Mechanomyogram from the different heads of the quadriceps muscle during incremental knee extension

To investigate the time- and frequency-domain responses of mechanomyograms (MMGs) during the progressive fatigue induced by intermittent incremental contractions, a surface MMG was obtained from the three muscle heads of the quadriceps muscle in seven subjects while they performed isometric knee extensions lasting 7.6 min. Isometric intermittent incremental contractions started at 1% of the maximal voluntary contraction (MVC) for 3 s, with a 3-s relaxation period in between each contraction, and the contraction level was increased by 1% of MVC for every contraction (by 10% of MVC per min) up to exhaustion. Separate contractions with sufficient rest periods were also conducted to serve for the MMG characteristics without fatigue. The integrated MMG (iMMG) was linearly related to force in all of the muscles when fatigue was not involved. With regard to the incremental contractions, the relationship exhibited an ascending-descending shape, but the behavior was not the same for the individual muscle heads, especially for the rectus femoris muscle. A steep increase in the median frequency of MMG from around 60% of MVC corresponded to a decrease in iMMG. These results suggest that analysis of MMG in the time- and frequency-domain during an incremental protocol is a useful way of characterizing the motor unit recruitment strategy and fatigue properties of individual muscles. Accepted: 19 March 1998     

The mechanomyography of persons after stroke during isometric voluntary contractions.

This study was to investigate the properties of mechanomyography (MMG), or muscle sound, of the paretic muscle in the affected side of hemiplegic subjects after stroke during isometric voluntary contractions, in comparison with those from the muscle in the unaffected side of the hemiplegic subjects and from the healthy muscle of unimpaired subjects. MMG and electromyography (EMG) signals were recorded simultaneously from the biceps brachii muscles of the dominant arm of unimpaired subjects (n=5) and the unaffected and affected arms of subjects after stroke (n=8), when performing a fatiguing maximal voluntary contraction (MVC) associated with the decrease in elbow flexion torque, and then submaximal elbow flexions at 20%, 40%, 60% and 80% MVCs. The root mean squared (RMS) values, the mean power frequencies (MPF, in the power density spectrum, PDS) of the EMG and MMG, and the high frequency rate (HF-rate, the ratio of the power above 15Hz in the MMG PDS) were used for the analysis. The MMG RMS decreased more slowly during the MVC in the affected muscle compared to the healthy and unaffected muscles. A transient increase could be observed in the MMG MPFs from the unaffected and healthy muscles during the MVC, associated with the decrease in their simultaneous EMG MPFs due to the muscular fatigue. No significant variation could be seen in the EMG and MMG MPFs in the affected muscles during the MVC. The values in the MPF and HF-rate of MMG from the affected muscles were significantly lower than those from the healthy and unaffected muscles (P<0.05) at the high contraction level (80% MVC). Both the MMG and EMG RMS values in the healthy and unaffected groups were found to be significantly higher than the affected group (P<0.05) at 60% and 80% MVCs. These observations were related to an atrophy of the fast-twitch fibers and a reduction of the neural input in the affected muscles of the hemiplegic subjects. The results in this study suggested MMG could be used as a complementary to EMG for the analysis on muscular characteristics in subjects after stroke.     

Bulk movement included in multi-channel mechanomyography: Similarity between mechanomyography of resting muscle and that of contracting muscle

Although mechanomyography (MMG) reflects local vibrations from contracting muscle fibers, it also includes bulk movement: deformation in global soft tissue around measuring points. To distinguish between them, we compared the multi-channel MMG of resting muscle, which dominantly reflected the bulk movement caused by arterial pulsations, to that of the contracting muscle. The MMG signals were measured at five points around the upper arms of 10 male subjects during resting and during isometric ramp contraction from 5% to 85% of maximal voluntary contraction (MVC) of the biceps brachii muscle. The characteristics of bulk movement were defined as the amplitude distribution and phase relation among the five MMG signals. The bulk movement characteristics during the rest state were not necessarily the same among the subjects. However, below 30 Hz, each subject’s characteristics remained the same from the rest state (0% MVC) to the contracting state (80% MVC), at which the bulk movement mainly originates from muscle contraction activity. Results show that the MMG of the low frequency domain (<30 Hz) includes bulk movement depending on the mechanical deformation characteristics of each subject’s body, for a wide range of muscle contraction intensities.     

Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans

To determine the non-uniform surface mechanical activity of human quadriceps muscle during fatiguing activity, surface mechanomyogram (MMG), or muscle sound, and surface electromyogram (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles of seven subjects during unilateral isometric knee extension exercise. Time- and frequency-domain analyses of MMG and of EMG fatigued by 50 repeated maximal voluntary contractions (MVC) for 3 s, with 3-s relaxation in between, were compared among the muscles. The mean MVC force fell to 49.5 (SEM 2.0)% at the end of the repeated MVC. Integrated EMG decreased in a similar manner in each muscle head, but a marked non-uniformity was found for the decline in integrated MMG (iMMG). The fall in iMMG was most prominent for RF, followed by VM and VL. Moreover, the median frequency of MMG and the relative decrease in that of EMG in RF were significantly greater (P < 0.05) than those recorded for VL and VM. These results would suggest a divergence of mechanical activity within the quadriceps muscle during fatiguing activity by repeated MVC. Accepted: 19 January 1999     

Influence of structure on the tissue dynamics of the human soleus muscle observed in MRI studies during isometric contractions

This article investigates how the internal structure of muscle and its relationship with tendon and even skeletal structures influence the translation of muscle fiber contractions into movement of a limb. Reconstructions of the anatomy of the human soleus muscle from the Visible Human Dataset (available from the National Library of Medicine), magnetic resonance images (MRI), and cadaver studies revealed a complex 3D connective tissue structure populated with pennate muscle fibers. The posterior aponeurosis and the median septum of the soleus form the insertion of the muscle and are continuous with the Achilles tendon. The distal extremities of the pennate muscle fibers attach to these structures. The anterior aponeurosis is located intramuscularly, between the posterior aponeurosis and the median septum. It forms the origin of the muscle and contacts the proximal extremities of the soleus muscle fibers. MRI measurements of in vivo tissue velocities during isometric contractions (20% and 40% maximum voluntary contractions) revealed a similarly complex 3D distribution of tissue movements. The distribution of velocities was similar to the distribution of major connective tissue structures within the muscle. During an isometric contraction, muscle fiber contractions move the median septum and posterior aponeurosis proximally, relative to the anterior aponeurosis. The pennate arrangement of muscle fibers probably amplifies muscle fiber length changes but not sufficiently to account for the twofold difference in muscle fiber length changes relative to excursion of the calcaneus. The discrepancy may be accounted for by an additional gain mechanism operating directly on the Achilles tendon by constraining the posterior movement of the tendon, which would otherwise occur due to the increasingly posterior location of the calcaneus in plantarflexeion.     

Time and frequency domain responses of the mechanomyogram and electromyogram during isometric ramp contractions: a comparison of the short-time Fourier and continuous wavelet transforms.

The purposes of this study were to examine the mechanomyographic (MMG) and electromyographic (EMG) time and frequency domain responses of the vastus lateralis (VL) and rectus femoris (RF) muscles during isometric ramp contractions and compare the time-frequency of the MMG and EMG signals generated by the short-time Fourier transform (STFT) and continuous wavelet transform (CWT). Nineteen healthy subjects (mean+/-SD age=24+/-4 years) performed two isometric maximal voluntary contractions (MVCs) before and after completing 2-3, 6-s isometric ramp contractions from 5% to 100% MVC with the right leg extensors. MMG and surface EMG signals were recorded from the VL and RF muscles. Time domains were represented as root mean squared amplitude values, and time-frequency representations were generated using the STFT and CWT. Polynomial regression analyses indicated cubic increases in MMG amplitude, MMG frequency, and EMG frequency, whereas EMG amplitude increased quadratically. From 5% to 24-28% MVC, MMG amplitude remained stable while MMG frequency increased. From 24-28% to 76-78% MVC, MMG amplitude increased rapidly while MMG frequency plateaued. From 76-78% to 100% MVC, MMG amplitude plateaued (VL) or decreased (RF) while MMG frequency increased. EMG amplitude increased while EMG frequency changed only marginally across the force spectrum with no clear deflection points. Overall, these findings suggested that MMG may offer more unique information regarding the interactions between motor unit recruitment and firing rate that control muscle force production during ramp contractions than traditional surface EMG. In addition, although the STFT frequency patterns were more pronounced than the CWT, both algorithms produced similar time-frequency representations for tracking changes in MMG or EMG frequency.     

