Low-intensity Isometric Contraction Research Articles

Myoelectric fatigue and motor-unit firing patterns during sinusoidal vibration superimposed on low-intensity isometric contraction.

Vibration exercise (VE) has shown promising results for improving muscle strength and power performance when superimposed on high-level muscle contraction. However, lowlevel contraction may be more preferable in many rehabilitation programs due to the weakness of the patients. Unfortunately, the effects and underlying physiological mechanisms of VE superimposed on low-level contraction are unclear. This study aims to investigate the fatiguing effects and motor unit (MU) firing patterns during VE with low-level muscle contraction. Twenty-one healthy participants performed 60-s isometric contraction of the upper limb under a baseline force at 30% maximum voluntary contraction and superimposed vibration with an amplitude of 50% baseline and different frequencies at 0 Hz (control), 15, 25, 35, and 45 Hz. High-density surface electromyography (EMG) was recorded on the biceps brachii. The decay in muscle fiber conduction velocity, calculated in 3-s sliding windows, was employed as an indicator of myoelectric fatigue. MU firing patterns were obtained by decomposing the high-density EMG into MU spike trains. VE, particularly at 25 Hz, produces increased myoelectric fatigue as compared to the control condition. Besides, synchronized MU discharges are observed at the vibration frequency for 15- and 25-Hz VE and the sub-harmonics for 35- and 45-Hz VE. Furthermore, VE-induced increase in MU synchronization (as compared to control) seems to decrease with myoelectric fatigue. Significance: Our findings suggest that VE may be a suitable modality for rehabilitation programs, providing useful insights for subscribing appropriate VE training protocols.

Low-load blood flow restriction reduces time-to-minimum single motor unit discharge rate.

Resistance training with low loads in combination with blood flow restriction (BFR) facilitates increases in muscle size and strength comparable with high-intensity exercise. We investigated the effects of BFR on single motor unit discharge behavior throughout a sustained low-intensity isometric contraction. Ten healthy individuals attended two experimental sessions: one with, the other without, BFR. Motor unit discharge rates from the tibialis anterior (TA) were recorded with intramuscular fine-wire electrodes throughout the duration of a sustained fatigue task. Three 5-s dorsiflexion maximal voluntary contractions (MVC) were performed before and after the fatigue task. Each participant held a target force of 20% MVC until endurance limit. A significant decrease in motor unit discharge rate was observed in both the non-BFR condition (from 13.13 ± 0.87Hz to 11.95 ± 0.43Hz, P = 0.03) and the BFR condition (from 12.95 ± 0.71Hz to 10.9 ± 0.75Hz, P = 0.03). BFR resulted in significantly shorter endurance time and time-to-minimum discharge rates and greater end-stage motor unit variability. Thus, low-load BFR causes an immediate steep decline in motor unit discharge rate that is greater than during contractions performed without BFR. This shortened neuromuscular response of time-to-minimum discharge rate likely contributes to the rapid rate of neuromuscular fatigue observed during BFR.

Low Intensity Isometric Contractions With Or Without Blood Flow Restriction Do Not Lower Blood Pressure

Low Intensity Isometric Contractions With Or Without Blood Flow Restriction Do Not Lower Blood Pressure

Myosin Heavy Chain Expression Influences Motor Unit Behavior During High- But Not Low-Intensity Isometric Contractions

Myosin Heavy Chain Expression Influences Motor Unit Behavior During High- But Not Low-Intensity Isometric Contractions

The effect of blood flow on tibialis anterior motor unit firing rates during sustained low-intensity isometric contractions.

