Low-level Muscle Contractions 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.

Lifelong Strength Versus Endurance Training: Differential Adaptations In Efferent Neural Drive And Spinal α-motoneuron Excitability

PURPOSE: Neural factors play a critical role in the age-related decline in maximal strength and rate of force development (RFD). However, it is uncertain how the neural attenuation may be mitigated by strength or endurance training, respectively. METHODS: In this study we applied evoked spinal motoneuron recordings to examine efferent neural drive (V/Msup during maximal voluntary contraction; MVC) and α-motoneuron excitability (Hmax/Mmax at 10% MVC) in a total of 69 older (>65 yrs) and younger (<35 yrs) strength athletes (n = 21), endurance athletes (n = 17) and control subjects (n = 31). RESULTS: Leg press strength (one repetition maximum: 1RM) but not maximal oxygen uptake (V̇O2max), were positively associated with V/Msup (r = 0.60, p < 0.001). Leg press 1RM and RFD were higher (all p < 0.05) in older strength athletes (1RM: 164 ± 18 kg; RFD (0-200 ms); 4262 ± 1022 N·s-1) than endurance athletes (1RM: 132 ± 25 kg; RFD: 3080 ± 669 N·s-1) and older controls (1RM: 109 ± 15 kg; RFD: 2202 ± 915 N·s-1), but lower compared to training-matched young groups (p < 0.05). V̇O2max was positively associated with Hmax/Mmax (r = 0.47, p < 0.001) while no correlation was observed for 1RM. V̇O2max was higher (all p < 0.05) in older endurance athletes (47.6 ± 7.6 ml·kg-1·min-1) than in older strength athletes (30.9 ± 6.0 ml·kg-1·min-1) and older controls (34.2 ± 4.4 ml·kg-1·min-1), but lower than all young groups (p < 0.001). CONCLUSIONS: Across the life span the plasticity in skeletal muscle strength and aerobic capacity, respectively, seems to be associated with differential neural factors. Specifically, chronic strength training seems to result in a higher efferent neural drive during maximal muscle contractions (elevated V-wave responses), largely dependent on corticospinal factors, whereas chronic endurance training appears to be related to a high α-motoneuron excitability, at least during low-level muscle contractions. Since high efferent neural drive is a key component for strong muscle contractions with age, strength training should be emphasized for the ability to carry out force-dependent tasks at older age.

A Compliant Ionic Adhesive Electrode with Ultralow Bioelectronic Impedance

Simultaneous implementation of high signal-to-noise ratio (SNR) but low crosstalk is of great importance for weak surface electromyography (sEMG) signals when precisely driving a prosthesis to perform sophisticated activities. However, due to gaps with the curved skin during muscle contraction, many electrodes have poor compliance with skin and suffer from high bioelectrical impedance. This causes serious noise and error in the signals, especially the signals from low-level muscle contractions. Here, the design of a compliant electrode based on an adhesive hydrogel, alginate-polyacrylamide (Alg-PAAm) is reported, which eliminates those large gaps through the strong electrostatic interaction and abundant hydrogen bond with the skin. The obtained compliant electrode, having an ultralow bioelectrical impedance of ≈20 kΩ, can monitor even 2.1% maximal voluntary contraction (MVC) of muscle. Furthermore, benefiting from the high SNR of >5:1 at low-level MVC, the crosstalk from irrelevant muscle is minimized through reducing the electrode size. Finally, a prosthesis is successfully demonstrated to precisely grasp a needle based on a 9 mm2 Alg-PAAm compliant electrode. The strategy to design such compliant electrodes provides the potential for improving the quality of dynamically weak sEMG signals to precisely control prosthesis in performing purposefully dexterous activity.

Torque Estimation Using Neural Drive for a Concentric Contraction.

