Muscle Activation Dynamics Research Articles

Comparison of dynamic muscle activity and 3d gait kinematics acquired while wearing an electronic stance control orthosis versus a locked knee-ankle-foot orthosis: A case study

Comparison of dynamic muscle activity and 3d gait kinematics acquired while wearing an electronic stance control orthosis versus a locked knee-ankle-foot orthosis: A case study

Muscle Synergies During a Single-Leg Drop-Landing in Boys and Girls

The purpose of this study was to investigate muscle activation patterns during a landing task in boys and girls through the use of muscle synergies. Electromyographical data from six lower extremity muscles were collected from 11 boys and 16 girls while they performed single-leg drop-landings. Electromyographical data from six leg muscles were rectified, smoothed, and normalized to maximum dynamic muscle activity during landing. Data from 100 ms before to 100 ms after touchdown were submitted to factor analyses to extract muscle synergies along with the associated activation and weighing coefficients. Boys and girls both used three muscle synergies. The activation coefficients of these synergies captured muscle activity during the prelanding, touchdown, and postlanding phases of the single-leg drop-landing. Analysis of the weighing coefficients indicated that within the extracted muscle synergies the girls emphasized activation of the medial hamstring muscle during the prelanding and touchdown synergy whereas boys emphasized activation of the vastus medialis during the postlanding synergy. Although boys and girls use similar muscle synergies during single-leg drop-landings, they differed in which muscles were emphasized within these synergies. The observed differences in aspects related to the muscle synergies during landing may have implications with respect to knee injury risk.

Primary motor cortical discharge during force field adaptation reflects muscle-like dynamics

We often make reaching movements having similar trajectories within very different mechanical environments, for example, with and without an added load in the hand. Under these varying conditions, our kinematic intentions must be transformed into muscle commands that move the limbs. Primary motor cortex (M1) has been implicated in the neural mechanism that mediates this adaptation to new movement dynamics, but our recent experiments suggest otherwise. We have recorded from electrode arrays that were chronically implanted in M1 as monkeys made reaching movements under two different dynamic conditions: the movements were opposed by either a clockwise or counterclockwise velocity-dependent force field acting at the hand. Under these conditions, the preferred direction (PD) of neural discharge for nearly all neurons rotated in the direction of the applied field, as did those of proximal limb electromyograms (EMGs), although the median neural rotation was significantly smaller than that of muscles. For a given neuron, the rotation angle was very consistent, even across multiple sessions. Within the limits of measurement uncertainty, both the neural and EMG changes occurred nearly instantaneously, reaching a steady state despite ongoing behavioral adaptation. Our results suggest that M1 is not directly involved in the adaptive changes that occurred within an experimental session. Rather, most M1 neurons are directly related to the dynamics of muscle activation that themselves reflect the external load. It appears as though gain modulation, the differential recruitment of M1 neurons by higher motor areas, can account for the load and behavioral adaptation-related changes in M1 discharge.

Influence of Muscle Activation and Deactivation Dynamics on Feasible Forearm Movements

Influence of Muscle Activation and Deactivation Dynamics on Feasible Forearm Movements

Numerical Methods and Applications in Biomechanical Modeling

Numerical Methods and Applications in Biomechanical Modeling

Dynamics of muscle activation during tonic–clonic seizures

The purpose of our study was to elucidate the dynamics of muscle activation during generalised tonic-clonic seizures (GTCS). We recorded surface electromyography (EMG) from the deltoid muscle during 26 GTCS from 13 patients and compared it with GTCS-like events acted by 10 control subjects. GTCS consisted of a sequence of phases best described quantitatively by dynamics of the low frequency (LF) wavelet component (2-8Hz). Contrary to the traditional view, the tonic phase started with a gradual increase in muscle activity. A longer clonic phase was associated with a shorter onset of the tonic phase and a higher seizure occurrence. Increase in LF occurred during the onset phase and during the transition from the tonic to the clonic phase, corresponding to the vibratory movements. The clonic phase consisted of EMG discharges of remarkably constant duration (0.2s) separated by silent periods (SP) of exponentially increasing duration - features that could not be reproduced voluntarily. The last SP was longer in seizures with higher EMG peak frequency whereas the energy of the last clonus was higher in seizures with a short clonic phase. We found specific features of muscle activation dynamics during GTCS. Our findings suggest that the same inhibitory mechanisms that contribute to GTCS termination counteract seizure initiation, accounting for the gradual onset. Both active inhibition and mechanisms related to metabolic depletion act synergistically to stop the seizure. Analysis of the ictal EMG dynamics is a valuable tool for monitoring the balance between pro-convulsive and anti-convulsive factors.