Low-intensity contractions with blood flow occlusion (BFO) result in neuromuscular adaptations comparable with high-intensity (>70% maximal voluntary contraction, MVC) exercise. Because BFO exercise can only be applied to limb muscles, it is of interest to explore whether muscles proximal to the occlusion site are affected. Therefore, the purpose of this study was to assess neural activation of the tibialis anterior (TA) when flow is occluded proximal and distal to the active muscle. Five males completed three protocols to observe the effect of BFO on motor unit firing rates (MUFR) of the TA at a fatiguing contraction intensity of ∼15% MVC. Two occlusion protocols, one proximal (BFOprox) to and one distal (BFOdis) to the TA, were compared with a control (free-flow) protocol time-matched to BFOdis. MVC was significantly reduced following the BFOprox (∼41%; P< 0.001) and BFOdis (∼27%, P< 0.001), but not following the control protocol (∼15%; P= 0.13). Surface electromyography (EMG) during BFOdis and BFOprox increased ∼14% and ∼28%, respectively, but was not different among protocols. MUFRs for BFOdis and BFOprox were significantly reduced (by ∼33% and ∼23%, respectively; P< 0.01) at task failure. Results indicate that although BFOprox results in the largest reductions of MUFRs, BFOdis shows greater impairments compared with the free-flow control condition. Novelty Effects on motor unit firing rates of proximal versus distal blood flow occlusion were compared during low-intensity fatiguing task. Proximal occlusion results in greatest fatigue and reduction in motor unit rates, but distal occlusion elicits more fatigue and rate reduction than a control task.

Neurophysiological correlates of force control improvement induced by sinusoidal vibrotactile stimulation

Objective. An optimal level of vibrotactile stimulation has been shown to improve sensorimotor control in healthy and diseased individuals. However, the underlying neurophysiological mechanisms behind the enhanced motor performance caused by vibrotactile stimulation are yet to be fully understood. Therefore, here we aim to evaluate the effect of a cutaneous vibration on the firing behavior of motor units in a condition of improved force steadiness. Approach. Participants performed a visuomotor task, which consisted of low-intensity isometric contractions of the first dorsal interosseous (FDI) muscle, while sinusoidal (175 Hz) vibrotactile stimuli with different intensities were applied to the index finger. High-density surface electromyogram was recorded from the FDI muscle, and a decomposition algorithm was used to extract the motor unit spike trains. Additionally, computer simulations were performed using a multiscale neuromuscular model to provide a potential explanation for the experimental findings. Main results. Experimental outcomes showed that an optimal level of vibration significantly improved force steadiness (estimated as the coefficient of variation of force). The decreased force variability was accompanied by a reduction in the variability of the smoothed cumulative spike train (as an estimation of the neural drive to the muscle), and the proportion of common inputs to the FDI motor nucleus. However, the interspike interval variability did not change significantly with the vibration. A mathematical approach, together with computer simulation results suggested that vibrotactile stimulation would reduce the variance of the common synaptic input to the motor neuron pool, thereby decreasing the low frequency fluctuations of the neural drive to the muscle and force steadiness. Significance. Our results demonstrate that the decreased variability in common input accounts for the enhancement in force control induced by vibrotactile stimulation.

Investigation of The Neural Drive During Vibration Exercise by High-density Surface-electromyography.

Mechanical vibration applied directly to the muscle belly or tendon has been reported to elicit a specific reflex loop named tonic vibration reflex (TVR), which involves motor unit (MU) activation synchronized and un-synchronized within the vibration cycle. Indirect application of vibration to the muscle by vibration exercise (VE) has also been suggested to evoke TVR, as evidenced by the spectral peaks observed at the vibration frequency in the surface electromyography (sEMG). However, other studies interpreted these spectral peaks as the result of motion artifacts (MAs). The aim of the present study is, therefore, to investigate MU activation patterns during VE in order to clarify the nature of those spectral peaks. To this end, low-intensity isometric contractions were executed with and without VE, and high-density sEMG measurements were performed during the contraction tasks. MU action potential (MUAP) trains were extracted by decomposing the recorded high-density sEMG signals. The spectra of the MUAP trains were then calculated and compared between vibration and no-vibration conditions. Clear MU synchronization was observed during VE, confirming the spectral peaks at the vibration frequency to be mainly due to the reflex loop rather than MAs.

