Tissue Doppler imaging for detecting onset of muscle activity

Rectus abdominis muscle thickness change and activation increase during planks performed on different surfaces

AIMS

The plank is a common exercise used to evaluate core function. Surface electromyography (sEMG) and ultrasound can be used simultaneously to measure muscle activity. We aimed to compare the %-thickness and %-activation during the plank performed on three surfaces and to determine agreement and relationship between rectus abdominis (RA) %-thickness of a rested tabletop position and %-activation normalized to quiet tabletop position during the plank on three surfaces.

METHODS

In this cross-sectional study, ultrasound and sEMG measured RA muscle function during the first 5-s and last 5-s of a plank performed on a table, yoga mat, and fitness ball. A repeated measures ANOVA compared differences in %-thickness change and Friedman's tests compared differences in %-activation, alpha set a priori p ≤ 0.05. Bland-Altman plots measured agreement between instruments. Spearman's rho determined relationships between instruments.

RESULTS

There was no difference between %-thickness change across surfaces during the first 5-s or last 5-s, or between %-activation during the last 5-s. The %-activation of the RA during the first 5-s performed on the fitness ball was higher than the table and yoga mat (p < 0.001). Ultrasound and sEMG had weak relationships across all surfaces (ρ = - 0.078 to 0.116).

CONCLUSION

The first 5-s of the plank performed on the fitness ball requires a greater RA activation. Ultrasound could not detect changes in %-thickness of the RA during the plank which may be influenced by the type of contraction. Comparison between these measurement tools during isometric exercise should be used with caution.

Toward a wearable monitor of local muscle fatigue during electrical muscle stimulation using tissue Doppler imaging

Electrical muscle stimulation (EMS) is widely used in rehabilitation and athletic training to generate involuntary muscle contractions. However, EMS leads to rapid muscle fatigue, limiting the force a muscle can produce during prolonged use. Currently available methods to monitor localized muscle fatigue and recovery are generally not compatible with EMS. The purpose of this study was to examine whether Doppler ultrasound imaging can assess changes in stimulated muscle twitches that are related to muscle fatigue from electrical stimulation. We stimulated five isometric muscle twitches in the medial and lateral gastrocnemius of 13 healthy subjects before and after a fatiguing EMS protocol. Tissue Doppler imaging of the medial gastrocnemius recorded muscle tissue velocities during each twitch. Features of the average muscle tissue velocity waveforms changed immediately after the fatiguing stimulation protocol (peak velocity: -38%, p = .022; time-to-zero velocity: +8%, p = .050). As the fatigued muscle recovered, the features of the average tissue velocity waveforms showed a return towards their baseline values similar to that of the normalized ankle torque. We also found that features of the average tissue velocity waveform could significantly predict the ankle twitch torque for each participant (R2 = 0.255-0.849, p < .001). Our results provide evidence that Doppler ultrasound imaging can detect changes in muscle tissue during isometric muscle twitch that are related to muscle fatigue, fatigue recovery, and the generated joint torque. Tissue Doppler imaging may be a feasible method to monitor localized muscle fatigue during EMS in a wearable device.

Comparison of single and multiple joint muscle functions and neural drive of trained athletes and untrained individuals

BACKGROUND: There is insufficient knowledge about the rate of force development (RFD) characteristics over both single and multiple joint movements and the electromechanical delay (EMD) values obtained in athletes and untrained individuals. OBJECTIVE: To compare single and multiple joint functions and the neural drive of trained athletes and untrained individuals. METHODS: Eight trained athletes and 10 untrained individuals voluntarily participated to the study. The neuromuscular performance was assessed during explosive and maximum voluntary isometric contractions during leg press and knee extension related to single and multiple joint. Explosive force and surface electromyography of eight superficial lower limb muscles were measured in five 50-ms time windows from their onset, and normalized to peak force and electromyography activity at maximum voluntary force, respectively. The EMD was determined from explosive voluntary contractions (EVC’s). RESULTS: The results showed that there were significant differences in absolute forces during knee extension maximum voluntary force and EVC’s (p< 0.01) while trained athletes achieved greater relative forces than untrained individuals of EVC at all five time points (p< 0.05). CONCLUSIONS: The differences in explosive performance between trained athletes and untrained individuals in both movements may be explained by different levels of muscle activation within groups, attributed to variation in biarticular muscle function over both activities.