Heterogeneous mechanomyographic absolute activation of paraspinal muscles assessed by a two-dimensional array during short and sustained contractions

Spatial dependency of paraspinal muscle activity was assessed using a new two-dimensional MMG recording system. MMG signals were detected over the left and right paraspinal muscles of 10 volunteers using a grid of 12 accelerometers. During two separate trials subjects maintained a 20 degrees flexed position and held loads that ranged from 0 to 15 kg (in 2.5 kg increments) for 20s; and 7.5 kg for 6 min. Maps of absolute and normalised (with respect to initial values) average rectified value, mean power frequency, variance and skewness of the power spectral density were obtained from the two-dimensional MMG recordings. For both the short duration and sustained contractions, the MMG absolute average rectified value, mean power frequency, variance and skewness depended on accelerometer location (P<0.05), while, with the exception of the skewness (P<0.05), normalised values did not. These results demonstrate both inhomogeneous MMG absolute activity and homogeneous MMG normalised activity in paraspinal muscles for short duration and sustained contractions. Moreover, the effect of accelerometer location on spectral variables confirmed the limited validity of general relationships between MMG spectral changes and motor unit recruitment strategies. This study underlines the importance of using multiple recording sites when assessing back muscle activity.     

Uncovering patterns of forearm muscle activity using multi-channel mechanomyography

A coordinated activation of distal forearm muscles allows the hand and fingers to be shaped during movement and grasp. However, little is known about how the muscle activation patterns are reflected in multi-channel mechanomyogram (MMG) signals. The purpose of this study is to determine if multi-site MMG signals exhibit distinctive patterns of forearm muscle activity. MMG signals were recorded from forearm muscle sites of nine able-bodied participants during hand movement. By using 14 features selected by a genetic algorithm and classified by a linear discriminant analysis classifier (LDA), we show that MMG patterns are specific and consistent enough to identify 7 ± 1 hand movements with an accuracy of 90 ± 4%. MMG-based movement recognition required a minimum of three recording sites. Further, by classifying five classes of contraction patterns with 98 ± 3% accuracy from MMG signals recorded from the residual limb of an amputee participant, we demonstrate that MMG shows pattern-specificity even in the absence of typical musculature. Multi-site monitoring of the RMS of MMG signals is suggested as a method of estimating the relative contributions of muscles to motor tasks. The patterns in MMG facilitate our understanding of the mechanical activity of muscles during movement.     

Muscle Plasticity During Sprouting and Reinnervation

SYNOPSIS. When peripheral nerves are cut, the axotomized nervesand denervated muscles undergo atrophic changes which are reversedonly when functional connections are remade in the periphery.The restored interaction completely reverses the effects ofaxotomy and denervation and leads to rematching of the sizeof the motoneuron, muscle unit force, speed and histochemicalproperties, according to the size principle. Differences inunit force and fatigue characteristics between motor unit typesare not fully restored in reinnervated muscles but do not obscuresize relationships between the motoneurons and their muscleunits. Although intact motoneurons will supply increased numbers ofmuscle fibers after partial nerve injuries, regenerating axonsappear to be limited in their ability to enlarge their muscleunits. Increased motor unit force in reinnervated slow motorunits is accounted for primarily by an increase in fiber diameter;fast motor units do not increase their mean force output. As a result of the rematching of muscle unit properties withthe size of the motoneurons that reinnervate them, motor unitproperties are appropriate for fine control of movement aftercomplete or partial nerve injuries. However, regenerating axonsdo not reinnervate their original muscle fibers and unless thefibers are injured close to the muscles, they often fail toreinnervate their original muscles. The mismatching of motorpools with inappropriate target muscles is probably the mainfactor responsible for poor recovery of motor function aftercomplete nerve injuries.     

A mathematical model of force transmission from intrafascicularly terminating muscle fibers

Many long skeletal muscles are comprised of fibers that terminate intrafascicularly. Force from terminating fibers can be transmitted through shear within the endomysium that surrounds fibers or through tension within the endomysium that extends from fibers to the tendon; however, it is unclear which pathway dominates in force transmission from terminating fibers. The purpose of this work was to develop mathematical models to (i) compare the efficacy of lateral (through shear) and longitudinal (through tension) force transmission in intrafascicularly terminating fibers, and (ii) determine how force transmission is affected by variations in the structure and properties of fibers and the endomysium. The models demonstrated that even though the amount of force that can be transmitted from an intrafascicularly terminating fiber is dependent on fiber resting length (the unstretched length at which passive stress is zero), endomysium shear modulus, and fiber volume fraction (the fraction of the muscle cross-sectional area that is occupied by fibers), fibers that have values of resting length, shear modulus, and volume fraction within physiologic ranges can transmit nearly all of their peak isometric force laterally through shearing of the endomysium. By contrast, the models predicted only limited force transmission ability through tension within the endomysium that extends from the fiber to the tendon. Moreover, when fiber volume fraction decreases to unhealthy ranges (less than 50%), the force-transmitting potential of terminating fibers through shearing of the endomysium decreases significantly. The models presented here support the hypothesis that lateral force transmission through shearing of the endomysium is an effective mode of force transmission in terminating fibers.     

