Comparison of the EMG power spectrum of the human soleus and gastrocnemius muscles

The influence of whole body vibration on the plantarflexors during heel raise exercise

Whole body vibration (WBV) during exercise offers potential to augment the effects of basic exercises. However, to date there is limited information on the basic physiological and biomechanical effects of WBV on skeletal muscles. The aim of this study was to determine the effects of WBV (40Hz, 1.9mm synchronous vertical displacement) on the myoelectrical activity of selected plantarflexors during heel raise exercise. 3D motion capture of the ankle, synchronised with sEMG of the lateral gastrocnemius and soleus, was obtained during repetitive heel raises carried out at 0.5Hz on 10 healthy male subjects (age 27±5 years, height 1.78±0.04m, weight 75.75±11.9kg). During both vibration and non vibration the soleus activation peaked earlier than that of the lateral gastrocnemius. The results indicate that WBV has no effect on the timing of exercise completion or the amplitude of the lateral gastrocnemius activity, however significant increases in amplitudes of the soleus muscle activity (77.5-90.4% MVC P<0.05). WBV had no significant effect on median frequencies of either muscle. The results indicate that the greatest effect of WBV during heel raise activity is in the soleus muscles during the early phases of heel raise.

The effects of muscle length and force output on the EMG power spectrum of the erector spinae

In many skeletal muscles the myoelectric power spectrum median frequency (MF) increases with increasing force output, possibly reflecting the greater size and conduction velocity of the later-recruited (fast twitch) fibres. Muscles, such as the erector spinae, in which fast twitch fibres are smaller than slow twitch, may display an atypical relationship between force output and median frequency. The present study sought to investigate this possibility. Ten healthy men held forces ranging from 20-80% maximal voluntary contraction (MVC) of the back extensors for 4-6 s, at muscle lengths corresponding to 30, 60 and 90% of the lumbar spine's range of flexion (ROF). MF was determined from surface electromyograms recorded from thoracic and lumbar regions of the erector spinae. In each region, MF was significantly higher at 30% ROF (short muscle length) than at 60 or 90% ROF (P < 0.005) and slightly (but not significantly) higher at 60 than 90% ROF. The muscle length effect on MF may reflect a reduction in conduction velocity of the stretched and narrowed muscle fibres. Force output had a significant effect on MF (P < 0.0004), although the shape of the relationship differed between the two levels of the erector spinae: in the thoracic region MF increased with force up to 40-50% MVC and then levelled off, whereas in the lumbar region MF was relatively stable up to 30-40% MVC and then declined with increasing force. The results suggest that the mean fibre size of the later recruited motor units is, in the thoracic region, larger, and in the lumbar region, smaller, than that of the earlier-recruited motor units.

Reliability of techniques to assess human neuromuscular function in vivo

The purpose of this study was to comprehensively evaluate the reliability of a large number of commonly utilized experimental tests of in vivo human neuromuscular function separated by 4-weeks. Numerous electrophysiological parameters (i.e., voluntary and evoked electromyogram [EMG] signals), contractile properties (i.e., evoked forces and rates of force development and relaxation), muscle morphology (i.e., MRI-derived cross-sectional area [CSA]) and performance tasks (i.e., steadiness and time to task failure) were assessed from the plantarflexor muscle group in 17 subjects before and following 4-weeks where they maintained their normal lifestyle. The reliability of the measured variables had wide-ranging levels of consistency, with coefficient of variations (CV) ranging from approximately 2% to 20%, and intraclass correlation coefficients (ICC) between 0.53 and 0.99. Overall, we observed moderate to high-levels of reliability in the vast majority of the variables we assessed (24 out of the 29 had ICC>0.70 and CV<15%). The variables demonstrating the highest reliability were: CSA (ICC=0.93-0.98), strength (ICC=0.97), an index of nerve conduction velocity (ICC=0.95), and H-reflex amplitude (ICC=0.93). Conversely, the variables demonstrating the lowest reliability were: the amplitude of voluntary EMG signal (ICC=0.53-0.88), and the time to task failure of a sustained submaximal contraction (ICC=0.64). Additionally, relatively little systematic bias (calculated through the limits of agreement) was observed in these measures over the repeat sessions. In conclusion, while the reliability differed between the various measures, in general it was rather high even when the testing sessions are separated by a relatively long duration of time.

Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion

This study compared magnetic resonance imaging (MRI) and surface electromyography (EMG) to evaluate the effect of knee angle upon plantar flexion activity in the triceps surae muscles [medial & lateral gastrocnemius (MG, LG) and the soleus (SOL)]. Two weight & height matched groups performed identical protocols, twelve (6M, 6F) in the MRI group, twelve (8M, 4F) in the EMG group. Subjects plantar flexed dynamically for 2 min at 25% of 1-repetition maximum voluntary contraction (1-RM). Exercise was performed with the knee extended (0 degrees flexion), flexed (90 degrees ), and partially flexed (45 degrees ). In the MRI group spin-echo images were acquired before and immediately following each exercise session. T(2) times, calculated at rest and after exercise by fitting the echoes to a monoexponential decay pattern with a least-squares algorithm, were compared with EMG data. In the EMG group a bipolar electrode was used to collect samples were from the MG, LG, SOL, and anterior tibialis (TA) during exercise at each knee angle, MRI also examined the peroneus (PER). At 0 degrees flexion MRI demonstrated a significant post-exercise T(2) increase in the MG (p < or = 0.001), LG (p < or = 0.001), and PER (p < or = 0.01), with no T(2) change in the SOL or TA. At 90 degrees flexion there was a significant T(2) increase in the SOL (p < or = 0.001) with no significant T(2) change in the MG, LG, PER, or TA. At 45 degrees T(2) increased significantly in the SOL (p < or = 0.001) and LG (p < or = 0.05), but not the MG, PER, or TA. EMG produced similar results with the exception that there was significant activity in the TA during the relaxation cycle of the 90 degrees protocol. We conclude that: 1) Soleus activity is measurable by MRI; and 2) MRI and EMG produce similar results from different physiological sources, and are therefore complementary tools for evaluating muscle activity.

Task dependent motor strategy of human triceps surae muscle

Even though many investigators have analyzed the functional difference of the three heads of triceps surae in human, none of them succeeded to clarify the distinctive functional difference of those three muscles. The aim of this study was to investigate whether the integrated EMGs (IEMGs) of the triceps surae muscle, gastrocnemius and soleus, were task dependent. IEMGs of the medial head of the gastrocnemius (GM), lateral head of the gastrocnemius (GL), and soleus (SO) were investigated at three different knee joint angles, at four different duration of ramp contraction, with the generation of a single ongoing force, from 0 to the maximum voluntary contraction (MVC). Three-way ANOVAs for repeated measures were used to estimate differences in IEMG values in each of the GM, GL, and SO, taken at four different durations of ramp contraction (5, 10, 15 and 20 s), at three different knee joint angles (0 deg, 30 deg and 90 deg), across ankle plantar flexion levels of force (10, 20, 30, 40, 50, 60 and 70% MVC). According to three-way ANOVAs for repeated measures, IEMG of the GM muscle showed a first-order interaction between force and knee joint angle. In addition, IEMG of the GL muscle showed first-order interactions between the level of force and knee joint angle, and between the level of force and duration of ramp contraction. Furthermore, IEMG of the SO showed a main effect only on level of force. These results suggest that the each head of the triceps surae may work task dependently.

MRI determination of muscle recruitment variations in dynamic ankle plantar flexion exercise

OBJECTIVE

The purpose of this study was to investigate the muscle recruitment variations in the dynamic ankle plantar flexion.

DESIGN

A total of 17 subjects participated in this study and performed the ankle plantar flexion exercise. Magnetic resonance T2-weighted images were obtained from the calf before and immediately after exercise to calculate each T2 relaxation time in the medial and lateral gastrocnemius, soleus, tibialis posterior, flexor digitorum/hallucis longus, peroneus longus, and dorsiflexors.

RESULTS

All the muscles except the dorsiflexors showed significantly increased T2 relaxation time and signal intensity on T2-weighted images after exercise. Above all, both gastrocnemius muscles showed significantly greater postexercise T2 relaxation time than the soleus, tibialis posterior, flexor digitorum/hallucis longus, and dorsiflexors. In addition, the peroneus longus had a tendency to show the greatest T2 relaxation time next to the gastrocnemius, but there was no significant difference between them.