The scope and relevance of wearable robotics spans across a number of research fields with a variety of applications. A challenge across these research areas is improving user-interface control. One established approach is using neural control interfaces derived from surface electromyography (sEMG). Although there has been some success with sEMG controlled prosthetics, the coarse nature of traditional sEMG processing has limited the development of fully functional prosthetics and wearable robotics. To solve this problem, blind source separation (BSS) techniques have been implemented to extract the user's movement intent from high-density sEMG (HDsEMG) measurements; however, current methods have only been well validated during static, low-level muscle contractions, and it is unclear how they will perform during movement. In this paper we present a neural drive based method for predicting output torque during a constant force, concentric contraction. This was achieved by modifying an existing HDsEMG decomposition algorithm to decompose 1 sec. overlapping windows. The neural drive profile was computed using both rate coding and kernel smoothing. Neither rate coding nor kernel smoothing performed as well as HDsEMG amplitude estimation, indicating that there are still significant limitations in adapting current methods to decompose dynamic contractions, and that sEMG amplitude estimation methods still remain highly reliable estimators.

Insertion and Presence of Fine-Wire Intramuscular Electrodes to the Lumbar Paraspinal Muscles Do Not Affect Muscle Performance and Activation During High-Exertion Spinal Extension Activities

BackgroundLow back pain (LBP) is commonly associated with paraspinal muscle dysfunctions. A method to study deep lumbar paraspinal (ie, multifidus) muscle function and neuromuscular activation pattern is intramuscular electromyography (EMG). Previous studies have shown that the procedure does not significantly impact muscle function during activities involving low-level muscle contractions. However, it is currently unknown how muscular function and activation are affected during high-exertion contractions. ObjectiveTo examine the effects of insertion and presence of fine-wire EMG electrodes in the lumbar multifidus on muscle strength, endurance, and activation profiles during high-exertion spinal extension muscle contractions. DesignSingle-blinded, repeated measures intervention trial. SettingUniversity clinical research laboratory ParticipantsTwenty individuals between the ages of 18-40 free of recent and current back pain. MethodsMuscle performance was assessed during 3 conditions (with [WI] and without [WO] presence of intramuscular electrodes, and insertion followed by removal [IO]). Isometric spinal extension strength was assessed with a motorized dynamometer. Muscle endurance was assessed using the Sorensen test with neuromuscular activation profiles analyzed during the endurance test. Main Outcome MeasurementsSpinal extensor muscle strength, endurance, and activation. ResultsOur data showed no significant difference in isometric strength (P = .20) between the 3 conditions. A significant difference in muscle endurance was found (P = .03). Post hoc analysis showed that the muscle endurance in the IO condition was significantly higher than the WO condition (161.3 ± 58.3 versus 142.1 ± 48.2 seconds, P = .04), likely due to a learning effect. All 3 conditions elicited minimal pain (range 0-4/10) and comparable muscle activation profiles. ConclusionOur findings suggested the sonographically guided insertion and presence of fine-wire intramuscular EMG electrodes in the lumbar multifidus muscles had no significant impact on spinal extension muscle function. This study provides evidence that implementing intramuscular EMG does not affect muscle performance during high-exertion contractions in individuals with no current back pain. Level of EvidenceII

Comparison of repeated transcranial stimulation and transcranial direct-current stimulation on primary motor cortex excitability and inhibition: A pilot study

Repeated transcranial magnetic stimulation (rTMS) is a well-known clinical neuromodulation technique, but transcranial direct-current stimulation (tDCS) is rapidly growing interest for neurorehabilitation applications. Both methods (contralesional hemisphere inhibitory low-frequency: LF-rTMS or lesional hemisphere excitatory anodal: a-tDCS) have been employed to modify the interhemispheric imbalance following stroke. The aim of this pilot study was to compare aHD-tDCS (anodal high-definition tDCS) of the left M1 (2 mA, 20 min) and LF-rTMS of the right M1 (1 Hz, 20 min) to enhance excitability and reduce inhibition of the left primary motor cortex (M1) in five healthy subjects. Single-pulse TMS was used to elicit resting and active (low level muscle contraction, 5% of maximal electromyographic signal) motor-evoked potentials (MEPs) and cortical silent periods (CSPs) from the right and left extensor carpi radialis muscles at Baseline, immediately and 20 min (Post-Stim-20) after the end of each stimulation protocol. LF-rTMS or aHD-tDCS significantly increased right M1 resting and active MEP amplitude at Post-Stim-20 without any CSP modulation and with no difference between methods. In conclusion, this pilot study reported unexpected M1 excitability changes, which most likely stems from variability, which is a major concern in the field to consider.

