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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↗
PubMed1989

[Changes of muscle activities in neck and shoulder region during sustained isometric contractions of jaw muscles].

Eight healthy participants performed horizontal isometric jaw movements with maximum voluntary contractions of the jaw muscles until they could not tolerate the task. Electromyographic activities bilaterally recorded from the superior and inferior regions of the sternocleidomastoid muscle and the nuchal and shoulder regions of the trapezius muscle were observed. The results obtained were as follows: 1. In more than 94% of 32 trials, the amplitude of EMGs during the task were higher than those at rest, and in more than 79%, temporal changes in EMG burst were synchronized with the onset-offset of the exercise. 2. In more than 72% of all trials, small amplitude EMG lasted after cessation of the task. 3. Muscle activities of the lateral cervical region during the exercise were superior to the activity of the nuchal and shoulder regions, and amplitude of EMGs in the neck and shoulder regions tended to be the highest during isometric retrusion. 4. These results don't contradict with our hypotheses that occlusal habits might be one cause of the unpleasant sensation in the neck and shoulder regions and that sustained isometric retrusion might have heavier effects on it.

Adult↗
PubMed2000

On the inter- and intra-subject variability of the electromyographic signal in isometric contractions.

The objective of the present study was to evaluate the variability of the surface EMG signal of the same muscle in healthy subjects, because of lack of reproducibility of the EMG signal for the same subject and muscle in different trials of maximal isometric voluntary contraction. The results showed an EMG coefficient of variability of 21.61%, indicating that this variability must be considered in experiments with an inappropriate condition for normalization procedures, such as EMG biofeedback in rehabilitation sessions, or normalization procedures by the maximal isometric voluntary contraction.

Adult↗
PubMed2003

Respiratory muscle electromyogram and mouth pressure during isometric contraction.

When extra-diaphragmatic muscles are activated progressively under approximately isometric conditions, we expect a corresponding increase in respiratory muscle output. Therefore, we examined relative recruitment shown as the latency to onset of EMG activity, and the relationship between mouth pressure and electromyogram activity of the neck accessory and transversus abdominis (TRANS) muscles during respiratory maneuvers against occlusion. Fine wire electrodes were inserted into the scalene (SCLN), sternocleidomastoid (STERNO), trapezius (TRAPZ) and TRANS in six awake, healthy subjects. Mouth pressure, raw and moving average EMG signals were recorded during gradual production of expiratory or inspiratory mouth pressure to maximum (MPmax) at FRC in the standing posture. Group mean linear regression lines of EMG activity versus mouth pressure were strongly significant for SCLN and TRANS, less for STERNO, and least for TRAPZ. The SCLN and STERNO showed EMG activities with low, and TRAPZ showed EMG activity only with high, mouth pressure. At 90% MPmax, TRAPZ was much less active compared with TRANS, SCLN, or STERNO. These results suggest that over a wide range of respiratory effort there is a significant difference in the relationship between mouth pressure and EMG activity in the accessory muscles, with differential recruitment of individual respiratory muscles.

Adult↗
PubMed1975

Tension decline during isometric contractions without visual cues.

Subjects tried to maintain 15%, 30% and 50% maximum voluntary contraction (MVC) of their handgrip for 60 sec and 70% MVC for 30 sec. When the subjects watched the tension record, they could maintain the contraction for the required time. However, without visual cues (uncued), the tests resulted in a tension decline. Ae-alpha t described the tension decline in the 15% tests, Be-beta t the 70% test, while the form in the 30% and 50% tests was Ae-alpha t + Be-beta t. In all cases alpha and/or beta were found to be dependent on the starting tension. The electrical activity obtained from surface electromyograms did not increase during the uncued tests which would be expected if muscle fatigue were involved in the tension decline. Instead the electrical activity was directly proportional to the decline in tension suggesting that the fall in tension was not due entirely to muscle fatigue. 15% MVC uncued tests in which fatigue is known not to be a factor also showed a tension decline. Precooling the hand in an attempt to alter sensory input produced no systematic change from the original uncued tests, indicating that sensory information from the hand was probably not a major factor in the tension decline. It is suggested that during the uncued tests the fast component of the tension decline may be related to a sensory adaptation involving the Golgi tendon organs. The slow component is due to forearm muscle fatigue which is related to the ischemia produced during the isometric contractions.

Adult↗
PubMedDate not supplied

Temporal effects of isometric contraction maneuvers on threshold sural amplitude.