CONCLUSIONS

The present study may suggest that the gastrocnemius muscle, especially the medial side, was best recruited in the dynamic ankle plantar flexion exercise. In addition, it is possible that the peroneus longus was most recruited next to the gastrocnemius in this exercise mode.

Gender dependent EMGs of runners resolved by time/frequency and principal pattern analysis

A promising approach for the analysis of surface electromyograms is to use wavelets to determine the spectral distribution of the signal intensity at any time. The authors have recently proposed using non-linearly scaled wavelets to obtain intensity patterns, which reflect the spectral distribution at any given time point. Further analysis of intensity-patterns is greatly facilitated by representing them as linear combinations of a base set of principal-patterns. The weight with which each principal-pattern contributes to the intensity-pattern can be represented on a set of orthogonal axes that span a previously introduced pattern space. The purpose of the present study was to show how to use pattern space to discriminate and classify male and female runners based on the electromyograms of five muscles of the limb. The results showed that there were significant gender specific differences, which allowed more than a 95% correct classification of the subjects as males or females. Classification was possible irrespective of the shod condition while running. Gender specific differences occurred at well-defined time periods during the movement. Common to both genders was that spectral changes did not parallel the changes in total signal intensity.

Amplitude and spectral characteristics of biceps Brachii sEMG depend upon speed of isometric force generation

In the present study the influence of speed of contraction on the interplay between recruitment and firing rate of motor units (MUs) was assessed. The surface electromyographic (sEMG) signal was recorded in nine healthy subjects from the right biceps brachii using a linear electrode array during ramp isometric contractions (from 0 to 100% of the maximal voluntary force, MVC) at 5, 10, and 20% MVC s(-1) (ramp phase), followed by 10 s of sustained MVC (hold phase). The median frequency (MDF), Root Mean Square (RMS) and conduction velocity (CV) of sEMG, were computed on adjacent epochs covering a force range of 5% MVC each. Full motor unit recruitment (FMUR) point was assessed as the force level at which MDF reached its maximum value; the MDF decay during the hold phase was taken as an index of localized muscle fatigue. At 5% MVC s(-1), FMUR was reached at 52.3% MVC. At 10%MVC s(-1) FMUR was achieved at 58% MVC; while at 20% MVC s(-1) FMUR point was located at 77% MVC, being statistically different from 5 and 10% MVCs(-1) ramps (p<0.05). The MDF decay was steeper at higher speed. CV modifications mirrored those reported for MDF. The RMS increased in a curvilinear fashion and the maximum value was always attained during the hold phase. Our findings suggest that MU recruitment strategies are significantly related to the speed of contraction even in a single muscle.

Variations in the relationship between the frequency content of EMG signals and the rate of torque development in voluntary and elicited contractions

Our purpose was to characterize the relationship between EMG mean power frequency (MPF) or median frequency (MF) and rate of torque development in voluntary ballistic and electrically elicited isometric contractions. Twenty-three healthy adults participated in two sets of experiments performed on elbow flexor muscles. For Experiment 1, subjects were asked to generate voluntary ballistic contractions by reaching four different target torque levels (20, 40, 60 and 100% of the maximal voluntary contraction (MVC)) as fast as they could. For Experiment 2, electrical (M-waves) and mechanical (twitches) responses to electrical stimulation of the nerves supplying the biceps brachii and brachioradialis muscles were recorded with the subjects at rest and with a background isometric contraction of 15% MVC. MPF, MF and rate of torque development (% MVC/s) were calculated for both voluntary and elicited contractions. Significant positive correlations were observed between MPF and rate of torque development for the voluntary contractions, whereas significant negative correlations were observed between the two variables for elicited contractions. This suggests that factors other than muscle fiber composition influence the frequency content of EMG signals and/or the rate of torque development, and that the effect of these factors will vary between voluntary and elicited contractions.