Differences in muscle contraction onset as determined by ultrasound and electromyography

INTRODUCTION

We characterize the agreement between the timing of muscle contraction onset detected by surface electromyography (sEMG), fine wire EMG (fwEMG), and motion-mode (M-mode) ultrasound for improved interpretations of clinical outcomes.

METHODS

Eighteen healthy adults participated. Differences in contraction onset were compared between sEMG, fwEMG, and M-mode ultrasound collected during concentric contractions of the vastus lateralis and biceps brachii.

RESULTS

The mean difference of 13.1 ms (-33.3-59.9) between sEMG and fwEMG was non-significant (intraclass correlation [ICC] = 0.60). Ultrasound was significantly different from surface and fine wire EMG (ICC = 0.65 and ICC = 0.40, respectively), occurring 98.6 ms (72.3-124.9) and 111.7 (60.3-163.0) before sEMG and fwEMG, respectively. Nonparametric interquartile ranges were also wide.

CONCLUSIONS

Due to high variability, comparisons between EMG methods should be interpreted with caution. Ultrasound detected onset before either EMG method, which may indicate motion from adjacent muscles during voluntary contractions. Muscle Nerve 59:494-500, 2019.

A technical note on variable inter-frame interval as a cause of non-physiological experimental artefacts in ultrasound

Ultrasound (US) imaging is a well-recognized technique for the study of static tissues but its suitability for studying tissue dynamics depends upon accurate frame time information, which may not always be available to users. Here we present methods to quantify the inter-frame interval (IFI) variability, and evaluate different procedures for collecting temporal information from two US-imaging devices. The devices tested exhibited variable IFIs that could only be confirmed by direct measures of timing signals, available by means of electrical signals (triggers) and/or temporal information contained in the software used for the US data collection. Interpolating frame-by-frame measures of dynamic changes within image sequences using individual IFI values provided improved synchronization between measures of skeletal muscle movement and activation; validating US as a valuable technique for the study of musculoskeletal tissue dynamics, when correctly implemented.

Computer-Based Algorithmic Determination of Muscle Movement Onset Using M-Mode Ultrasonography

The study purpose was to evaluate the use of computer-automated algorithms as a replacement for subjective, visual determination of muscle contraction onset using M-mode ultrasonography. Biceps and quadriceps contraction images were analyzed visually and with three different classes of algorithms: pixel standard deviation (SD), high-pass filter and Teager Kaiser energy operator transformation. Algorithmic parameters and muscle onset threshold criteria were systematically varied within each class of algorithm. Linear relationships and agreements between computed and visual muscle onset were calculated. The top algorithms were high-pass filtered with a 30 Hz cutoff frequency and 20 SD above baseline, Teager Kaiser energy operator transformation with a 1200 absolute SD above baseline and SD at 10% pixel deviation with intra-class correlation coefficients (mean difference) of 0.74 (37.7 ms), 0.80 (61.8 ms) and 0.72 (109.8 ms), respectively. The results suggest that computer automated determination using high-pass filtering is a potential objective alternative to visual determination in human movement science.

Spatial variation and inconsistency between estimates of onset of muscle activation from EMG and ultrasound

Delayed onset of muscle activation can be a descriptor of impaired motor control. Activation onset can be estimated from electromyography (EMG)-registered muscle excitation and from ultrasound-registered muscle motion, which enables non-invasive measurements in deep muscles. However, in voluntary activation, EMG- and ultrasound-detected activation onsets may not correspond. To evaluate this, ten healthy men performed isometric elbow flexion at 20% to 70% of their maximal force. Utilising a multi-channel electrode transparent to ultrasound, EMG and M(otion)-mode ultrasound were recorded simultaneously over the biceps brachii muscle. The time intervals between automated and visually estimated activation onsets were correlated with the regional variation of EMG and muscle motion onset, contraction level and speed. Automated and visual onsets indicated variable time intervals between EMG- and motion onset, median (interquartile range) 96 (121) ms and 48 (72) ms, respectively. In 17% (computed analysis) or 23% (visual analysis) of trials, motion onset was detected before local EMG onset. Multi-channel EMG and M-mode ultrasound revealed regional differences in activation onset, which decreased with higher contraction speed (Spearman ρ ≥ 0.45, P < 0.001). In voluntary activation the heterogeneous motor unit recruitment together with immediate motion transmission may explain the high variation of the time intervals between local EMG- and ultrasound-detected activation onset.

