Non-invasive characterization of single motor unit electromyographic and mechanomyographic activities in the biceps brachii muscle.

The aim of the study was to investigate amplitude and frequency content of single motor unit (MU) electromyographic (EMG) and mechanomyographic (MMG) responses. Multi-channel surface EMG and MMG signals were detected from the dominant biceps brachii muscle of 10 volunteers during isometric voluntary contractions at 20%, 50%, and 80% of the maximal voluntary contraction (MVC) force. Each contraction was performed three times in the experimental session which was repeated in three non-consecutive days. Single MU action potentials were identified from the surface EMG signals and their times of occurrence used to trigger the averaging of the MMG signal. At each contraction level, the MUs with action potentials of highest amplitude were identified. Single MU EMG and MMG amplitude and mean frequency were estimated with normalized standard error of the mean within subjects (due to repetition of the measure in different trials and experimental sessions) smaller than 15% and 7%, respectively, in all conditions. The amplitude of the action potentials of the detected MUs increased with increasing force (mean +/- SD, 244 +/- 116 microV at 20% MVC, and 1426 +/- 638 microV at 80% MVC; P < 0.001) while MU MMG amplitude increased from 20% to 50% MVC (40.5 +/- 20.9 and 150 +/- 88.4 mm/s(2), respectively; P<0.001) and did not change significantly between 50% and 80% MVC (129 +/ -82.7 mm/s(2) at 80% MVC). MU EMG mean frequency decreased with contraction level (20% MVC: 97.2 +/- 13.9 Hz; 80% MVC: 86.2 +/- 11.4 Hz; P < 0.001) while MU MMG mean frequency increased (20% MVC: 33.2 +/- 6.8 Hz; 80% MVC: 40.1 +/- 6.1 Hz; P < 0.001). EMG peak-to-peak amplitude and mean frequency of individual MUs were not correlated with the corresponding variables of MMG at any contraction level.     

Effect of conditioning contraction intensity on postactivation potentiation is muscle dependent

We aimed to examine whether the influence of conditioning contraction intensity on the extent of postactivation potentiation (PAP) is muscle dependent. Eleven healthy males performed both thumb adduction and plantar flexion as a conditioning contraction. The conditioning contraction intensities were set at 20%, 40%, 60%, 80%, or 100% of the maximal voluntary isometric contraction (MVC).Before and after the conditioning contraction, twitch torque was measured for the respective joint to calculate the extent of PAP. In plantar flexion, the extent of PAP became significantly larger as the conditioning contraction intensity increased up to 80% MVC (p < 0.05). In contrast, the extent of PAP in thumb adduction increased significantly only up to 60% MVC (p < 0.05), but not at higher intensities.These results indicate that the influence of the conditioning contraction intensity on the extent of PAP is muscle dependent. Our results suggest that a conditioning contraction with submaximal intensity can sufficiently evoke sizable PAP in the muscle where most of muscle fibers are recruited at submaximal intensities, thereby attenuating muscle fatigue induced by the conditioning contraction.     

Mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this investigation was to determine the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque (or force) relationships during isokinetic and isometric muscle actions of the biceps brachii. Ten adults (mean +/- SD age = 21.6 +/- 1.7 years) performed submaximal to maximal isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects randomly performed submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Polynomial regression analyses indicated that MMG amplitude increased linearly with torque during both the isokinetic (r2 = 0.982) and isometric (r2 = 0.956) muscle actions. From 80% to 100% of isometric MVC, however, MMG amplitude appeared to plateau. Cubic models provided the best fit for the MMG MPF versus isokinetic (R2 = 0.786) and isometric (R2 = 0.940) torque relationships, although no significant increase in MMG MPF was found from 10% to 100% of isokinetic PT. For the isometric muscle actions, however, MMG MPF remained relatively stable from 10% to 50% MVC, increased from 50% to 80% MVC, and decreased from 80% to 100% MVC. The results demonstrated differences in the MMG amplitude and MPF versus torque relationships between the isokinetic and isometric muscle actions. These findings suggested that the time and frequency domains of the MMG signal may be useful for describing the unique motor control strategies that modulate dynamic versus isometric torque production.     