Variability of some SEMG parameter estimates with electrode location

Muscular action potential conduction velocity (CV) and mean power frequency (MPF) are commonly used parameters to describe the surface electromyographic signal (SEMG). The discrepancies concerning the behavior and interpretation of these main parameters in the literature have motivated this work. Our objective was to evaluate within- and between-individual reproducibility, sensitivity and variation of CV and MPF depending on the electrode location with respect to various contraction modalities. The results present evidence for significant influence of electrode location on CV and MPF, not only in their initial values but also in their changes during fatiguing efforts. This influence appears to be subject-dependent. This variability seems to be essentially due to the relative displacements of myotendinous and neuromuscular junctions with respect to the electrode set. Moreover, this study shows that MPF can be seen as force-dependent under certain conditions and that the CV-MPF relationship is strongly influenced by methodological factors. In conclusion, it seems irrelevant to derive reliable SEMG parameter estimates without considering electrode location. There is a strong need for proper standardization based on anatomical and methological aspects before attempting any individual characterization. Finally, we suggest a procedure for assessment of measurement quality.

Is spectral analysis of the surface electromyographic signal a clinically useful tool for evaluation of skeletal muscle fatigue?

Normal values for initial median frequency (IMF) of the electromyographic (EMG) power-density spectrum must be determined before EMG spectral analysis can be used to evaluate clinically muscle fatigue. This study attempts to establish normal values in four muscles. Thirty-one healthy subjects performed isometric contractions of the biceps brachii, triceps, deltoid, and tibialis anterior muscles at 50% maximum voluntary contraction. Linear regression analysis was used to compute the IMF and the slope of the median frequency as it decayed with fatigue over 45 seconds. The IMF of the tibialis anterior (mean +/- standard deviation, 116 +/- 20 Hz) was significantly higher (p < 0.001) than that of the biceps (90 +/- 18 Hz), triceps (85 +/- 18 Hz), and deltoid (87 +/- 15 Hz). The deltoid and tibialis anterior had the steepest slopes. The IMF of all muscles was greater in men than in women, but gender did not affect the slope. This study attempted to establish normal values for IMF and slope in specific muscles. However, the range of normal values is so broad that it may preclude the clinical use of spectral analysis to evaluate muscle fatigue.

EMG power spectrum and integrated EMG of ankle plantarflexors during stepwise and ramp contractions

The aim of this study was to investigate whether the median frequencies (MF) of the electromyogram (EMG) and the integrated EMG (IEMG) of histochemically differentiated ankle plantarflexors, the gastrocnemius and soleus, were force dependent. Bipolar intramuscular wire electrodes were used to measure EMG of the soleus (SO), medial head of gastrocnemius (GM), and lateral head of gastrocnemius (GL) during ramp (single ongoing contractions) with the force increasing linearly from 0 to 100% of maximum voluntary contraction (MVC) and stepwise (steady force levels) ankle plantarflexion at 10, 20, 30, 40, 60, and 80% MVC. EMG and force were measured simultaneously. Power spectral analysis of these signals was performed to calculate MF on 1024-point by fast Fourier transform (FFT) technique. IEMG value of each muscle was also obtained at the same levels of force. While IEMG of three heads of triceps surae in both stepwise and ramp contractions increased significantly with increasing force, MF values of GL during stepwise contraction increased significantly (20, 40, 60, 80% MVC). These results suggest that the sensitivity of EMG power spectrum might be influenced by the proportion of fast twitch muscle fibers, which histochemically corresponds to type II fibers.

Physiology and interpretation of the electromyogram

The purpose of this review is to consider some issues in the interpretation of the electromyogram (EMG) and to discuss current areas of controversy regarding use of the EMG. We consider the underlying physiology and origin of the EMG signal and offer an abbreviated discussion of measurement issues and selected factors that affect the characteristics of the EMG signal. We discuss many of the problems affecting interpretation, including normalization, crosstalk, and issues specific to contraction. In the final section, we consider topics of current interest in electromyography, such as muscle fatigue, task specificity, multichannel representations, and muscle fiber conduction velocity. We present, in addition, alternative analysis techniques. This review should interest researchers and clinicians who seek to obtain the valuable information inherent in the EMG while respecting the potential sources of variance and misinterpretation.