Effect of Verbal Instruction on Muscle Activity During the Bench Press Exercise

Recent research suggests that humans have some ability to selectively activate or relax some muscles during isometric or dynamic muscle actions without changing posture or position. This study sought to reveal whether trained athletes could isolate either the pectoral or triceps muscles, respectively, at different intensities when given verbal technique instruction. Eleven male Division III football players performed 3 sets of bench press at 50% 1-repetition max (1RM) and 80% 1RM while electromyographic (EMG) activity was recorded from the pectoralis major (PM), anterior deltoid (AD), and triceps brachii (TB). In the first set, the subjects performed the exercise without instruction. In the second set, the subjects were given verbal instructions to use only chest muscles. In the third set, the subjects were instructed to use only triceps muscles. Mean normalized root mean square EMG activity was calculated during 3 repetitions in each condition. Repeated-measures analysis of variance was used to detect differences from the preinstruction condition, with significance set to p ≤ 0.017 as indicated by a Bonferroni correction for multiple comparisons. During the 50% max lift with verbal instructions to focus on chest muscles, PM EMG activity increased by 22% over preinstruction activity (p = 0.005), whereas AD and TB activities were statistically unchanged. When the subjects were instructed to focus on only the triceps muscles, PM returned to baseline activity, whereas TB activity was increased by 26% (p = 0.005). When the lift was increased to 80% max, PM and AD activities were both increased with verbal instructions to use only chest muscles. The TB activity was unchanged during the 80% lifts, regardless of instructions. In conclusion, it is found that verbal technique instruction is effective in shifting muscle activity during a basic lift, but it may be less effective at higher intensities.

A neuromechanical computational model of spinal control of locomotion

We developed a neuromechanical computational model of cat locomotion that simulated the locomotor movements of cat hindlimbs controlled by spinal locomotor central pattern generators (CPGs, one per limb). In the closed-loop model, CPG operation was adjusted by afferent feedback from the hindlimbs. The CPG model was based on the previous two-level model [4] and included a half-center rhythm generator (RG), producing alternating flexor and extensor activities, and a pattern formation (PF) network operating under control of RG and controlling the synergetic activities of different hindlimb motoneuron pools. This basic model [4] was extended by incorporating additional neural circuits at the PF level allowing the CPG to generate the complex activity patterns of motoneurons controlling two-joint muscles. The model included reflex circuits, mediating reciprocal inhibition between antagonistic motoneurons, recurrent inhibition of motoneurons via Renshaw cells, and di-synaptic excitation of extensors by extensor afferents during stance phase of locomotion. The hindlimbs with the pelvis and trunk were modeled as a 10 degree-of-freedom sagittal plane system of rigid segments interconnected by frictionless revolute joints. The hindlimb interactions with the ground and other body segments were modeled as linear springs and dampers [1,2]. The two hindlimbs were driven by 18 muscle actuators representing major hindlimb muscles in the cat. The dynamics of each muscle-tendon unit was described by a Hill-type model extended to account for muscle mass, pennation angle, and force-length-velocity properties of muscle and tendon [1,2]. All muscles generated motion-dependent afferent signals which were calculated as functions of muscle length, velocity and force using modified regression equations from [3]. Parameters of the musculoskeletal model were identified by minimizing a mismatch between the computed (based on the recorded hindlimb muscle activity) and experimentally recorded changes in the joint angles, joint moments and ground reaction forces using the simulated annealing optimization algorithm [1,2]. The developed neuromechanical model demonstrated stable locomotion and exhibited realistic patterns of muscle activation, limb kinematics, and ground reaction force dynamics. The model was used for investigation of the role of afferent feedback and the CPG in control of locomotion under different conditions.

Muscle co-contraction modulates damping and joint stability in a three-link biomechanical limb.