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.     

Changes in fascicle length from rest to maximal voluntary contraction affect the assessment of voluntary activation

The purpose of this study was to investigate the effect of the differences between the actual fascicle length during a voluntary contraction and the fascicle length at rest of the triceps surae muscle on the determination of the voluntary activation (VA) by using the interpolated twitch technique. Twelve participants performed isometric voluntary maximal (MVC) and submaximal (20%, 40%, 60% and 80% MVC) contractions at two different ankle angles (75 degrees and 90 degrees ) under application of the interpolated twitch technique. Two ultrasound probes were used to determine the fascicle length of soleus, gastrocnemius medialis and gastrocnemius lateralis muscles. Further, the MVCs and the twitches were repeated for six more ankle angles (85 degrees , 95 degrees , 100 degrees , 105 degrees , 110 degrees and 115 degrees ). The VA of the triceps surae muscle were calculated (a) using the rest twitch force (RTF) measured during the same trial as the interpolated twitch force (ITF; traditional method) and (b) using the RTF at an ankle angle where the fascicle length showed similar values between ITF and RTF (fascicle length consideration method). The continuous changes in fascicle length from rest to MVC affect the accuracy of the assessment of the VA. The traditional method overestimates the assessment of the VA on average 4% to 12%, especially at 90 degrees ankle angle (i.e. short muscle length). The reason for this influence is the unequal force-length potential of the muscle at twitch application by the measure of ITF and RTF. These findings provide evidence that the fascicle length consideration method permits a more precise prediction (an improvement of 4-12%) of the voluntary contraction compared to the traditional method.     

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.     

Changes in contractile properties and neuromuscular propagation evaluated by simultaneous mechanomyogram and electromyogram during experimentally induced hypothermia.

The present study examined, whether or not mechanomyogram (MMG) amplitude and frequency component could reflect the contractile properties of the triceps surae muscles, composed of relatively slow soleus (SOL) and fast medial gastrocnemius (MG), during experimentally induced hypothermia condition. In eight male subjects, lying in prone position, supramaximal single twitch and repetitive electrical stimulations at 10 Hz were applied at the intramuscular temperatures of control (34 degrees C), 15, 20, and 25 degrees C, respectively. The hypothermia induced substantial reduction in muscle contractile properties, e.g. prolonged twitch contraction and half relaxation times, resulted in a highly significant reduction in the fluctuation of force signal during the repetitive stimulations. These changes were almost mirrored by the similar and significant reductions in the MMG amplitude in both SOL and MG. Power spectrum analysis revealed that peak frequency components of MMG and fluctuation of force were almost matched with the applied stimulation frequencies, independent of the temperature condition. These results strongly suggest that MMG analysis could be employed to study muscle contractile properties varying across different physiological conditions.     

Changes in the properties of mechanomyographic signals and in the tension during the fatigue test of rat medial gastrocnemius muscle motor units

The influence of activity-related changes in tension on properties of the mechanomyogram (MMG) was investigated in fast fatigable, fast resistant and slow motor units (MUs). A standard fatigue test was used in which rhythmically repeated unfused tetani were evoked. The amplitudes of the rise in tension of the first and the last contraction within the unfused tetanus and the amplitudes of accompanying signals in MMG were calculated. For fast fatigable MUs a parallel decrease in the amplitudes of both analysed contractions and in the amplitudes of accompanying MMG signals during the fatigue test was observed. For majority of fast resistant MUs at the beginning of the fatigue test a potentiation occurred and this phenomenon increased the tension of the first contraction and of the peak tetanic tension. However, the potentiation coincided also with a decrease of the amplitude of the last contraction in the tension recording of an unfused tetanus. The MMG reflected both, the increase of amplitude of the first contraction and the decrease of the amplitude of the further contractions within the tetanus. The single twitch contraction evoked immediately before and after the fatigue test was additionally recorded. A decrease (fatigue) or an increase (potentiation) of the twitch tension after the fatigue test was reflected by a decrease or an increase in the amplitude of MMG, respectively. However, the fatigue failed to change significantly the time parameters of MMG. To conclude, fatigue and potentiation can occur during activity of fast MUs and both these phenomena involve changes in the amplitude of oscillations in tension of unfused tetani which are reflected in MMG.     