Effects of Carrying Out a Low-Intensity Isometric Contraction of the Proximal Thigh Muscles on the Electromyographic Activity of the Leg Muscles when Simulating an Ankle Sprain

Background: The patterns of activation of the leg musculature can be altered after suffering from a sprained ankle, which can contribute negatively to the biomechanics of the leg. Ankle sprains can generate changes in muscle electromyographic activity in both the thigh and ankle regions, for which physiotherapy is the most common treatment. This study aimed to determine the effect of a low-intensity isometric contraction of the proximal thigh muscles (gluteus maximus (Gmax), gluteus medius (Gmed), tensor fasciae latae (TFL) on the electromyographic response of the tibialis anterior (TA), peroneus longus (PL), lateral gastrocnemius (LG), and soleus muscles (SL) in a sudden ankle supination. Methods: Fifteen healthy volunteers were subjected to 2 series of three trials on a sudden ankle supination platform (50°). This involved a multiple analysis of variance per trial and muscle for the dependent variables and an analysis of variance in the measures repeated under study for the different time windows evaluated, according to the intended factors and muscle studied. Results: No significant differences were observed in the variables concerning the time of activation, the moment of maximum EMG registration or registered peak value. Conclusion: Inducing a low-intensity isometric contraction of the abductor leg muscles did not generate changes in the muscular values studied.

Sex differences in variances of multi-channel surface electromyography distribution of the vastus lateralis muscle during isometric knee extension in young adults.

The aim of the present study was to compare spatial electromyographic potential distribution during force production between healthy young female and male using multi-channel surface electromyography (multi-SEMG). Thirty healthy subjects (15 females) performed sustained isometric knee extension at 10% maximal voluntary contraction (MVC) task for 120s. Multi-SEMG signals from the vastus lateralis muscle were detected and the modified entropy, coefficient of variation (CV), and correlation coefficient determined. The modified entropy and CV showed significant interaction and difference between females and males at all time points during the 10% MVC task. The correlation coefficient in females was significantly lower at 90 and 120s than that of males. The multi-SEMG potential distribution pattern in females showed more varied motor unit recruitment during sustained low-intensity isometric contraction than that of males. Variations in motor unit recruitment may result from recruitment and/or de-recruitment of motor units.

Effects of electrocardiography contamination and comparison of ECG removal methods on upper trapezius electromyography recordings

Electromyography (EMG) recordings from the trapezius are often contaminated by the electrocardiography (ECG) signal, making it difficult to distinguish low-level muscle activity from muscular rest. This study investigates the influence of ECG contamination on EMG amplitude and frequency estimations in the upper trapezius during muscular rest and low-level contractions. A new method of ECG contamination removal, filtered template subtraction (FTS), is described and compared to 30Hz high-pass filter (HPF) and averaged template subtraction (ATS) methods. FTS creates a unique template of each ECG artifact using a low-pass filtered copy of the contaminated signal, which is subtracted from contaminated periods in the original signal. ECG contamination results in an over-estimation of EMG amplitude during rest in the upper trapezius, with negligible effects on amplitude and frequency estimations during low-intensity isometric contractions. FTS and HPF successfully removed ECG contamination from periods of muscular rest, yet introduced errors during muscle contraction. Conversely, ATS failed to fully remove ECG contamination during muscular rest, yet did not introduce errors during muscle contraction. The relative advantages and disadvantages of different ECG contamination removal methods should be considered in the context of the specific motor tasks that require analysis.

Dynamic training of the lumbar musculature to prevent recurrence of acute low back pain: a randomized controlled trial using a daily pain recall for 1 year

Purpose: The purpose of this experiment was to quantify lumbar muscle endurance training for individuals with a recent episode of acute low back pain (LBP) (≥ moderate pain for ≥ 2 days) and to observe whether the training would reduce the rate and severity of recurrent LBP episodes. Method: Twenty-six participants who were pain-free at the time of the study were randomly divided into a high intensity back endurance (HIBE)-trained or a low intensity abdominal (LOAB)-trained (control) group. The HIBE-trained group performed preloaded maximum isokinetic exertions of the back extensors (five sets of 10 repetitions, 3 days a week for 4 weeks, totaling 12 sessions). The LOAB-trained group performed low intensity isometric contractions on their abdominals that had minimal effect on their back musculature. The two groups reported daily pain logs on a weekly basis on an interactive voice response telephone system for 1 year. Results: The HIBE-trained group experienced more pain days (p = 0.038) in the minor and moderate categories and more episodes of acute LBP than the LOAB-trained group. However, there was a trend of less pain in the severe, intense and excruciating categories in the HIBE-trained group. Conclusions: The results of this experiment did not provide evidence that short-term intense training of the low back musculature provides protection against future episodes of LBP.Implications for RehabilitationShort-term high intensity back endurance (HIBE) training of the back musculature did not provide protection against future episodes of acute low back pain compared to the low intensity abdominal (LOAB) – trained group.HIBE training may have sensitized the subjects to report significantly more minor and moderate pain and less pain-free days compared to the LOAB-trained group.There was a strong trend that the HIBE-trained group experienced less pain in the severe, intense and excruciating categories compared to the LOAB-trained group.Future studies in rehabilitation must require daily recall of pain and quick reporting in order to capture the subtle effects training can elicit from pain reporting.