52. Lower limbs motor evoked potentials intra-session reliability: A coil comparison study

Our aim is to investigate intra-session reliability of lower limbs (LL) motor evoked potentials (MEPs) amplitudes elicited by Transcranial Magnetic Stimulation (TMS) and measured from Tibialis-Anterior (TA) and Abductor-Hallucis (AH), comparing double cone and circular coil. TMS was used in healthy subjects (HS) (n = 7) to elicit MEPs in TA and AH of both LL, at rest and during low level muscle contraction. Double cone and circular coil were used. Two sets of eight responses were recorded for each coil in a random order, using a stimulation intensity of 100% of the maximum stimulator output. Intra-session reliability was calculated using Intraclass correlation coefficients (ICC). Double cone and circular coil showed excellent intra-session reliability in TA and AH of both LL, at rest and during low level muscle contraction (ICC ⩾ 0.75 in all experimental conditions). Interestingly, circular coil failed to elicit measurable MEPs at rest in non-dominant limb in 3 out of 7 HS. Double cone coil is an efficient alternative to circular coil in order to measure LL MEPs amplitudes in HS. Since circular coil did not elicit MEPs in all HS, double cone coil could be more useful in neurological patients to better detect MEPs in cortico-spinal tract pathological conditions.

Effects of muscle fatigue on the usability of a myoelectric human-computer interface

Electromyography-based human-computer interface development is an active field of research. However, knowledge on the effects of muscle fatigue for specific devices is limited. We have developed a novel myoelectric human-computer interface in which subjects continuously navigate a cursor to targets by manipulating a single surface electromyography (sEMG) signal. Two-dimensional control is achieved through simultaneous adjustments of power in two frequency bands through a series of dynamic low-level muscle contractions. Here, we investigate the potential effects of muscle fatigue during the use of our interface. In the first session, eight subjects completed 300 cursor-to-target trials without breaks; four using a wrist muscle and four using a head muscle. The wrist subjects returned for a second session in which a static fatiguing exercise took place at regular intervals in-between cursor-to-target trials. In the first session we observed no declines in performance as a function of use, even after the long period of use. In the second session, we observed clear changes in cursor trajectories, paired with a target-specific decrease in hit rates.

Evaluation of ultrasound Tissue Velocity Imaging: a phantom study of velocity estimation in skeletal muscle low-level contractions

BackgroundTissue Velocity Imaging (TVI) is an ultrasound based technique used for quantitative analysis of the cardiac function and has earlier been evaluated according to myocardial velocities. Recent years several studies have reported applying TVI in the analysis of skeletal muscles. Skeletal tissue velocities can be very low. In particular, when performing isometric contractions or contractions of low force level the velocities may be much lower compared to the myocardial tissue velocities.MethodsIn this study TVI was evaluated for estimation of tissue velocities below the typical myocardial velocities. An in-house phantom was used to see how different PRF-settings affected the accuracy of the velocity estimations.ResultsWith phantom peak velocity at 0.03 cm/s the error ranged from 31% up to 313% with the different PRF-settings in this study. For the peak velocities at 0.17 cm/s and 0.26 cm/s there was no difference in error with tested PFR settings, it is kept approximately around 20%.ConclusionsThe results from the present study showed that the PRF setting did not seem to affect the accuracy of the velocity estimation at tissue velocities above 0.17 cm/s. However at lower velocities (0.03 cm/s) the setting was crucial for the accuracy. The PRF should therefore preferable be reduced when the method is applied in low-level muscle contraction.

Etiology of myofascial trigger points.

Myofascial pain syndrome (MPS) is described as the sensory, motor, and autonomic symptoms caused by myofascial trigger points (TrPs). Knowing the potential causes of TrPs is important to prevent their development and recurrence, but also to inactivate and eliminate existing TrPs. There is general agreement that muscle overuse or direct trauma to the muscle can lead to the development of TrPs. Muscle overload is hypothesized to be the result of sustained or repetitive low-level muscle contractions, eccentric muscle contractions, and maximal or submaximal concentric muscle contractions. TrPs may develop during occupational, recreational, or sports activities when muscle use exceeds muscle capacity and normal recovery is disturbed.

