Muscle activation and its distribution within human triceps surae muscles

Alpha and beta/low-gamma frequency bands may have distinct neural origin and function during post-stroke walking

Plantarflexors provide propulsion during walking and receive input from both corticospinal and corticoreticulospinal tracts, which exhibit some frequency-specificity that allows potential differentiation of each tract's descending drive. Given that stroke may differentially affect each tract and impair the function of plantarflexors during walking; here, we examined this frequency-specificity and its relation to walking-specific measures during post-stroke walking. Fourteen individuals with chronic stroke walked on an instrumented treadmill at self-selected and fast walking speed (SSWS and FWS, respectively) while surface electromyography (sEMG) from soleus (SOL), lateral gastrocnemius (LG), and medial gastrocnemius (MG) and ground reaction forces (GRF) were collected. We calculated the intermuscular coherences (IMC; alpha, beta, and low-gamma bands between SOL-LG, SOL-MG, LG-MG) and propulsive impulse using sEMG and GRF, respectively. We examined the interlimb and intralimb IMC comparisons and their relationships with propulsive impulse and walking speed. Interlimb IMC comparisons revealed that beta LG-MG (SSWS) and low-gamma SOL-LG (FWS) IMCs were degraded on the paretic side. Intralimb IMC comparisons revealed that only alpha IMCs (both speeds) exhibited a statistically significant difference to random coherence. Further, alpha LG-MG IMC was positively correlated with propulsive impulse in the paretic limb (SSWS). Alpha and beta/low-gamma bands may have a differential functional role, which may be related to the frequency-specificity of the underlying descending drives. The persistence of alpha band in plantarflexors and its strong positive relationship with propulsive impulse suggests relative alteration of corticoreticulospinal tract after stroke. These findings imply the presence of frequency-specific descending drives to walking-specific muscles in chronic stroke.

Progression of partial to complete ruptures of the Achilles tendon during rehabilitation: A study using a finite element model

Substantial research on complete Achilles tendon ruptures is available, but guidance on partial ruptures is comparatively sparse. Conservative management is considered acceptable in partial tendon ruptures affecting less than 50% of the tendon's width, but supporting experimental evidence is currently lacking. Using a previously validated finite element model of the Achilles tendon, this study aimed to assess whether loading conditions simulating an early functional rehabilitation protocol could elicit progression to a complete rupture in partial ruptures of varying severity. In silico tendon rupture simulations were performed to locate the most likely rupture site for least, moderate, and extreme subtendon twist configurations. These three models were split at the corresponding rupture site and two sets of partial ruptures were created for each, starting from the medial and lateral sides, and ranging from 10% to 50% loss of continuity. Simulations were conducted with material parameters from healthy and tendinopathic tendons. Partial ruptures were considered to progress if the volume of elements showing a maximum principal strain above 10% exceeded 3 mm3. To assess whether the tendinopathic tendons typical geometric characteristics could compensate for the inferior material properties found in tendinopathy, an additional model with increased cross-sectional area in the free tendon region was developed. Progression to complete ruptures occurred even with less than a 50% loss of continuity, regardless of subtendon twisting, and material parameters. The tendinopathic tendon model with increased cross-sectional area showed similar results. These findings suggest the current criteria for surgical treatment of partial ruptures should be reconsidered. Statement of clinical significance: The clinical significance and most appropriate treatment of partial ruptures of the Achilles tendon is unclear. Despite the widespread use of the "50% rule" in treatment decisions of partial tendon ruptures, experimental evidence supporting it is missing. The present study provides new data, from a validated aponeurotic and free Achilles tendon finite element model, showing that partial ruptures may progress to complete ruptures under loading conditions elicited from functional rehabilitation protocols, even for partial ruptures affecting less than 50% of the tendon's width. Under these novel findings, the current criteria for surgical treatment of partial ruptures should be reconsidered.

Towards standardisation of surface electromyography measurements in the horse: Bipolar electrode location

The use of surface electromyography in the field of animal locomotion has increased considerably over the past decade. However, no consensus exists on the methodology for data collection in horses. This study aimed to start the development of recommendations for bipolar electrode locations to collect surface electromyographic data from horses during dynamic tasks. Data were collected from 21 superficial muscles of three horses during trot on a treadmill using linear electrode arrays. The data were assessed both quantitatively (signal-to-noise ratio (SNR) and coefficient of variation (CoV)) and qualitatively (presence of crosstalk and activation patterns) to compare and select electrode locations for each muscle. For most muscles and horses, the highest SNR values were detected near or cranial/proximal to the central region of the muscle. Concerning the CoV, there were larger differences between muscles and horses than within muscles. Qualitatively, crosstalk was suspected to be present in the signals of twelve muscles but not in all locations in the arrays. With this study, a first attempt is made to develop recommendations for bipolar electrode locations for muscle activity measurements during dynamic contractions in horses. The results may help to improve the reliability and reproducibility of study results in equine biomechanics.