Resposta da frequência cardíaca durante o exercício isométrico de pacientes submetidos à reabilitação cardíaca fase III

the magnitude of cardiovascular responses is dependent on the static and dynamic components as well as the duration and intensity of the contraction performed. to evaluate the heart rate responses to different percentages of isometric contractions in 12 patients (63 ± 11.6 years) with coronary artery disease and/or risk factors for coronary artery disease that were participating in a phase III cardiac rehabilitation program. heart rate variation (ΔHR) was evaluated during maximum (MVC, five and ten seconds in duration) and submaximal (SMVC, 30 and 60% of MVC-5, until muscle exhaustion) voluntary contraction, using a handgrip dynamometer. Additionally, the representative index of cardiac vagal modulation (RMSSD index) was calculated at rest (pre-contraction), at the final 30 seconds of SMVC and during recovery (post-contraction). ΔHR showed higher values in MVC-10 versus MVC-5 (17 ± 5.5 vs 12 ± 4.2 bpm, p<0.05) and the SMVC-60 vs SMVC-30 (19 ± 5.8 vs 15 ± 5.1 bpm, p<0.05). However, results for CVM-10 showed similar ΔHR compared to results for CVSM (p> 0.05). RICVM at rest decreased (p<0.05) during SMVC-30 (30% = 27.9 ± 17.1 vs 12.9 ± 8.5 ms) and SMVC-60 (60% =25.8 ± 18.2 vs 9.96 ± 4.2 ms), but returned to the baseline values when the contraction was interrupted. in patients with coronary artery disease and/or risk factors for coronary heart disease, low intensity isometric contraction, maintained over long periods of time, presents the same effect on the responses of HR, compared to a high intensity or maximal isometric contraction of briefly duration.

Asymmetry of Force Fluctuation During Low Intensity Isometric Contraction In Leg Muscle

The purpose of this study was to investigate the asymmetry of force fluctuation in the leg muscles during isometric knee extension and flexion. Twenty healthy males (21±2 years) performed the maximal voluntary isometric contraction (MVC) in knee extensor and flexor. On the basis of MVC measurement, the subjects performed sustained isometric knee extension and flexion for 15s at levels corresponding to 10%, 20% and 30% MVC. The main findings of this study were: (1) a greater force fluctuation was found in the stronger MVC limb than in the weaker MVC limb at 30% MVC; (2) no difference was found in the force fluctuation between the stronger and weaker MVC limbs at 10% and 20% MVC; and (3) significant positive correlations were found between the target force values and the force fluctuation at each contraction intensity. These results suggest that: (1) asymmetry of force fluctuation increases with load, (2) asymmetry of the force fluctuation is observed at more than 30% MVC intensity; and (3) if the contraction intensity is same relatively loads (% MVC), force fluctuation is increase with absolute load (target force value). Force fluctuation influence the functional ability of an individual in controlling finger or limb movements in daily life. Further, asymmetry of force fluctuation might influence for more than 30% MVC of daily activities.

Laterality and Influence of Contraction Time during Low-intensity Isometric Contraction in the Case of Upper Limb Physiological Tremor

Laterality and Influence of Contraction Time during Low-intensity Isometric Contraction in the Case of Upper Limb Physiological Tremor

Dose Response of Isometric Contractions on Pain Perception in Healthy Adults

The purpose of the study was to compare pain ratings and thresholds in men and women before and after isometric contractions of varying intensity and duration performed with the elbow flexor muscles. Pain perception was assessed, using a pressure device applied to the contralateral finger, before and after the following isometric contractions: 1) three brief maximal voluntary contractions (MVC), 2) 25% MVC sustained until task failure, 3) 25% MVC sustained for 2 min, and 4) 80% MVC sustained until task failure. High-intensity and long-duration, low-intensity isometric contractions produced an analgesic response. The greatest change in pain threshold and pain ratings, when pressure was applied to the contralateral finger, was after the long-duration, low-intensity contraction sustained until failure. Sex differences were present with women reporting 1) lower pain thresholds and higher pain ratings during quiet rest and 2) higher pain ratings before and after isometric contractions. These results suggest that activation of high-threshold motor units is involved in exercise-induced analgesia.