The image of motor units architecture in the mechanomyographic signal during the single motor unit contraction: in vivo and simulation study

The mechanomyographic (MMG) signal analysis has been performed during single motor unit (MU) contractions of the rat medial gastrocnemius muscle. The MMG has been recorded as a muscle surface displacement by using a laser distance sensor. The profiles of the MMG signal let to categorize these signals for particular MUs into three classes. Class MMG-P (positive) comprises MUs with the MMG signal similar to the force signal profile, where the distance between the muscle surface and the laser sensor increases with the force increase. The class MMG-N (negative) has also the MMG profile similar to the force profile, however the MMG is inverted in comparison to the force signal and the distance measured by using laser sensor decreases with the force increase. The third class MMG-M (mixed) characterize the MMG which initially increases with the force increases and when the force exceeds some level it starts to decrease towards the negative values. The semi-pennate muscle model has been proposed, enabling estimation of the MMG generated by a single MU depending on its localization. The analysis have shown that in the semi-pennate muscle the localization of the MU and the relative position of the laser distance sensor determine the MMG profile and amplitude. Thus, proposed classification of the MMG recordings is not related to the physiological types of MUs, but only to the MU localization and mentioned sensor position. When the distance sensor is located over the middle of the muscle belly, a part of the muscle fibers have endings near the location of the sensor beam. For the MU MMG of class MMG-N the deflection of the muscle surface proximal to the sensor mainly influences the MMG recording, whereas for the MU MMG class MMG-P, it is mainly the distal muscle surface deformation. For the MU MMG of MMG-M type the effects of deformation within the proximal and distal muscle surfaces overlap. The model has been verified with experimental recordings, and its responses are consistent and adequate in comparison to the experimental data.     

The effect of skinfold on frequency of human muscle mechanomyogram.

The aim of the study was to assess the effect of skinfold thickness on median and peak frequency of mechanomyographic (MMG) signal in relation to subject's age, gender and force during voluntary contraction of elbow flexor and extensor muscles. Seventy-nine healthy subjects participated in the study: 22 young females (age 20.1+/-1.1 years), 22 young males (age 23.4+/-1.1 years), 17 elderly females (age 64.9+/-5.1 years), and 18 elderly males (age 67.4+/-6.2 years). Three identical MMG probes were used to record MMG signals from above the triceps brachii (TB), biceps brachii (BB), and brachioradialis (BR) muscles simultaneously with the force signal. The results showed that the tissue between the muscle and the skin surface has a major contribution to the median and a minor contribution to the peak MMG frequencies independent of subjects' age (with force having more than 2 folds a smaller effect). During antagonistic function of the main elbow flexors and extensors, there is a decreasing effect of skinfold thickness and an increasing effect of force on the MMG frequency, and the relative contribution of both factors to the MMG signal is age related, especially in the TB and BR muscles. The BR muscle differs from the TB and BB muscles in regard to the effects of skinfold thickness and force on the MMG frequency, as well as in the effect of age on the relationship between the MMG frequency and skinfold thickness and force. The effect of age on the relative contribution of skinfolds and force to MMG frequency is specific for muscle and its function. It was concluded that studies that report MMG frequency with different values of skinfold thickness cannot be easily compared, especially when maximally activated prime movers are tested. A use of force and skinfold thickness as covariates is recommended when an MMG frequency is analyzed in subjects differing in the skinfold thickness.     

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms.

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.     