OBJECTIVE: To investigate the effect of isometric biceps brachii contraction and neck flexion on the time course of threshold sural amplitude. DESIGN: Twelve healthy subjects, who were asked to lie supine on an examination bench, performed 1 min of muscle contraction. The sural sensory nerve action potential was recorded before, immediately after, and at 2-min intervals after muscle contraction. The preexercise level of stimulus intensity remained unchanged for sural readings throughout the entire course of the experiment. RESULTS: The temporal changes in sensory nerve action potential amplitudes for both maneuvers were similar (P = 0.9734, two-way interaction). The mean sural amplitude after neck flexion increased from 6.0 +/- 2.9 microV (SD) to 10.6 +/- 6.6 microV (SD) 10 min after contraction. Similarly, mean sural amplitude increased from 6.5 +/- 1.8 microV (SD) to 14.5 +/- 9.7 microV (SD) 8 min after biceps brachii contraction. Statistical analysis performed using repeated measures with post hoc least significant difference showed a significant temporal effect in the two groups (P = 0.04). CONCLUSION: The temporal responses of threshold sural amplitudes after isometric biceps brachii contraction and central reinforcement neck flexion maneuvers are nearly identical with regard to increase in the amplitude.

Action Potentials↗
PubMed1988

Time-resolved 31P nuclear magnetic resonance studies on isometric contraction of frog skeletal muscle.

The time course of changes of inorganic phosphate (Pi) and phosphocreatine (PCr) was studied during and after brief isometric tetanic contractions using phosphorus nuclear magnetic resonance on isolated semitendinosus muscles of the bullfrog at 4 degrees C. We followed essentially the method of Dawson, Gadian and Wilkie and the signal-to-noise ratio was improved by using many muscles. Immediately following the relaxation of contraction, the fall of PCr was not sufficient to account for the Pi released. The level of PCr remained unaltered, or decreased further, in the early post-contractile period. The amount of Pi that appeared without PCr splitting was similar to the amount of myosin sites (0.3 mmol kg-1 muscle) irrespective of the duration of contraction. The post-contractile utilization of PCr was strongly temperature dependent. It was prolonged at 1 degree C, whereas it almost disappeared at 10 degrees C. When the filament overlap was much reduced by stretching the muscles the phenomenon became undetectable. All these results have indicated that the post-contractile utilization of PCr is closely associated with the actin-myosin ATPase cycle.

Animals↗
PubMed1989

Influence of vibration on endurance of maximal isometric contraction.

In order to investigate how vibration affects endurance during muscular contraction, knee-joint extension efforts were performed with and without superimposed vibrations. Fourteen healthy non-smoking 20-year-old males performed maximal isometric and sustained knee-joint extension efforts (angle 90 degrees) in sitting posture three times with each leg, with or without vibration. The tests were done once with each leg in a randomly chosen order. The frequency of the vibration was 20 Hz and the acceleration 20 m/s2 RMS, applied in a horizontal sagittal direction to the ankle. The endurance was defined as the time in seconds that it took for the exerted force to decrease by 10% of the initial value. The endurance time averages 22.5 s without vibration and 15.8 s with vibration. The vibratory stress reduced endurance by 6.7 +/- 1.84 s (mean +/- SEM) (P less than 0.005). The difference in maximal force recorded initially was 34 +/- 1.9 N (P less than 0.1). Our conclusion is that vibration may decrease the endurance of maximal och sustained isometric muscular contraction.

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↗
PubMed1986

Isometric contractions in the patellofemoral pain syndrome. An electromyographic study.

The role played by the vastus lateralis and vastus medialis obliquus in their alignment of the patella during maximal isometric quadriceps contractions was studied by electromyography in 28 patients with unilateral symptoms of the patellofemoral pain syndrome. Eleven knees were classified as subluxation of the patella and 17 as idiopathic chondromalacia patellae. The muscular activity pattern was the same in the two groups but decreased as compared with the contralateral nonsymptomatic knees. However, none of the groups revealed differences in the activity of vastus lateralis and vastus medialis obliquus suggesting muscular imbalance.

Adolescent↗
PubMed1997

Discharge of human muscle spindle afferents innervating ankle dorsiflexors during target isometric contractions.

1. There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the 'acceleration threshold' and the factors contributing to variability of 'acceleration threshold'. 2. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed 'ramp-and-hold' template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required 'ramp' rate and 'hold' force was adjusted between trials (range, 0.5-5% MVCs-1 and 0.5-20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any 'after-effects' on spindle afferent responses in subsequent contractions. 3. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the 'acceleration threshold'. Of twenty-six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01-11.9% MVC). 4. Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. 5. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after-effects are controlled, much of the residual variability in 'acceleration threshold' for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

Acceleration↗
PubMed1993

Birefringence changes associated with isometric contraction and rapid shortening steps in frog skeletal muscle fibres.