Mechanomyogram amplitude vs. isometric ankle plantarflexion torque of human medial gastrocnemius muscle at different ankle joint angles

The purpose of this study was to investigate the influence of changes in ankle joint angle on the mechanomyogram (MMG) amplitude of the human medial gastrocnemius (MG) muscle during voluntary isometric plantarflexion contractions. Ten healthy individuals were asked to perform voluntary isometric contractions at six different contraction intensities (from 10% to 100%) and at three different ankle joint angles (plantarflexion of 26°; plantarflexion of 10°; dorsiflexion of 3°). MMG signals were recorded from the surface over the MG muscle, using a 3-axis accelerometer. The relations between root mean square (RMS) MMG and isometric plantarflexion torque at different ankle joint angles were characterized to evaluate the effects of altered muscle mechanical properties on RMS MMG.We found that the relation between RMS MMG and plantarflexion torque is changed at different ankle joint angles: RMS MMG increases monotonically with increasing the plantarflexion torque but decreases as the ankle joint became dorsiflexed. Moreover, RMS MMG shows a negative correlation with muscle length, with passive torque, and with maximum voluntary torque, which were all changed significantly at different ankle joint angles.Our findings demonstrate the potential effects of changing muscle mechanical properties on muscle vibration amplitude. Future studies are required to explore the major sources of this muscle vibration from the perspective of muscle mechanics and muscle activation level, attributable to changes in the neural command.     

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.     

Mechanomyographic and electromyographic responses of the triceps surae during maximal voluntary contractions.

The objective of this study was to examine the effect of joint angle on the electromyogram (EMG) and mechanomyogram (MMG) during maximal voluntary contraction (MVC). Eight subjects performed maximal isometric plantar flexor torque productions at varying knee and/or ankle angles. Maximal voluntary torque, EMG, and MMG from the soleus (Sol), medial (MG) and lateral gastrocnemius (LG) muscles were measured at different joint angles. At varying knee angles, the root mean squared (rms) MMG amplitude of the MG and LG increased with knee joint extension from 60 degrees to 180 degrees (full extension) in steps of 30 degrees, whereas that of the Sol was constant. At varying ankle angles, the rms-MMG of all muscles (Sol, MG, and LG) decreased with torque as ankle joint extending from 80 degrees (10 degrees dorsiflexion position) to 120 degrees (30 degrees plantar flexion position) in steps of 10 degrees. In each case, changes in the rms-MMG of the three muscles were almost parallel to those in torque. In contrast, there were no significant differences in the rms-EMG of all muscles among all joint angles. Our data suggest that the MMG amplitudes recorded from individual muscles during MVCs can represent relative torque-angle relationships that cannot be represented by the EMG signals.     

Relationship between mechanomyogram and force during voluntary contractions reinvestigated using spectral decomposition

A mechanomyogram (MMG) is considered to represent the pressure waves resulting from the lateral expansion of contracting muscle fibers. However, the actual MMG recording appears not only to reflect lateral changes of active fibers, but also to include the effect of their longitudinal shortening, because the fiber orientation, particularly in pennate muscles, is not parallel with the MMG transducer attached at the skin surface. In the present investigation, a spectral decomposition method was developed to eliminate the interference due to fiber longitudinal movement from the MMG recording. The MMG was recorded over the belly of the rectus femoris muscle, which is a pennate muscle. Vibration over the tibial tuberosity (VTT) was used as a measure of the integrated longitudinal movement of the muscle fibers. The lateral and longitudinal components included in the MMG were separated by a spectral decomposition method that is based on the coherence function of the MMG and VTT. The MMG/force relationship was compared between the original and decomposed MMG. One-third of the 12 subjects demonstrated a curvilinear relationship between the original MMG and force throughout the range of force. In the other two-thirds, the MMG saturated or reduced beyond 70% of the maximum voluntary contraction (MVC). After decomposition, the MMG increased progressively with force up to 70% MVC, beyond which it decreased in all subjects. The spectral decomposition method described here is considered to be a useful tool with which to examine in more detail the MMG/force relationship of different pennate muscles.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.     