Computational models of neuromotor control require forward models of limb movement that can replicate the natural relationships between muscle activation and joint dynamics without the burdens of excessive anatomical detail. We present a model of a three-link biomechanical limb that emphasizes the dynamics of limb movement within a simplified two-dimensional framework. Muscle co-contraction effects were incorporated into the model by flanking each joint with a pair of antagonist muscles that may be activated independently. Muscle co-contraction is known to alter the damping and stiffness of limb joints without altering net joint torque. Idealized muscle actuators were implemented using the Voigt muscle model which incorporates the parallel elasticity of muscle and tendon but omits series elasticity. The natural force-length-velocity relationships of contractile muscle tissue were incorporated into the actuators using ideal mathematical forms. Numerical stability analysis confirmed that co-contraction of these simplified actuators increased damping in the biomechanical limb consistent with observations of human motor control. Dynamic changes in joint stiffness were excluded by the omission of series elasticity. The analysis also revealed the unexpected finding that distinct stable (bistable) equilibrium positions can co-exist under identical levels of muscle co-contraction. We map the conditions under which bistability arises and prove analytically that monostability (equifinality) is guaranteed when the antagonist muscles are identical. Lastly we verify these analytic findings in the full biomechanical limb model.

Comparison of different methods for estimating muscle forces in human movement

The aim of this study was to compare muscle-force estimates derived for human locomotion using three different methods commonly reported in the literature: static optimisation (SO), computed muscle control (CMC) and neuromusculoskeletal tracking (NMT). In contrast with SO, CMC and NMT calculate muscle forces dynamically by including muscle activation dynamics. Furthermore, NMT utilises a time-dependent performance criterion, wherein a single optimisation problem is solved over the entire time interval of the task. Each of these methods was used in conjunction with musculoskeletal modelling and experimental gait data to determine lower-limb muscle forces for self-selected speeds of walking and running. Correlation analyses were performed for each muscle to quantify differences between the various muscle-force solutions. The patterns of muscle loading predicted by the three methods were similar for both walking and running. The correlation coefficient between any two sets of muscle-force solutions ranged from 0.46 to 0.99 (p < 0.001 for all muscles). These results suggest that the robustness and efficiency of static optimisation make it the most attractive method for estimating muscle forces in human locomotion.

Mechanisms underlying rhythmic locomotion: dynamics of muscle activation

We have studied the dynamical properties of tension development in leech longitudinal muscle during swimming. A new method is proposed for modeling muscle properties under functionally relevant conditions where the muscle is subjected to both periodic activation and rhythmic length changes. The 'dual-sinusoid' experiments were conducted on preparations of leech nerve cord and body wall. The longitudinal muscle was activated periodically by injection of sinusoidal currents into an identified motoneuron. Simultaneously, sinusoidal length changes were imposed on the body wall with prescribed phase differences (12 values equally spaced over 2π radians) with respect to the current injection. Through the singular value decomposition of appropriately constructed tension data matrices, the leech muscle was found to have a multiplicative structure in which the tension was expressed as the product of activation and length factors. The time courses of activation and length factors were determined from the tension data and were used to develop component models. The proposed modeling method is a general one and is applicable to contractile elements for which the effects of series elasticity are negligible.

Relationship Between Force-Time and Velocity-Time Characteristics of Dynamic and Isometric Muscle Actions

Previous research has investigated the force-time curve characteristics of isometric and dynamic muscle actions; however, few studies have addressed their relationship to dynamic exercise velocity-time variables. The purpose of this study was to investigate relationships between velocity-time characteristics (high pull and vertical jump peak velocity and rate of velocity development [HPPV, HPRVD, VJPV, VJRVD]), force-time characteristics (isometric peak force [IsoPF], body mass adjusted isometric peak force [IsoPF/BM], isometric rate of force development at different millisecond windows [IsoRFD50-250], dynamic peak force [HPPF], body mass adjusted dynamic peak force [HPPF/BM]), and vertical jump height (VJHeight). Nineteen recreationally trained men (age 23.89 ± 2.92 yr; height 176.32 ± 7.06 cm; mass 78.76 ± 16.50 kg) completed 2 testing sessions. The first session consisted of 3 isometric mid-thigh pulls on a force plate with each repetition held for 3 seconds. On the second testing session, subjects completed 3 dynamic mid-thigh high pulls with 30% IsoPF followed by 3 vertical jumps on a force plate. The HPRVD correlated with IsoRFD50 (r = 0.52) and IsoRFD100 (r = 0.49). The HPPV correlated with IsoPF/BM (r = -0.60), IsoRFD50 (r = 0.56), and IsoRFD100 (r = 0.56). The VJHeight correlated with IsoPF/BM (r = 0.61), whereas VJPV correlated with IsoPF/BM (r = 0.62). These correlations suggest that explosive isometric force production within 50 to 100 milliseconds may influence the ability to accelerate an implement or body and attain high velocity, albeit in a moderate fashion. In addition, body mass adjusted strength may positively influence vertical jump parameters.