M-Mode Ultrasound Reveals Earlier Gluteus Minimus Activity in Individuals With Chronic Hip Pain During a Step-down Task

STUDY DESIGN

Controlled laboratory study.

BACKGROUND

The hip abductor muscles are important hip joint stabilizers. Hip joint pain may alter muscle recruitment. Motion-mode (M-mode) ultrasound enables noninvasive measurements of the onset of deep and superficial muscle motion, which is associated with activation onset.

OBJECTIVES

To compare (1) the onset of superficial and deep gluteus medius and gluteus minimus muscle motion relative to the instant of peak ground reaction force and (2) the level of swing-phase muscle motion during step-down between subjects with chronic hip pain and controls using M-mode ultrasound.

METHODS

Thirty-five subjects with anterior, nontraumatic hip pain for more than 6 months (mean ± SD age, 54 ± 9 years) and 35 controls (age, 57 ± 7 years) were scanned on the lateral hip of the leading leg during frontal step-down onto a force platform using M-mode ultrasound. Computerized motion detection with the Teager-Kaiser energy operator was applied on the gluteus minimus and the deep and superficial gluteus medius to determine the time lag between muscle motion onset and instant of peak ground reaction force and the level of gluteus minimus motion during the swing phase. Time lags and motion levels were averaged per subject, and t tests were used to determine between-group differences.

RESULTS

In participants with hip pain, gluteus minimus motion onset was 103 milliseconds earlier (P = .002) and superficial gluteus medius motion was 70 milliseconds earlier (P = .047) than those in healthy control participants. The level of gluteus minimus swing-phase motion was higher with pain (P = .006).

CONCLUSION

Increased gluteus minimus motion during the swing phase and earlier gluteus minimus and superficial gluteus medius motion in individuals with hip pain suggest an overall increase of muscle activity, possibly a protective behavior.

Observation of Age-Related Decline in the Performance of the Transverse Abdominis Muscle

BACKGROUND

Previous research has shown that the performance of skeletal muscle declines with advancing age. Coordination of the transverse abdominis (TrA), a deep postural muscle, has been shown to be reduced in persons with low back pain. No previous research has studied the effect of age on the activation on this muscle.

OBJECTIVE

To assess the effect of age on TrA activation in response to rapid arm abduction.

DESIGN

Cross-sectional cohort study.

SETTING

University exercise physiology laboratory.

PARTICIPANTS

A total of 18 adult men (aged 27 ± 7.0 years) for the younger group and 11 older adults (5 men and 6 women, aged 59.6 ± 4.0 years) were recruited for this study.

METHOD

Participants were positioned on a treatment table and performed a series of rapid arm abduction movements with their right arm while the activation of the TrA muscle was recorded using ultrasound imaging. Onset of arm abduction was measured using surface electromyography and synchronized with the ultrasound through the ultrasound unit's electrocardiogram channel. The mean time difference between the 2 events was calculated during post-hoc analysis.

MAIN OUTCOME MEASUREMENTS

A Mann-Whitney test was performed to test for differences in the onset performance of the TrA muscle between the 2 groups.

RESULTS

Results showed that the older group was significantly slower than the younger group in engaging their TrA in response to the rapid arm abduction (P = .036). A separate analysis of the older group data showed that no significant differences existed between the male and female participants that could potentially have acted as a confounding factor for the main finding (P = .126).

CONCLUSIONS

This study shows that older adults were slower than younger adults in activating their TrA muscle in response to rapid arm abduction. This delay has the potential to lead to increased occasions when the low back is unprotected, increasing the likelihood of injury or low back pain.