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.     

Twitch interpolation technique in testing of maximal muscle strength: Influence of potentiation, force level, stimulus intensity and preload

The aim was to study the methodological aspects of the muscle twitch interpolation technique in estimating the maximal force of contraction in the quadriceps muscle utilizing commercial muscle testing equipment. Six healthy subjects participated in seven sets of experiments testing the effects on twitch size of potentiation, time lag after potentiation, magnitude of voluntary force, stimulus amplitude, stimulus duration, angle of the knee, and angle of the hip. In addition, the consequences of submaximal potentiation on the estimation of maximal force from twitch sizes were studied in five healthy subjects. We found an increase in twitch size with increasing levels of potentiation and twitch size decreased exponentially following potentiation. We found a curvilinear relationship between twitch size and voluntary force, and these properties were more obvious when the stimulation intensity of the preload was reduced. The relationship between twitch size and force was only linear, for force levels greater than 25% of maximum. It was concluded that to achieve an accurate estimate of true maximal force of muscle contraction, it would be necessary for the subject to be able to perform at least 75% of the true maximal force.     

Comparison of a piezoelectric contact sensor and an accelerometer for examining mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to compare a piezoelectric contact sensor with an accelerometer for measuring the mechanomyographic (MMG) signal from the biceps brachii during submaximal to maximal isokinetic and isometric forearm flexion muscle actions. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), 10 adults (mean+/-SD age=22.8+/-2.7yrs) performed randomly ordered, submaximal step muscle actions of the dominant forearm flexors in 20% increments from 20% to 80% PT and MVC. Surface MMG signals were recorded simultaneously from a contact sensor and an accelerometer placed over the belly of the biceps brachii muscle. During the isokinetic and isometric muscle actions, the contact sensor and accelerometer resulted in linear increases in normalized MMG amplitude with torque (r(2) range=0.84-0.97) but the linear slope of the normalized MMG amplitude versus isokinetic torque relationship for the accelerometer was less (p<0.10) than that of the contact sensor. There was no significant (p>0.05) relationship for normalized MMG mean power frequency (MPF, %max) versus isokinetic and isometric torque for the contact sensor, but the accelerometer demonstrated a quadratic (R(2)=0.94) or linear (r(2)=0.83) relationship for the isokinetic and isometric muscle actions, respectively. There were also a number of significant (p<0.05) mean differences between the contact sensor and accelerometer for normalized MMG amplitude or MPF values. These findings indicated that in some cases involving dynamic and isometric muscle actions, the contact sensor and accelerometer resulted in different torque-related responses that may affect the interpretation of the motor control strategies involved.     

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography.

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC10 min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC10 min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC10 min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067+/-0.028 mV before 10%MVC10 min to 0.107+/-0.049 and 0.087+/-0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054+/-0.039 ms(-2) to 0.133+/-0.104 and 0.127+/-0.099 ms(-2), respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.     

Influence of the intensity of a conditioning contraction on the subsequent twitch torque and maximal voluntary concentric torque

This study aimed to clarify the influence of the intensity of a conditioning contraction on subsequent isometric twitch and maximal voluntary concentric torques. Subjects (n=12men) performed voluntary isometric plantar flexion for six seconds as a conditioning contraction, at intensities of 40%, 60%, 80% and 100% of a maximal voluntary isometric contraction (MVIC). Before and immediately after the conditioning contraction, isometric twitch and maximal voluntary concentric (180°/s) plantar flexion torques were determined. Surface electromyograms were recorded from the triceps surae muscles and M-wave amplitudes and root-mean-square values of the electromyographic signals (RMS(EMG)) were calculated. The isometric twitch torque increased significantly after conditioning contraction at all intensities (P<0.05), whereas maximal voluntary concentric torque increased significantly only at 80% and 100% MVIC conditions (P<0.05). It is concluded that during a six second conditioning contraction, the effect of the intensity of a conditioning contraction on subsequent torque development is different between an isometric twitch and maximal voluntary concentric contractions, with the latter being less affected.     

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions.

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.     

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