Effects of Prolonged Vibration on H-Reflexes, Muscle Activation, and Dynamic Strength

Neural activation is generally lower during maximal voluntary lengthening compared with shortening and isometric muscle actions, but the mechanisms underlying these differences are unclear. In maximal voluntary isometric actions, reduced Ia-afferent input induced by prolonged tendon vibration has been shown to impair neural activation and strength. This study aimed to investigate whether reducing Ia-afferent input influences neural activation in maximal voluntary dynamic muscle actions and, if so, whether it affects shortening and lengthening muscle actions differently. Eight women participated in three familiarization sessions and two randomly ordered experiments. In one experiment, 30-min vibration at 100 Hz was applied to the Achilles tendon to decrease Ia-afferent input as measured by the H-reflex. In the control experiment, rest substituted the vibration. Root mean square EMG from plantar and dorsiflexor muscles and plantar flexor strength were measured during maximal voluntary plantar flexor shortening and lengthening actions (20°·s(-1)) before and after vibration and rest, respectively. Soleus H-reflexes and M-waves were elicited before each set of strength tests. The vibration caused a decrease in H-reflex amplitude by, on the average, 33%, but root mean square EMG and plantar flexor strength remained largely unaffected in both action types. The findings suggest that Ia-afferent input may not substantially contribute to maximal voluntary dynamic muscle strength of the plantar flexor muscles, as tested here, and thus, the results do not support the notion that Ia-afferent excitation would contribute differently to neural activation in maximal voluntary lengthening and shortening muscle actions.

A phase-reduced neuro-mechanical model for insect locomotion: feed-forward stability and proprioceptive feedback

In earlier work, we have developed an integrated model for insect locomotion that includes a central pattern generator (CPG), nonlinear muscles, hexapedal geometry and a representative proprioceptive sensory pathway. Here, we employ phase reduction and averaging theory to replace 264 ordinary differential equations (ODEs), describing bursting neurons in the CPG, their synaptic connections to motoneurons, muscle activation dynamics and sensory neurons, with 24 one-dimensional phase oscillators that describe motoneuronal activation of agonist-antagonist muscle pairs driving the jointed legs. Reflexive feedback is represented by stereotypical spike trains with rates proportional to joint torques, which change phase relationships among the motoneuronal oscillators. Restriction to the horizontal plane, neglect of leg mass and use of Hill-type muscle models yield a biomechanical body-limb system with only three degrees of freedom, and the resulting hybrid dynamical system involves 30 ODEs: reduction by an order of magnitude. We show that this reduced model captures the dynamics of unperturbed gaits and the effects of an impulsive perturbation as accurately as the original one. Moreover, the phase response and coupling functions provide an improved understanding of reflexive feedback mechanisms.

Case Report: Quantitative MRI of Tibial Tubercle Transfer During Active Quadriceps Contraction

The evaluation of distal transfer procedures relies primarily on qualitative clinical findings. Although quantitative MRI measurements provide an objective supplement to qualitative clinical findings, an association between qualitative clinical findings and quantitative patellofemoral indices has yet to be established. A 21-year-old man presented with frequent episodes of patellar dislocation. Clinical and radiographic findings identified the patient as a candidate for realignment. MR images were obtained with the quadriceps contracted at 30° flexion and in full extension preoperatively and postoperatively. The procedure eliminated the clinical J-sign, defined as lateral movement of the patella over the lateral femoral condyle during active leg extension, which was confirmed quantitatively using established patellofemoral indices in conjunction with MRI. Movement of the patellofemoral joint is complex and dependent on many factors, including skeletal geometry, dynamic muscle action, and soft tissue restraints. Therefore, proper clinical and radiographic evaluation of patellar tracking requires observation during active quadriceps contraction. However, it is unclear whether there is any association between quantitative radiographic indices and qualitative clinical assessment during active quadriceps contraction. We believe objective measurements are valuable in determining the radiographic assessment of patellar realignment procedures. Lateral patellar edge appears to quantify the clinical J-sign. The association between radiographic medialization of the patella and clinical improvement postoperatively in this patient warrants additional investigation in a larger group of patients with longer followup.