1. Muscle birefringence, the difference between the refractive indices of light polarized parallel and perpendicular to the muscle fibre axis, was measured at 3 degrees C in intact single fibres isolated from frog muscle. Resting birefringence was 2.20 +/- 0.02 x 10(-3) (mean +/- S.E.M., n = 44) at sarcomere length 2.4-2.7 microns and 2.35 +/- 0.03 x 10(-3) (n = 19) at 3.5-3.8 microns. 2. Birefringence decreased during isometric twitch or tetanic contractions. The peak change in a twitch at sarcomere length 2.6 microns, determined by two independent methods, was 0.150 +/- 0.017 x 10(-3) (mean +/- S.E.M., n = 6). The corresponding value after 0.4 s of tetanic stimulation was 0.167 +/- 0.012 x 10(-3) (n = 6). 3. The birefringence change had a shorter latency than tension and reached its half-maximum value earlier than tension. The difference in time to half-maximum in tetani was 11.5 +/- 1.3 ms (mean +/- S.E.M., n = 6) at 3 degrees C. After stimulation birefringence recovered to its pre-stimulus baseline more slowly than tension. 4. The birefringence decrease after 0.4 s of tetanic stimulation was linearly related to the expected degree of overlap between actin and myosin filaments in the sarcomere length range 2.6-3.6 microns. The amplitude of the birefringence decrease at full filament overlap (sarcomere length 2.2 microns) was estimated to be 0.235 +/- 0.015 x 10(-3). 5. Birefringence changes associated with shortening steps of 0.9% fibre length at sarcomere length 2.6 microns exhibited four phases corresponding to those of the tension transient. There was no consistent birefringence change during the length step itself. During the rapid tension recovery birefringence increased by 0.014 +/- 0.001 x 10(-3) (n = 3), measured from the end of the length step to 2 ms later. Birefringence continued to increase as tension recovery slowed, reaching a peak about 10 ms after the step, then recovered with a rate similar to that of the final tension recovery. 6. These birefringence changes are likely to be caused by axial rotation of the head domain of the myosin cross-bridge. During isometric contraction heads bind to actin with their long axes more perpendicular to the fibre axis than in resting muscle, although there is likely to be a wide range of head orientations during contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1996

[Recruitment of the bi-articular gastrocnemius muscles during maximal isometric contractions in humans].

The main purpose of this study was to investigate the maximal isometric plantarflexion moment as a function of the triceps surae's length. As the gastrocnemius are bi-articular actuators, their respective lengths were modified by variations of the ankle and knee joints angles. The mechanical results agreed ones previously published in the literature. The maximal and minimal isometric moments were attained at maximal lengthening and shortening of the gastrocnemius respectively. The gastrocnemius electromyographic (EMG) activities showed paradoxal results. Their EMG activities decreased with their lengthening when this was induced by ankle joint angle variations. Inversely, their EMG activities increased with their lengthening when this was induced by knee joint angle variations. These results allowed the authors to hypothesize the existence of specific recruitment of neuro-muscular compartments of the gastrocnemius with knee extension.

Ankle Joint↗
PubMedDate not supplied

Effect of isometric contraction on threshold somatosensory evoked potentials.

We previously described the augmentation of sensory nerve action potential amplitudes after near and remote isometric muscle contraction. In this study, we wished to determine if the sensory cortex was involved in this process. In this prospective, intrinsically controlled study, we studied threshold somatosensory evoked potentials in 12 normal subjects with stimulation of the median nerve at 5.1 Hz. The subjects were tested during the following conditions: baseline, 25%, and 75% maximum isometric abductor digiti minimi contraction for 4 min. Each of these conditions was recorded before, during, and 4 min and 8 min after contraction. Results showed that at 25% contraction, there was a significant temporal increase in N9 amplitude (2.1-2.6 microV; P = 0.05, analysis of variance, repeated measures) and a decrease in N20 amplitude with 75% contraction (1.9-1.6 microV; P = 0.03, analysis of variance, repeated measure). No significant changes were noted in the spinal cord or brainstem recordings. In conclusion, it appears that augmentation of the brachial plexus peripheral nervous system recording occurs concurrently with central inhibitory gating. The possibility of peripheral nervous system adaptability will be discussed.

Adult↗
PubMed1999

Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle.

The repeatability of initial value and rate of change of mean spectral frequency (MNF), average rectified values (ARV) and muscle fiber conduction velocity (CV) was investigated in the dominant biceps brachii of ten normal subjects during sustained isometric voluntary contractions. Four levels of contraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary contraction level (MVC). Each contraction was repeated three times in each of three different days for a total of nine contractions/level/subject and 90 contractions per level across the ten subjects. Repeatability was investigated using the Intraclass Correlation Coefficient (ICC) and the standard error of the mean (SEM) of the estimates for each subject. Contrary to observations in other muscles, CV estimates appeared to be very repeatable both within and between subjects. CV showed a small but significant increase when contraction force increased from 10% to 50% MVC but no change for further increase of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement of MNF and CV increased with the level of contraction. The normalized decrement (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A final observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC.

Adult↗