Broader Estimates of Gastrocnemius Activity Generated a More Representative Cocontraction Index: A Study in Pediatric Population

The electromyography (EMG) cocontraction index (CCI) given by the antagonistic/agonistic Root Mean Square (RMS) amplitude ratio of the same muscle is a qualified biomarker used for spastic cocontraction quantification and management in cerebral palsy children. However, this normative EMG ratio is likely subject to a potential source of errors with biased estimates when measuring the gastrocnemius plantar flexors activity. Due to the uneven distribution of electrical activity within the muscle volume, cocontraction levels can be misestimated, if EMGs are obtained from the sole traditional bipolar sensor location recommended by SENIAM. This preliminary study, on 10 healthy children (mean age 10 yr), investigated whether surface EMG detected proximally and distally via two pairs of bipolar electrodes, within the medial gastrocnemius (MG), provides a significant difference in CCI estimates during non-dynamic (isometric dorsiflexion) and dynamic (swing phases of gait) conditions. Gait cycles were extracted from Inertial Measurement Unit sensors. Medial gastrocnemius activity was greater distally than proximally during plantar flexion when it acts as an agonist (~24±18%) and it was greater proximally during dorsiflexion (~23±9%) when it is acting as an antagonist. As a direct consequence, CCI estimates from the conventional sensor location were significantly different (~36%) from the CCIs computed by considering broader MG regions. This difference arose in all subjects during isometric efforts and in two of 10 healthy children during the swing phase of gait who presented cocontraction patterns ( [Formula: see text]). EMG bipolar sampling encompassing proximal and distal gastrocnemius muscle regions may reduce bias in CCI computation and provide a more representative and accurate cocontraction index that is especially important for comparisons to the diseased state.

Muscle Swelling of the Triceps Surae in Response to Straight-Leg and Bent-Leg Calf Raise Exercises in Young Women

ABSTRACT

Kassiano, W, Costa, B, Kunevaliki, G, Soares, D, Stavinski, N, Francsuel, J, Carneiro, MAS, Tricoli, I, Nunes, JP, Ribeiro, AS, and Cyrino, ES. Muscle swelling of the triceps surae in response to straight-leg and bent-leg calf raise exercises in young women. J Strength Cond Res 37(7): e438-e443, 2023-Triceps surae muscle swelling in response to different resistance exercises remains to be determined. This study compared the effects of straight-leg (STRA-leg) calf raise vs. bent-leg (BENT-leg) calf raise exercises on triceps surae muscle swelling. Seventeen young women (23.7 ± 4.0 years; 67.4 ± 16.0 kg; and 163.5 ± 7.2 cm) performed 2 resistance training sessions; in one, they performed the STRA-leg calf raise, and in another, they performed the BENT-leg calf raise. A randomized, cross-over, and counterbalanced design was adopted for this investigation. The subjects performed 4 sets of 20 repetitions maximum until concentric muscular failure. The muscle thickness of the gastrocnemius medial (GM), gastrocnemius lateral (GL), and soleus (SOL) was taken through B-mode ultrasound before and immediately after the calf raise exercises. STRA-leg calf raise elicited greater increases in muscle thickness of GM (+8.8% vs. -0.9%, p < 0.001) and GL (+14.5% vs. +7.0%, p < 0.001) than BENT-leg calf raise. Conversely, BENT-leg calf raise elicited greater increases in SOL muscle thickness than STRA-leg calf raise (+15.4% vs. +7.7%, p < 0.001). From a practical perspective, the STRA-leg calf raise should be preferred if the aim is stimulating the 3 muscles that comprise the triceps surae, whereas the BENT-leg calf raise should be prioritized if the target muscle is the SOL.