The Influence of Intrinsic Muscle Properties on Musculoskeletal System Stability: A Modelling Study

The objective of this study is to investigate how the intrinsic mechanical properties of muscles will affect the musculoskeletal system stability. A typical musculoskeletal joint driven by a pair of antagonist muscles confined only in the sigittal plane was constructed. The dynamic characteristics of the flexor and extensor muscles induced by neural inputs were represented by three dynamic processes: neural excitation, muscle activation and muscle contraction dynamics. The muscle contraction mechanics was described using a modified Hill's model with a Contractile Element (CE), a parallel elastic element and a serial elastic element. Additionally, the change of muscle Physiological Cross-Sectional Area (PCSA) and pennation angle during muscle contraction were also considered. A set of dynamic simulations were conducted by applying an external impulsive force at the distal part of the musculoskeletal system. Sensitivity analysis was conducted to investigate the effect of the CE's force-length relationship, the CE's force-velocity relationship, the force-length relationship of the serial elastic element, the parallel elastic element and the pennation angle on the system stability. The results show that the muscles with full intrinsic mechanical properties are sufficient to stabilize the system subject to an impulsive force perturbation without reflexive changes in activations. To guarantee a self-stabilizing ability, a proper CE's force-velocity relationship, the existence of a series elastic element and a sufficient muscle co-contraction level are necessary. This study would provide insight into the intrinsic design and function of the musculoskeletal system, and also give implications for the design of bionic actuators, biomimetic robotics and prosthetic devices.

Relationships among peak power output, peak bar velocity, and mechanomyographic amplitude during the free-weight bench press exercise

The purpose of this study was to examine the relationships among mechanomyographic (MMG) amplitude, power output, and bar velocity during the free-weight bench press exercise. Twenty-one resistance-trained men [one-repetition maximum (1-RM) bench press = 125.4 ± 18.4 kg] performed bench press muscle actions as explosively as possible from 10% to 90% of the 1-RM while peak power output and peak bar velocity were assessed with a TENDO Weightlifting Analyzer. During each muscle action, surface MMG signals were detected from the right and left pectoralis major and triceps brachii, and the concentric portion of the range of motion was selected for analysis. Results indicated that power output increased from 10% to 50% 1-RM, followed by decreases from 50% to 90% 1-RM, but MMG amplitude for each of the muscles increased from 10 to 80%1-RM. The results of this study indicate that during the free-weight bench press exercise, MMG amplitude was not related to power output, but was inversely related to bar velocity and directly related to the external load being lifted. In future research, coaches and sport scientists may be able to estimate force/torque production from individual muscles during multi-joint, dynamic constant external resistance muscle actions.

Effect of Functional Isometric Squats on Vertical Jump in Trained and Untrained Men

Functional isometrics (FIs) combine dynamic and isometric muscle actions and may hyperstimulate the nervous system leading to an enhanced postactivation potentiation (PAP) and improved subsequent performance. The purpose of this study was to investigate the impact of an FI squat on the countermovement vertical jump (CMVJ) in resistance trained and untrained men. Thirteen trained men (age: 22.8 +/- 3.2 years, mass: 90.0 +/- 16.3 kg, and height: 178.9 +/- 7.1 cm) and 8 untrained men (age: 28.5 +/- 5.9 years, mass: 101.5 +/- 23.0 kg, and height: 177.0 +/- 4.8 cm) participated. On separate days, subjects performed CMVJs after 2 different warm-up conditions. The warm-up conditions consisted of either 5 minutes of low-intensity cycling or 5 minutes of low-intensity cycling plus a 3-second FI squat with 150% of their 1 repetition maximum (1RM). A 2 x 3 repeated-measures analysis of variance with Bonferroni post hoc revealed that when comparing the 2 warm-up conditions in the trained subjects, a significant increase (p < 0.05) in CMVJ occurred at 4 minutes (2.4 cm, +5.1%) post-FI squat. This increase was maintained when subjects were retested at 5 minutes post (2.6 cm, + 5.5%). No significant difference in CMVJ was detected in the untrained group (p = 0.49). Results support the addition of an FI squat performed at 150% of 1RM to a low-intensity cycling warm-up to enhance PAP in resistance trained but not in untrained men as measured by CMVJ. Practically, adding functional isometrics to a warm-up scheme may significantly enhance acute, short-term power output in resistance trained men.

The Relationship Between The Eccentric Utilization Ratio, Reactive Strength, And Pre-Stretch Augmentation And Selected Dynamic And Isometric Muscle Actions.