Aging alters contractile properties and fiber morphology in pigeon skeletal muscle

In this study, we tested the hypothesis that skeletal muscle from pigeons would display age-related alterations in isometric force and contractile parameters as well as a shift of the single muscle fiber cross-sectional area (CSA) distribution toward smaller fiber sizes. Maximal force output, twitch contraction durations and the force–frequency relationship were determined in tensor propatagialis pars biceps muscle from young 3-year-old pigeons, middle-aged 18-year-old pigeons, and aged 30-year-old pigeons. The fiber CSA distribution was determined by planimetry from muscle sections stained with hematoxylin and eosin. Maximal force output of twitch and tetanic contractions was greatest in muscles from young pigeons, while the time to peak force of twitch contractions was longest in muscles from aged pigeons. There were no changes in the force–frequency relationship between the age groups. Interestingly, the fiber CSA distribution in aged muscles revealed a greater number of larger sized muscle fibers, which was verified visually in histological images. Middle-aged and aged muscles also displayed a greater amount of slow myosin containing muscle fibers. These data demonstrate that muscles from middle-aged and aged pigeons are susceptible to alterations in contractile properties that are consistent with aging, including lower force production and longer contraction durations. These functional changes were supported by the appearance of slow myosin containing muscle fibers in muscles from middle-aged and aged pigeons. Therefore, the pigeon may represent an appropriate animal model for the study of aging-related alterations in skeletal muscle function and structure.     

Inferences on force transmission from muscle fiber architecture of the canine diaphragm

Functional properties of the diaphragm are mediated by muscle structure. Modeling of force transmission necessitates a precise knowledge of muscle fiber architecture. Because the diaphragm experiences loads both along and transverse to the long axes of its muscle fibers in vivo, the mechanism of force transmission may be more complex than in other skeletal muscles that are loaded uniaxially along the muscle fibers. Using a combination of fiber microdissections and histological and morphological methods, we determined regional muscle fiber architecture and measured the shape of the cell membrane of single fibers isolated from diaphragm muscles from 11 mongrel dogs. We found that muscle fibers were either spanning fibers (SPF), running uninterrupted between central tendon (CT) and chest wall (CW), or were non-spanning fibers (NSF) that ended within the muscle fascicle. NSF accounted for the majority of fibers in the midcostal, dorsal costal, and lateral crural regions but were only 25-41% of fibers in the sternal region. In the midcostal and dorsal costal regions, only approximately 1% of the NSF terminated within the fascicle at both ends; the lateral crural region contained no such fibers. We measured fiber length, tapered length, fiber diameters along fiber length, and the taper angle for 271 fibers. The lateral crural region had the longest mean length of SPF, which is equivalent to the mean muscle length, followed by the costal and sternal regions. For the midcostal and crural regions, the percentage of tapered length of NSF was 45.9 +/- 5.3 and 40.6 +/- 7.5, respectively. The taper angle was approximately 0.15 degrees for both, and, therefore, the shear component of force was approximately 380 times greater than the tensile component. When the diaphragm is submaximally activated, as during normal breathing and maximal inspiratory efforts, muscle forces could be transmitted to the cell membrane and to the extracellular intramuscular connective tissue by shear linkage, presumably via structural transmembrane proteins.     

Long-term activity in upper- and lower-limb muscles of humans.

Despite limited data on humans, previous studies suggest that there is an association between the duration of daily muscle activity and the proportion of type I muscle fibers. We quantified the activity of limb muscles in healthy men and women during normal use and compared these measurements with published reports on fiber-type proportions. Seven men (age range = 21-28 yr) and seven women (age range = 18-26 yr) participated in two 10-h recording sessions. Electromyogram (EMG) activity of four muscles in nondominant upper (first dorsal interosseus and biceps brachii) and lower limbs (vastus medialis and vastus lateralis) was recorded with surface electrodes. Hand and arm muscles were active for 18% of the recording time, whereas leg muscles were active for only 10% of the recording time. On average, upper-limb muscles were activated 67% more often than lower-limb muscles. When lower-limb muscles were activated, however, the mean amplitude of each burst was greater in leg muscles [18 and 17% maximum voluntary contraction (MVC)] compared with hand (8% MVC) and arm (6% MVC) muscles. Temporal association in activity between pairs of muscles was high for the two lower-limb muscles (r2 = 0.7) and relatively weak for the two upper-limb muscles (r2 = 0.09). Long-term muscle activity was only different between men and women for the biceps brachii muscle. We found no relation between duration of muscle activity in 10-h recordings and the reported values of type I fibers in men and women.     