Design and validation of a finite element model of the aponeurotic and free Achilles tendon

The Achilles tendon (AT) is a common injury site. Ruptures are usually located in the free tendon but may cross the myotendinous junction into the aponeurotic region. Considering the possibility of aponeurotic region involvement in AT ruptures, a novel three dimensional (3D) finite element (FE) model that includes both the aponeurotic and free AT regions and features subtendon twisting and sliding was developed. It was hypothesized that the model would be able to predict in vivo data collected from the literature, thus being considered valid, and that model outputs would be most sensitive to subtendon twist configurations. The 3D model was constructed using magnetic resonance images. The model was divided into soleus and gastrocnemius subtendons. In addition to a frictionless contact condition, the interaction between subtendons was modeled using two contact formulations: sliding with anisotropic friction and no sliding. Loads were applied on the tendon's most proximal cross-section and anterior surface, with magnitudes estimated from in vivo studies. Model outputs were compared with experimental data regarding 3D deformation, transverse plane rotation, and nodal displacements in the free tendon. The FE model adequately simulated the free tendon behavior regarding longitudinal strain, cross-section area variation, transverse plane rotation, and sagittal nodal displacements, provided that subtendon sliding was allowed. The frictionless model exhibited noticeable medial transverse sliding of the soleus subtendon, which was present to a much lesser degree in the anisotropic friction model. Model outputs were most sensitive to variations in subtendon twist and dispersion of the collagen fiber orientations. Clinical Significance: This Achilles tendon finite element model, validated using in vivo experimental data, may be used to study its mechanical behavior, injury mechanisms, and rupture risk factors.

E-textile based modular sEMG suit for large area level of effort analysis

We present a novel design for an e-textile based surface electromyography (sEMG) suit that incorporates stretchable conductive textiles as electrodes and interconnects within an athletic compression garment. The fabrication and assembly approach is a facile combination of laser cutting and heat-press lamination that provides for rapid prototyping of designs in a typical research environment without need for any specialized textile or garment manufacturing equipment. The materials used are robust to wear, resilient to the high strains encountered in clothing, and can be machine laundered. The suit produces sEMG signal quality comparable to conventional adhesive electrodes, but with improved comfort, longevity, and reusability. The embedded electronics provide signal conditioning, amplification, digitization, and processing power to convert the raw EMG signals to a level-of-effort estimation for flexion and extension of the elbow and knee joints. The approach we detail herein is also expected to be extensible to a variety of other electrophysiological sensors.

Measuring Muscle Activity in Sprinters Using T2-Weighted Magnetic Resonance Imaging

PURPOSE

This study aimed to investigate the level of muscle activity during sprint running using T2-weighted magnetic resonance imaging.

METHODS

Fourteen male sprinters (age 21.2 [4.0] y; height 171.8 [4.2] cm, weight 65.5 [5.3] kg, 100-m personal record 11.01 [0.41] s; mean [SD]) performed 3 sets of three 60-m round-trip sprints. Before and after the round-trip sprints, 3 T magnetic resonance imaging scans were performed to obtain the T2 values of the 14 athletes' lower-extremity muscles.

RESULTS

After the 60-m round-trip sprints, the T2 value of the gluteus maximus, long head of biceps femoris, semitendinosus, semimembranosus, adductor brevis, adductor longus, adductor magnus, and gracilis increased significantly. The rate of change in the T2 values before and after the 60-m round-trip sprints was notably higher in the semitendinosus and gluteus maximus than in the other muscles.

CONCLUSIONS

These findings demonstrate the specific physiological metabolism of the lower-extremity muscles during fast sprinting. There are particularly high levels of muscle activity in the gluteus maximus and semitendinosus during sprint performance.

The effects of triceps surae muscle stimulation on localized Achilles subtendon tissue displacements