Central fatigue contributes to the greater reductions in explosive than maximal strength with high-intensity fatigue

The study aimed to assess the influence of fatigue induced by repeated high-force explosive contractions on explosive and maximal isometric strength of the human knee extensors and to examine the neural and contractile mechanisms for the expected decrement. Eleven healthy untrained males completed 10 sets of voluntary maximal explosive contractions (five times 3 s, interspersed with 2 s rest). Sets were separated by 5 s, during which supramaximal twitch and octet contractions [eight pulses at 300 Hz that elicit the contractile peak rate of force development (pRFD)] were evoked. Explosive force, at specific time points, and pRFD were assessed for voluntary and evoked efforts, expressed in absolute terms and normalized to maximal/peak force. Maximal voluntary contraction force (MVCF) and peak evoked forces were also determined. Surface EMG amplitude was measured from three superficial agonists and normalized to maximal compound action potential area. By set 10, explosive force (47-52%, P < 0.001) and MVCF (42%, P < 0.001) had declined markedly. Explosive force declined more rapidly than MVCF, with lower normalized explosive force at 50 ms (29%, P = 0.038) that resulted in reduced normalized explosive force from 0 to 150 ms (11-29%, P ≤ 0.038). Neural efficacy declined by 34%, whilst there was a 15-28% reduction in quadriceps EMG amplitude during voluntary efforts (all P ≤ 0.03). There was demonstrable contractile fatigue (pRFD: octet, 27%; twitch, 66%; both P < 0.001). Fatigue reduced normalized pRFD for the twitch (21%, P = 0.001) but not the octet (P = 0.803). Fatigue exerted a more rapid and pronounced effect on explosive force than on MVCF, particularly during the initial 50 ms of contraction, which may explain the greater incidence of injuries associated with fatigue. Both neural and contractile fatigue mechanisms appeared to contribute to impaired explosive voluntary performance.

Human capacity for explosive force production: neural and contractile determinants

This study assessed the integrative neural and contractile determinants of human knee extension explosive force production. Forty untrained participants performed voluntary and involuntary (supramaximally evoked twitches and octets - eight pulses at 300 Hz that elicit the maximum possible rate of force development) explosive isometric contractions of the knee extensors. Explosive force (F0-150 ms) and sequential rate of force development (RFD, 50-ms epochs) were measured. Surface electromyography (EMG) amplitude was recorded (superficial quadriceps and hamstrings, 50-ms epochs) and normalized (quadriceps to Mmax, hamstrings to EMGmax). Maximum voluntary force (MVF) was also assessed. Multiple linear regressions assessed the significant neural and contractile determinants of absolute and relative (%MVF) explosive force and sequential RFD. Explosive force production exhibited substantial interindividual variability, particularly during the early phase of contraction [F50, 13-fold (absolute); 7.5-fold (relative)]. Multiple regression explained 59-93% (absolute) and 35-60% (relative) of the variance in explosive force production. The primary determinants of explosive force changed during the contraction (F0-50, quadriceps EMG and Twitch F; RFD50-100, Octet RFD0-50; F100-150, MVF). In conclusion, explosive force production was largely explained by predictor neural and contractile variables, but the specific determinants changed during the phase of contraction.

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

Background

Tissue 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.

Methods

In 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.

Results

With 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%.

Conclusions

The 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.

Bilateral deficit in explosive force production is not caused by changes in agonist neural drive

Bilateral deficit (BLD) describes the phenomenon of a reduction in performance during synchronous bilateral (BL) movements when compared to the sum of identical unilateral (UL) movements. Despite a large body of research investigating BLD of maximal voluntary force (MVF) there exist a paucity of research examining the BLD for explosive strength. Therefore, this study investigated the BLD in voluntary and electrically-evoked explosive isometric contractions of the knee extensors and assessed agonist and antagonist neuromuscular activation and measurement artefacts as potential mechanisms. Thirteen healthy untrained males performed a series of maximum and explosive voluntary contractions bilaterally (BL) and unilaterally (UL). UL and BL evoked twitch and octet contractions were also elicited. Two separate load cells were used to measure MVF and explosive force at 50, 100 and 150 ms after force onset. Surface EMG amplitude was measured from three superficial agonists and an antagonist. Rate of force development (RFD) and EMG were reported over consecutive 50 ms periods (0–50, 50–100 and 100–150 ms). Performance during UL contractions was compared to combined BL performance to measure BLD. Single limb performance during the BL contractions was assessed and potential measurement artefacts, including synchronisation of force onset from the two limbs, controlled for. MVF showed no BLD (P = 0.551), but there was a BLD for explosive force at 100 ms (11.2%, P = 0.007). There was a BLD in RFD 50–100 ms (14.9%, P = 0.004), but not for the other periods. Interestingly, there was a BLD in evoked force measures (6.3–9.0%, P<0.001). There was no difference in agonist or antagonist EMG for any condition (P≥0.233). Measurement artefacts contributed minimally to the observed BLD. The BLD in volitional explosive force found here could not be explained by measurement issues, or agonist and antagonist neuromuscular activation. The BLD in voluntary and evoked explosive force might indicate insufficient stabiliser muscle activation during BL explosive contractions.