Basic characteristics between mechanomyogram and muscle force during twitch and tetanic contractions in rat skeletal muscles

The mechanomyogram (MMG) is a signal measured by various vibration sensors for slight vibrations induced by muscle contraction, and it reflects the muscle force during electrically induced-contraction or until 60%–70% maximum voluntary contraction, so the MMG is considered an alternative and novel measurement tool for muscle strength. We simultaneously measured the MMG and muscle force in the gastrocnemius (GC), vastus intermedius (VI), and soleus (SOL) muscles of rats. The muscle force was measured by attaching a hook to the tendon using a load cell, and the MMG was measured using a charged-coupled device-type displacement sensor at the middle of the target muscle. The MMG-twitch waveform was very similar to that of the muscle force; however, the half relaxation time and relaxation time (10%), which are relaxation parameters, were prolonged compared to those of the muscle force. The MMG amplitude correlated with the muscle force. Since stimulation frequencies that are necessary to evoke tetanic progression have a significant correlation with the twitch parameter, there is a close relationship between twitch and tetanus in the MMG signal. Therefore, we suggest that the MMG, which is electrically induced and detected by a laser displacement sensor, may be an alternative tool for measuring muscle strength.     

Mechanomyogram and electromyogram responses of upper limb during sustained isometric fatigue with varying shoulder and elbow postures

To investigate the behavior of mechanomyogram (MMG) and electromyogram (EMG) signals in the time and frequency domains during sustained isometric contraction, MMG and surface EMG were obtained simultaneously from four muscles: upper trapezius (TP), anterior deltoid (DL), biceps brachii (BB), and brachioradialis (BR) of 10 healthy male subjects. Experimental conditions consisted of 27 combinations of 9 postures [3 shoulder angles (SA): 0 degree, 30 degrees, 60 degrees and 3 elbow angles (EA): 120 degrees, 90 degrees, 60 degrees] and 3 contraction levels: 20%, 40%, and 60% of maximum voluntary contraction (MVC). Subjective evaluations of fatigue were also assessed using the Borg scale at intervals of 60, 30, and 10 sec at 20%, 40%, and 60% MVC tests, respectively. The mean power frequency (MPF) and root mean square (RMS) of both signals were calculated. The current study found clear and significant relationships among physiological and psychological parameters on the one hand and SA and EA on the other. EA's effect on MVC was found to be significant. SA had a highly significant effect on both endurance time and Borg scale. In all experimental conditions, significant correlations were found between the changes in MPF and RMS of EMG in BB with SA and EA (or muscle length). In all four muscles, MMG frequency content was two or three times lower than EMG frequency content. During sustained isometric contraction, the EMG signal showed the well-known shift to lower frequencies (a continuous decrease from onset to completion of the contraction). In contrast, the MMG spectra did not show any shift, although its form changed (generally remaining about constant). Throughout the contraction, increased RMS of EMG was found for all tests, whereas in the MMG signal, a significant progressive increase in RMS was observed only at 20% MVC in all four muscles. This supports the hypothesis that the RMS amplitude of the MMG signal produced during contraction is highly correlated with force production. Possible explanations for this behavioral difference between the MMG and EMG signals are discussed.     

Gender comparisons of mechanomyographic amplitude and mean power frequency versus isometric torque relationships

This study compared the patterns of mechanomyographic (MMG) amplitude and mean power frequency vs. torque relationships in men and women during isometric muscle actions of the biceps brachii. Seven men (mean age 23.9 +/- 3.5 yrs) and 8 women (mean 21.0 +/- 1.3 yrs) performed submaximal to maximal isometric muscle actions of the dominant forearm flexors. Following determination of the isometric maximum voluntary contraction (MVC), they randomly performed submaximal step muscle actions in 10% increments from 10% to 90% MVC. Polynomial regression analyses indicated that the MMG amplitude vs. isometric torque relationship for the men was best fit with a cubic model (R(2) = 0.983),,where MMG amplitude increased slightly from 10% to 20% MVC, increased rapidly from 20% to 80% MVC, and plateaued from 80% to 100% MVC. For the women, MMG amplitude increased linearly (r(2) = 0.949) from 10% to 100% MVC. Linear models also provided the best fit for the MMG mean power frequency vs. isometric torque relationship in both the men (r(2) = 0.813) and women (r(2) = 0.578). The results demonstrated gender differences in the MMG amplitude vs. isometric torque relationship, but similar torque-related patterns for MMG mean power frequency. These findings suggested that the plateau in MMG amplitude at high levels of isometric torque production for the biceps brachii in the men, but not the women, may have been due to greater isometric torque, muscle stiffness, and/or intramuscular fluid pressure in the men, rather than to differences in motor unit activation strategies for modulating isometric torque production.     

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions.

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.