The triceps surae muscle-tendon unit is composed of the lateral and medial gastrocnemius (MG) and soleus (SOL) muscles and three in-series elastic 'subtendons' that form the Achilles tendon. Comparative literature and our own in vivo evidence suggest that sliding between adjacent subtendons may facilitate independent muscle actuation. We aim to more clearly define the relationship between individual muscle activation and subtendon tissue displacements. Here, during fixed-end contractions, electrical muscle stimulation controlled the magnitude of force transmitted via individual triceps surae muscles while ultrasound imaging recorded resultant subtendon tissue displacements. We hypothesized that MG and SOL stimulation would elicit larger displacements in their associated subtendon. Ten young adults completed four experimental activations at three ankle angles (-20, 0 and 20 deg) with the knee flexed to approximately 20 deg: MG stimulation (STIMMG), SOL stimulation (STIMSOL), combined stimulation, and volitional contraction. At 20 deg plantarflexion, STIMSOL elicited 49% larger tendon non-uniformity (SOL-MG subtendon tissue displacement) than that of STIMMG (P=0.004). For STIMSOL, a one-way post hoc ANOVA revealed a significant main effect of ankle angle (P=0.009) on Achilles tendon non-uniformity. However, peak tendon non-uniformity decreased by an average of 61% from plantarflexion to dorsiflexion, likely due to an increase in passive tension. Our results suggest that localized tissue displacements within the Achilles tendon respond in anatomically consistent ways to differential patterns of triceps surae muscle activation, but these relations are highly susceptible to ankle angle. This in vivo evidence points to at least some mechanical independence in actuation between the human triceps surae muscle-subtendon units.

Novel Insights Into Biarticular Muscle Actions Gained From High-Density Electromyogram

Biarticular muscles have traditionally been considered to exhibit homogeneous neuromuscular activation. The regional activation of biarticular muscles, as revealed from high-density surface electromyograms, seems however to discredit this notion. We thus hypothesize the regional activation of biarticular muscles may contribute to different actions about the joints they span. We then discuss the mechanistic basis and methodological implications underpinning our hypothesis.

Regional changes in muscle activity do not underlie the repeated bout effect in the human gastrocnemius muscle

The repeated bout effect (RBE) confers protection following exercise-induced muscle damage. Typical signs of this protective effect are significantly less muscle soreness and faster recovery of strength after the second bout. The aim of this study was to compare regional changes in medial gastrocnemius (MG) muscle activity and mechanical hyperalgesia after repeated bouts of eccentric exercise. Twelve healthy male participants performed two bouts of eccentric heel drop exercise (separated by 7 days) while wearing a vest equivalent to 20% of their body weight. High-density MG electromyographic amplitude maps and topographical pressure pain sensitivity maps were created before, two hours (2H), and two days (2D) after both exercise bouts. Statistical parametric mapping was used to identify RBE effects on muscle activity and mechanical hyperalgesia, using pixel-level statistics when comparing maps. The results revealed a RBE, as a lower strength loss (17% less; P < .01) and less soreness (50% less; P < .01) were found after the second bout. However, different muscle regions were activated 2H and 2D after the initial bout but not following the repeated bout. Further, no overall changes in EMG distribution or mechanical hyperalgesia were found between bouts. These results indicate that muscle activation is unevenly distributed during the initial bout, possibly to maintain muscle function during localized mechanical fatigue. However, this does not reflect a strategy to confer protection during the repeated bout by activating undamaged/non-fatigued muscle areas.

Myoelectric activity of the gastrocnemius during plantar flexion in a standing versus seated position and with a neutral or dorsiflexed ankle: A pilot study

OBJECTIVE

The purpose of this study was to investigate the surface electromyographic (EMG) activity of the medial (GM) and lateral (GL) heads of the gastrocnemius muscle during plantar flexion performed in standing or seated positions, and with a neutral or dorsiflexed ankle.

INTRODUCTION

The tríceps surae (gastrocnemius and soleus) is an essential muscle group that promotes propulsion during walking, running and tasks of daily living.

METHODS

Ten male volunteers (24.5 ± 3.6 years, weight: 75.8 ± 10 kg; height: 1.74 ± 0.07 m) performed ten repetitions of plantar flexion under four different conditions: a) standing position (SP); b) standing position with passive stretch of the gastrocnemius muscle (SPS); c) seated with the knee flexed (KF); d) seated with the knee flexed and a passive stretch of the gastrocnemius (KFS).

RESULTS

The muscle activity expressed as the root mean square (RMS) was significantly higher for the GL in the SPS versus the SP condition, while there was no difference for the GM between conditions. However, for the KF condition, GM activity was significantly higher versus the KFS condition, which was not verified for the GL.

CONCLUSIONS

Therefore, the standing position with passive stretching of the triceps surae may be more efficient in increasing the myoelectric activity of the GL. However, the sitting position without passive stretching promoted greater GM muscle recruitment. The length/tension relationship of gastrocnemius could be used as a prescription variable during rehabilitation or conditioning programs.