Correlation functions and power spectra of Doppler response signals in ultrasonic medical applications

Ultrasound Doppler methods are widely used in clinical practice as prospective investigational tool to study the vascular system and soft biological tissues. Meanwhile, the most general relationship between the power Doppler spectra, spectral characteristics of the scattering fluctuations and the probing ultrasound field parameters for some clinical implementations are still unexplored. Based upon the continuum model of scattering inhomogeneities, a set of the closed-form expressions for the correlation functions and the spectra of Doppler response of soft tissues and blood have been derived. The influence of the correlation among inhomogeneities and the diffusion processes on the Doppler power spectra formed by stationary flows have been examined. Computer simulations of Doppler spectra were performed for different values of correlation radius and diffusion coefficient. With simulation results the effects of the correlation among inhomogeneities and the diffusion processes on the spectral width and mean frequency are established and discussed in respect to turbulent flows. Closed-form expressions for correlation functions and Doppler spectra for the vibrational sonoelastography technique for visualizing malignant tumors in tissues have been derived. Based on the peculiarities of the obtained Doppler spectra, it is shown that the differentiation of soft tissues with respect to the amplitude value of constrained oscillations is feasible. The expressions were derived for the cases of non-stationary accelerated blood movement. It has been found that the frequency dependence reveals solely at a finite time of observation and depends on the initial phase of the accelerated movement.

Ultrasound tissue Doppler imaging reveals no delay in abdominal muscle feed-forward activity during rapid arm movements in patients with chronic low back pain

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

Comparison of the timing of onset of lateral abdominal muscle activity during rapid arm movements in patients with nonspecific chronic low back pain (cLBP) and back-pain-free controls.

SUMMARY OF BACKGROUND DATA

Rapid movements of the arm are normally associated with prior activation of trunk-stabilizing muscles in readiness for the impending postural perturbation. Using invasive intramuscular electromyography techniques, studies have shown that this feed-forward function is delayed in some patients with low back pain (LBP). Ultrasound tissue Doppler imaging (TDI) provides an ultrasound method for quantifying muscle activation in a noninvasive manner, allowing investigation of larger groups of patients and controls.

METHODS

Ninety-six individuals participated (48 patients with cLBP and 48 matched LBP-free controls). During rapid shoulder flexion, abduction, and extension, surface electromyographic signals from the deltoid and motion-mode TDI images from the contralateral lateral abdominal muscles were recorded simultaneously. The onset of muscle activity was given by changes in the tissue velocity of the abdominal muscles, as measured with TDI. Pain and disability in the patients were assessed using standardized questionnaires. Data were analyzed using repeated measures analysis of variance.

RESULTS

In both groups, feed-forward activity of the lateral abdominal muscles was recorded during arm movements in all directions. The main effect of "group membership" revealed no significant difference between the groups for the earliest onset of abdominal muscle activity (P = 0.398). However, a significant "group x body side" interaction (P = 0.015) was observed, and this was the result of earlier onsets in the cLBP group than controls for the abdominal muscles on the right (but not left) body side. No relationship was found between the time of onset of the earliest abdominal muscle activity and pain intensity, pain frequency, pain medication usage, or Roland Morris disability scores.

CONCLUSION

Patients with cLBP did not show a delayed onset of feed-forward activation of the lateral abdominal muscles during rapid arm movements. Earlier activation was observed for one body side compared with the controls. However, the clinical relevance of this finding remains obscure, especially because there was no relationship between the onset of activation and any clinical parameters.