Myofascial Trigger Points: An Evidence-Informed Review

This article provides a best evidence-informed review of the current scientific understanding of myofascial trigger points with regard to their etiology, pathophysiology, and clinical implications. Evidence-informed manual therapy integrates the best available scientific evidence with individual clinicians' judgments, expertise, and clinical decision-making. After abrief historical review, the clinical aspects of myofascial trigger points, the interrater reliability for identifying myofascial trigger points, and several characteristic features are discussed, including the taut band, local twitch response, and referred pain patterns. The etiology of myofascial trigger points is discussed with a detailed and comprehensive review of the most common mechanisms, including low-level muscle contractions, uneven intramuscular pressure distribution, direct trauma, unaccustomed eccentric contractions, eccentric contractions in unconditioned muscle, and maximal or sub-maximal concentric contractions. Many current scientific studies are included and provide support for considering myofascial trigger points in the clinical decision-making process. The article concludes with a summary of frequently encountered precipitating and perpetuating mechanical, nutritional, metabolic, and psychological factors relevant for physical therapy practice. Current scientific evidence strongly supports that awareness and working knowledge of muscle dysfunction and in particular myofascial trigger points should be incorporated into manual physical therapy practice consistent with the guidelines for clinical practice developed by the International Federation of Orthopaedic Manipulative Therapists. While there are still many unanswered questions in explaining the etiology of myofascial trigger points, this article provides manual therapists with an up-to-date evidence-informed review of the current scientific knowledge.

Continuous, intermitted and sporadic motor unit activity in the trapezius muscle during prolonged computer work

The Cinderella hypothesis postulates the continuous activity of specific motor units (MUs) during low-level muscle contraction. The MUs may become metabolically overloaded, with the subject developing muscle pain and strain. The hypothesis requires MUs that are active for a time long enough to actually damage muscle fibers. The aim of this study was to determine if there are continuously active MUs in the right trapezius muscle during normal computer work using a computer mouse. Fourteen healthy subjects executed an interactive computer-learning program (ErgoLight) for 30 min. Six-channel intramuscular EMG and two-channel surface EMG signals were recorded from two positions of the trapezius muscle. Decomposition was achieved with automated, multi-channel, long-term decomposition software (EMG-LODEC). In two out of the 14 subjects, three MUs were continuously active throughout the 30 min. Although the majority of the MUs were active during only part of the experimental session, an ordered on–off behavior (e.g. substitution) pattern was not observed. As long-lasting activity was verified in some subjects, the results support the Cinderella hypothesis. However, it cannot be concluded here how long the MUs could stay active. If continuous activity overloads low threshold MUs, the potential exists for selective fibre injuries in low threshold MUs of the trapezius muscle in subjects exposed to long-term computer work.

Motor-unit activity in the trapezius muscle during rest, while inputting data, and during fast finger tapping.

The Cinderella hypothesis postulates the continuous activity of specific motor units during low-level muscle contraction and contradicts the concept of motor-unit substitution. Constant trapezius muscle activity has been reported in typical visual-display-unit-related tasks. If it can be shown that constant muscle activity can be caused by the continuous firing of single motor units, this could explain the frequent complaints of muscular neck pain reported by computer users. The present study was undertaken to investigate motor-unit activity in the trapezius muscle during resting with closed eyes, while inputting three-digit numbers with auditory presentation at a rate of 0.5 Hz, and while tapping on a key with the right index finger at a rate of 5 Hz. Electrodes with four fine wires were inserted into the right upper trapezius muscle of six healthy subjects, and three-channel intramuscular electromyography was recorded. The decomposition programme MAPQuest, developed to analyse short-term one-channel signals, was complemented with MAPView, a programme that merges the short-term results of 10 s to a 3-min analysis. The results showed that activity in the trapezius muscle was induced in one subject while resting, in two subjects while inputting data, and in five subjects while finger tapping. Long-lasting single motor-unit firing was observed in two subjects while inputting data and in one subject while finger tapping. Whilst our findings may support the Cinderella hypothesis, the measurement periods are too short to confirm it fully, and for further discussion it is necessary to record and analyse for longer periods.