The ability to use the stretch shortening cycle (SSC) is essential for many sporting activities. There are several approaches by which to assess the SSC. These include the eccentric utilization ratio (EUR), the reactive strength calculation (RSC), and the percent pre-stretch augmentation (PPA). These measures are typically quantified for vertical jump displacements and power outputs. The assessment of the SSC activity via jumping activities may yield valuable information of the athletes training status and potential training interventions to improve SSC performance. To determine if EUR, RSC and PPA are related to dynamic and isometric muscle actions. Twenty seven college track athletes (age = 19.6 ± 1.0 y; body mass = 83.0 ± 25.2 kg; height = 176.9 ± 8.8 cm) performed three types of countermovement vertical jumps (CMJ) and static vertical jumps (SJ) on a force plate. Both CMJ and SJ were performed in unloaded and loaded (11 kg and 20 kg) conditions. All jump data were analyzed for vertical displacement, peak force (PF), and rate of force development (RFD). The EUR (CMJ/SJ), RSC (CMJ-SJ), and PPA ((CMJ-SJ)/SJ) × 100) were calculated for jump height, peak power, and peak force Additionally, subjects performed 2 isometric mid-thigh pull with previously established methods. The isometric mid-thigh pulls were analyzed for PF and RFD. Finally, each subject performed a one repetition maximum back squat test (1-RM) and a maximal ball throw test. There were no significant correlations between the EUR (jump height, PP, PF) for loaded or unloaded jumps and back squat, ball throw, isometric PF, and RFD. All EUR values were above 1, which is indicative of well trained athletes. Significant inverse relationships were found between the PPA calculation for PF during the unloaded condition and squat 1-RM (r = −0.85), ball throw distance (r = −0.74), and isometric PF (r = −0.55). Additionally, during the loaded conditions (11 and 20 kg) the PPA calculation for PF was significantly correlated with the back squat 1-RM (11 kg: r = −0.89; 20kg: r = −0.81), ball throw distance (11 kg: r = −0.824; 20kg: r = −0.65), isometric PF (11kg:r = −0.60; 20kg: r = −0.56), and the isometric RFD (11kg: r = −0.43; 20kg:r = −0.36). The PPA calculation for peak power during the loaded conditions was significantly correlated with the 1-RM back squat (11kg; r = −0.89; 20kg: r = −0.62), ball throw (11kg: r = −0.72; 20kg: r = −0.56), isometric PF (11kg: r = −0.47; 20kg:r = −0.40), isometric RFD (11kg: r = −0.43; 20kg: r = −0.42). The isometric RFD can be estimated by the equation: RFD = −58168.7 (PPA for PP 11kg) + 10866.415 or RFD = −142636.9 (PPA for PP 20 kg) + 10340.226. This study suggests that unloaded or loaded PPA has a relationship with an athlete's 1-RM in the back squat, the ball throw distance, and isometric PF generating capacity. Additionally, under loaded conditions the PPA appears to be related to the isometric RFD. The utilization of CMJ and SJ testing is common place in strength and conditioning and the data calculated from these measures may yield valuable information about performance capacity. The significant relationship between the 1-RM back squat and PPA suggests that strength is an underlying mechanism in the ability to utilize the SSC. Additionally, the PPA assessment may be useful in predicting the isometric RFD.

Linearity and reliability of the mechanomyographic amplitude versus dynamic torque relationships for the superficial quadriceps femoris muscles

The purpose of this investigation was to examine the linearity and reliability of the mechanomyographic (MMG) amplitude versus dynamic torque relationships for the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles. Nine healthy men and 11 healthy women performed submaximal to maximal, concentric, isokinetic muscle actions of the leg extensors at 30 degrees s(-1) on two occasions. Surface MMG signals were detected from the VL, RF, and VM of the dominant thigh during both trials. The ranges of the coefficients of determination for the MMG amplitude versus dynamic torque relationships were 0.01-0.94 for the VL, 0.01-0.84 for the RF, and 0.19-0.96 for the VM. The intraclass correlation coefficients for the linear MMG amplitude versus torque slope coefficients were 0.823 (VL), 0.792 (RF), and 0.927 (VM). These results indicate that, when analyzed for individual subjects, the MMG amplitude versus dynamic torque relationships demonstrated inconsistent linearity. When using MMG in the clinical setting, dynamic muscle actions of the superficial quadriceps femoris muscles do not appear to be appropriate for assessing changes in muscle function during strength training.