(854): Pain decreases after long duration and low intensity isometric contractions in men and women

(854): Pain decreases after long duration and low intensity isometric contractions in men and women

Changes in the Reflex Excitability During and after a Sustained, Low-Intensity Muscle Contraction

The purpose of the study was to evaluate the soleus H-reflex amplitude during and after a low-intensity isometric contraction. Twelve healthy, untrained subjects performed a 10-minute isometric plantar flexion at 20% of their maximum voluntary contraction torque output. The electromyogram, H-reflex, and maximum M-wave (Mmax) of the soleus muscle was recorded during and 10 minutes after the end of the contraction. The results indicated that the H-reflex increased significantly (mean +/- SEM: 44.7 +/- 16.6%, p < 0.05), but when the fatigue protocol was over, the H-reflex was depressed for the first 3 minutes, relative to the H-reflex that was recorded before fatigue, when the muscle was relaxed. The Mmax did not change significantly during the whole experiment. Furthermore, the stimulation frequency (0.1 vs. 0.3 Hz) did not have any significant effect on the H-reflex modulation. The results of the current study suggest that the reflex excitability is increased as fatigue develops, whereas this increase turns to depression for the first minutes of the recovery phase. The functional significance of these changes and the neural mechanisms which might be responsible are discussed.

Physiological response in the forearm during and after isometric intermittent handgrip : Europ J Appl Physiol and Occupat Physiol, 1989, 60.6, 457–466, 33 refs

The aim of the present paper was to study the development of fatigue during isometric intermittent handgrip exercise. Using a handgrip dynamometer, four combinations of contraction-relaxation periods were studied (10+10, 10+5, 10+2s and continuous contraction) at three contraction intensities (10, 25 and 40% maximum voluntary contraction, MVC). Local blood flow (BF) in the forearm (venous occlusion plethysmography) was followed before, during and after the exercise period. Electromyography (EMG) (frequency analysis) and the perceived effort and pain were recorded during the exercise period. Forearm BF is insufficient even at isometric contractions of low intensity (10% MVC). The results indicate that vasodilating metabolites play an active role for BF in low-intensity isometric contractions. It is shown that maximal BF in the forearm during relaxation periods (25–30 ml min−1 · 100 ml−1) is already reached at 25% MVC. Only intermittent exercise at 10% MVC and (10+5s) and (10+10s) at 25% MVC was considered acceptable with regard to local fatigue, which was defined as a switch of local BF to the post-exercise period, a decrease in the number of zero-crossings (EMG) and marked increases in subjective ratings.

Physiological response in the forearm during and after isometric intermittent handgrip

The aim of the present paper was to study the development of fatigue during isometric intermittent handgrip exercise. Using a handgrip dynamometer, four combinations of contraction-relaxation periods were studied (10 + 10, 10 + 5, 10 + 2 s and continuous contraction) at three contraction intensities (10, 25 and 40% maximum voluntary contraction, MVC). Local blood flow (BF) in the forearm (venous occlusion plethysmography) was followed before, during and after the exercise period. Electromyography (EMG) (frequency analysis) and the perceived effort and pain were recorded during the exercise period. Forearm BF is insufficient even at isometric contractions of low intensity (10% MVC). The results indicate that vasodilating metabolites play an active role for BF in low-intensity isometric contractions. It is shown that maximal BF in the forearm during relaxation periods (25-30 ml.min-1.100 ml-1) is already reached at 25% MVC. Only intermittent exercise at 10% MVC and (10 + 5 s) and (10 + 10 s) at 25% MVC was considered acceptable with regard to local fatigue, which was defined as a switch of local BF to the post-exercise period, a decrease in the number of zero-crossings (EMG) and marked increases in subjective ratings.