Changes in Muscle Morphology Following Gastrocnemius Recession for Achilles Tendinopathy: A Prospective Cohort Imaging Study

Background. The purpose of this study was to evaluate changes in posterior compartment muscle volume and intramuscular fat content following gastrocnemius recession in people with Achilles tendinopathy (AT). Methods. Eight patients diagnosed with unilateral recalcitrant AT and an isolated gastrocnemius contracture participated in this prospective cohort study. Magnetic resonance imaging was performed on both limbs of each participant before and 6 months following an isolated gastrocnemius recession. Involved limb muscle volumes and fat fractions (FFs) of the medial gastrocnemius, lateral gastrocnemius, and soleus muscle were normalized to the uninvolved limb. Preoperative to postoperative comparisons were made with Wilcoxon signed-rank tests. Results. Soleus or lateral gastrocnemius muscle volumes or FFs were not significantly different between study time points. A significant difference was found in medial gastrocnemius muscle volume (decrease; P = .012) and FF (increase; P = .017). Conclusion. A major goal of the Strayer gastrocnemius recession, selective lengthening of the posterior compartment while preserving soleus muscle morphology, was supported. The observed changes isolated to the medial gastrocnemius muscle may reduce ankle plantarflexion torque capacity. Study findings may help inform selection of surgical candidates, refine anticipated outcomes, and better direct postoperative rehabilitation following gastrocnemius recession for AT.Levels of Evidence: Level IV: Prospective cohort study.

T2 Distribution in the Forearm Muscles and the T2 Threshold for Defining Activated Muscle

Purpose:

In order to detect exercised muscles by the increase in T₂, we have defined a Gaussian T₂ distribution and reference values (T_2r and SD_r) in resting state muscles, and a threshold for detecting exercised muscles.

Methods:

The subjects were healthy adult volunteers (14 males and 12 females). Multiple-spin-echo (MSE) MR images were obtained with 10 TE values from 10 to 100 ms using a 0.2T MRI system. T₂ values for 10 forearm muscles were obtained in the resting state and after isometric wrist flexion exercise with 5%, 15%, and 25% of the maximum voluntary contraction (MVC). Z values were obtained by (T_2e − T_2r)/SD_r, where T_2e was T₂ after exercise. Based on sample size calculations, three thresholds (Z_T = 1.00, 2.56, and 3.07) were applied to agonist and antagonist muscles.

Results:

A normal distribution of T₂ was detected in resting muscles at 34 ± 3 ms (mean ± standard deviation [SD]) in 26 subjects using the Kolmogorov–Smirnov test, the Shapiro–Wilk test, and the Jarque–Bera test (P > 0.05). No gender differences were shown between the T₂ or SD, and a similar result was obtained in 12 measurements on a single subject (P < 0.01). The T_2r and SD_r were used for reference values. The threshold Z_T = 1.00 showed the highest sensitivity (0.86) even with 5% MVC, but it showed a lower specificity (0.85) than the other thresholds. Z_T = 3.07 showed the highest specificity (1.0), but it showed a lower sensitivity (0.36) with the 5% MVC, compared with Z_T = 2.56 (0.50). The receiver operating characteristics analysis also supported these results.

Conclusion:

We found that the T₂ distribution in muscles was Gaussian, suggesting that a one-sample t-test can be applied, and that Z_T = 2.56 could cover low-intensity exercise with high specificity and a low false-positive rate.

Relationship between Achilles tendon length and running performance in well-trained male endurance runners

This study aimed to determine the relationship between Achilles tendon (AT) length and running performance, including running economy, in well-trained endurance runners. We also examined the reasonable portion of the AT related to running performance among AT lengths measured in three different portions. The AT lengths at three portions and cross-sectional area (CSA) of 30 endurance runners were measured using magnetic resonance imaging. Each AT length was calculated as the distance from the calcaneal tuberosity to the muscle-tendon junction of the soleus, gastrocnemius medialis (GM_AT ), and gastrocnemius lateralis, respectively. These AT lengths were normalized with shank length. The AT CSA was calculated as the average of 10, 20, and 30 mm above the distal insertion of the AT and normalized with body mass. Running economy was evaluated by measuring energy cost during three 4-minutes submaximal treadmill running trials at 14, 16, and 18 km/h, respectively. Among three AT lengths, only a GM_AT correlated significantly with personal best 5000-m race time (r=-.376, P=.046). Furthermore, GM_AT correlated significantly with energy cost during submaximal treadmill running trials at 14 km/h and 18 km/h (r=-.446 and -.429, respectively, P<.05 for both), and a trend toward such significance was observed at 16 km/h (r=-.360, P=.050). In contrast, there was no correlation between AT CSA and running performance. These findings suggest that longer AT, especially GM_AT , may be advantageous to achieve superior running performance, with better running economy, in endurance runners.

