-related indexes such as MPF, MF and Lempel-ziv complexity were analyzed, and curve estimation was also explored. RESULT: A strong positive correlation was found between Borg ratings and endurance time, and negative correlations at different degrees were found between Borg ratings and each of sEMG signal indexes. Curve estimation (Logarithmic curve and Power curve) was also significant. CONCLUSION: The significant correlation between the Borg scale, sEMG signal indexes and endurance time indicates a close relationship between subjective and objective assessment of muscle fatigue. The numerical relationship between them accord with the power law of Stevens in psychophysics relatively.

Adult↗

PubMedDate not supplied

Elbow torques and EMG patterns of flexor muscles during different isometric tasks.

This paper examines the torque responses and EMG activity levels in four muscles acting at the elbow joint during different combinations of one- and two- degree of freedom isometric torque production (single and dual tasks, respectively). Flexor and supinator/pronator torques and surface EMG signals from m. biceps brachii, m. brachialis, m. brachioradialis and m. triceps brachii were measured in 16 male subjects while they performed maximal effort isometric contractions of pure flexion, pure supination, pure pronation, combined flexion and supination and combined flexion and pronation. In the single tasks, the torque responses were consistent with task requirements, but the dual task results were surprising in that flexor torque levels were reduced as compared to pure flexion, while supinator/pronator torque levels were as high or higher than in pure supination or pronation. Muscle activity levels varied with task, and could not always explain the differences observed in torque responses. These data are discussed within the framework of subpopulations of task-specific motor units within each muscle. The implications of such task-specific muscle units are related to musculoskeletal modelling and previous EMG - torque relationships found at the elbow.

Analysis of Variance↗

PubMed1991

Isokinetic and isometric muscle strength combined with transcutaneous electrical muscle stimulation in primary fibromyalgia syndrome.

Twenty women with primary fibromyalgia syndrome and 20 age matched healthy women were investigated. The subjects performed maximum voluntary isokinetic contractions of the right quadriceps in an isokinetic dynamometer. Maximum voluntary isometric contractions of the right quadriceps were performed with superimposed transcutaneous electrical stimulation. The examination protocol was repeated after 1 h of resting. Isokinetic and isometric muscle strength was found to be, respectively, 45% (p = 0.0001) and 44% (p = 0.0001) lower in the patient group compared to the healthy subjects. The frequency of superimposed twitches was 65% in the patient group and 15% in the control group (p = 0.003). Patients with primary fibromyalgia have a lower maximum voluntary muscle strength than expected. The increased presence of superimposed electrically elicited twitches during maximum voluntary contraction indicates submaximal force application in primary fibromyalgia syndrome.

Adult↗

PubMed1991

The relationship between isometric contraction durations and improvement in shoulder joint range of motion.

Proprioceptive Neuromuscular Facilitation (PNF) flexibility techniques are now being used in health and sports related activities, yet it is unclear as to the relationship between various isometric contraction time increments and joint range of motion. The purpose of this study, therefore, was to determine the relationship between a three-second, six-second, and ten-second maximum voluntary isometric contraction (MVIC). A modified PNF procedure referred to as the slow-reversal-hold-relax (SRHR) flexibility technique was employed in the investigation. It was hypothesized that longer MVIC time increments used with the SRHR flexibility technique would provide greater range of motion (ROM). Specifically, the ten-second MVIC was believed to be superior to the six-second and three-second MVIC. Furthermore, it was hypothesized that the six-second MVIC was superior to the three-second MVIC. Sixty subjects, ages 14-57 were randomly assigned to one of three treatment groups. Using a Leighton Flexometer, acute internal rotation of the shoulder joint was measured in degrees for six trials. Three passive stretch trials served as the baseline measurement for each subject (trials 1-3). The SRHR flexibility technique was used as the treatment for trials 4-6. A sixty-second rest interval common to clinical settings was integrated between each trial. The hypothesis was not accepted that a positive correlation existed between increased MVIC time and greater ROM.

Adolescent↗