Location and sequence of muscle onset in deep abdominal muscles measured by different modes of ultrasound imaging

Various modes of ultrasound (US) imaging have been introduced as an alternative to electromyography for determining muscle onset. The purpose of this study was to compare the agreement between US motion-mode (US(m-mode)) and US strain rate (US(SR)) derived from tissue velocity imaging in determining latency time, location and sequence of muscle onset in abdominal muscles using the same data set (contractions). Twenty-four subjects performed four rapid arm flexions in response to a light signal while US recordings were made from the abdominal muscles on the contralateral side. The examined muscles were transversus abdominis (TrA), superficial and deep obliquus internus abdominis (OI(deep) and OI(sup)), and obliquus externus abdominis (OE). The results showed that the two methods detected the first muscle onset on average within 0.1 ms (95% CI; +/-1.4 ms) of each other. US(SR) detected the second muscle onset on average 27 ms after US(m-mode). While US(SR) and US(m-mode) can be used interchangeably to detect the first muscle onset, the location of both first onset and subsequent muscle onsets can be reliably detected by US(SR) only. Furthermore, this study indicates that OI may be functionally subdivided into a superficial and deep region, with onset in OI(deep) occurring on average 53 ms before OI(sup). First onset was detected more frequently in OI than in TrA (65% versus 25% of detected onsets, 10% were equal).

Measurement of rectus femoris muscle velocities during patellar tendon jerk using vector tissue doppler imaging

We have developed a vector tissue Doppler imaging (TDI) system based on a clinical scanner that can be used to measure muscle velocities independent of the direction of motion. This method overcomes the limitations of conventional Doppler ultrasound, which can only measure velocity components along the ultrasound beam. In this study, we utilized this method to investigate the rectus femoris muscle velocities during a patellar tendon jerk test. Our goal was to investigate whether the muscle elongation velocities during a brisk tendon tap fall within the normal range of velocities that are expected due to rapid stretch of limb segments. In a preliminary study, we recruited six healthy volunteers (three men and three women) following informed consent. The stretch reflex response to tendon tap was evaluated by measuring: (1) the tapping force using an accelerometer instrumented to the neurological hammer (2) the angular velocities of the knee extension and flexion using a electrogoniometer (3) reflex activation using electromyography (EMG) and (4) muscle elongation, extension and flexion velocities using vector TDI. The passive joint angular velocity was linearly related to the passive muscle elongation velocity (R(2)=0.88). The maximum estimated joint angular velocity corresponding to muscle elongation due to tendon tap was less than 8.25 radians/s. This preliminary study demonstrates the feasibility of vector TDI for measuring longitudinal muscle velocities and indicates that the muscle elongation velocities during a clinical tendon tap test are within the normal range of values for rapid limb stretch encountered in daily life. With further refinement, vector TDI could become a powerful method for quantitative evaluation of muscle motion in musculoskeletal disorders.

Electromechanical delay revisited using very high frame rate ultrasound

Electromechanical delay (EMD) represents the time lag between muscle activation and muscle force production and is used to assess muscle function in healthy and pathological subjects. There is no experimental methodology to quantify the actual contribution of each series elastic component structures that together contribute to the EMD. We designed the present study to determine, using very high frame rate ultrasound (4 kHz), the onset of muscle fascicles and tendon motion induced by electrical stimulation. Nine subjects underwent two bouts composed of five electrically evoked contractions with the echographic probe maintained over 1) the gastrocnemius medialis muscle belly (muscle trials) and 2) the myotendinous junction of the gastrocnemius medialis muscle (tendon trials). EMD was 11.63 ± 1.51 and 11.67 ± 1.27 ms for muscle trials and tendon trials, respectively. Significant difference ( P < 0.001) was found between the onset of muscle fascicles motion (6.05 ± 0.64 ms) and the onset of myotendinous junction motion (8.42 ± 1.63 ms). The noninvasive methodology used in the present study enabled us to determine the relative contribution of the passive part of the series elastic component (47.5 ± 6.0% of EMD) and each of the two main structures of this component (aponeurosis and tendon, representing 20.3 ± 10.7% and 27.6 ± 11.4% of EMD, respectively). The relative contributions of the synaptic transmission, the excitation-contraction coupling, and the active part of the series elastic component could not be directly quantified with our results. However, they suggest a minor role of the active part of the series elastic component that needs to be confirmed by further experiments.