Optimization of surface electrodes location for H-reflex recordings in soleus muscle

The Hoffmann reflex (H reflex) is extensively used to investigate the spinal motor neuron excitability in healthy and pathological subjects. Obtaining a representative and robust amplitude estimation of the H reflex is of marked relevance in clinical as well as in research applications. As for the motor responses, this issue is strictly related to the electrode positioning, especially for large, pinnate muscles such as the triceps surae. In this study we investigated the effect of electrode position on soleus H-reflex amplitude. A grid of 96 electrodes was used to identify maximal H reflexes (Hmax) across the whole soleus region available for surface recording. The spatial distribution of Hmax amplitude detected in monopolar and single-differential derivations was used to determine where greatest reflex responses were detected from soleus. For both derivations and for all participants, largest Hmax were detected consistently over the central soleus region, in correspondence of the muscle superficial aponeurosis. Indeed, the amplitude of Hmax provided by conventional electrodes (1cm² area, 2cm apart) located centrally was significantly greater (median: 35% for monopolar and 79% for single-differential derivations) than that obtained medially, where surface electromyograms are typically recorded from soleus. Computer simulations, used to assist in the interpretation of results, suggest the soleus pinnate architecture was the key determinant of the medio-lateral variability observed for the experimental Hmax. The presented results provide a clear indication for electrode positioning, of crucial relevance in applied studies aimed at eliciting H reflexes.

Behavior of human gastrocnemius muscle fascicles during ramped submaximal isometric contractions

Precise estimates of muscle architecture are necessary to understand and model muscle mechanics. The primary aim of this study was to estimate continuous changes in fascicle length and pennation angle in human gastrocnemius muscles during ramped plantar flexor contractions at two ankle angles. The secondary aim was to determine whether these changes differ between proximal and distal fascicles. Fifteen healthy subjects performed ramped contractions (0–25% MVC) as ultrasound images were recorded from the medial (MG, eight sites) and lateral (LG, six sites) gastrocnemius muscle with the ankle at 90° and 120° (larger angles correspond to shorter muscle lengths). In all subjects, fascicles progressively shortened with increasing torque. MG fascicles shortened 5.8 mm (11.1%) at 90° and 4.5 mm (12.1%) at 120°, whereas LG muscle fascicles shortened 5.1 mm (8.8%) at both ankle angles. MG pennation angle increased 1.4° at 90° and 4.9° at 120°, and LG pennation angle decreased 0.3° at 90° and increased 2.6° at 120°. Muscle architecture changes were similar in proximal and distal fascicles at both ankle angles. This is the first study to describe continuous changes in fascicle length and pennation angle in the human gastrocnemius muscle during ramped isometric contractions. Very similar changes occurred in proximal and distal muscle regions. These findings are relevant to studies modeling active muscle mechanics.

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

KEY POINTS

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low-threshold units preferentially located distally. We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion). The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action. The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

ABSTRACT

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories, with low-threshold units preferentially located distally. In this study, subjects (n = 8) performed ramped and sustained isometric contractions (ankle plantar flexion and knee flexion; range: ∼1-40% maximal voluntary contraction) and we measured MU territory size with spike-triggered averages from fine-wire electrodes inserted along the length (seven electrodes) or across the width (five electrodes) of the MG muscle. Of 69 MUs identified along the length of the muscle, 32 spanned at least half the muscle length (≥ 6.9 cm), 11 of which spanned all recording sites (13.6-17.9 cm). Distal fibres had smaller pennation angles (P < 0.05), which were accompanied by larger territories in MUs with fibres located distally (P < 0.05). There was no distal-to-proximal pattern of muscle activation in ramp contraction (P = 0.93). Of 36 MUs identified across the width of the muscle, 24 spanned at least half the muscle width (≥ 4.0 cm), 13 of which spanned all recording sites (8.0-10.8 cm). MUs were not localized (length or width) based on recruitment threshold or contraction type, nor was there a relationship between MU territory size and recruitment threshold (Spearman's rho = -0.20 and 0.13, P > 0.18). MUs in the human MG have larger territories than previously reported and are not localized based on recruitment threshold or joint action. This indicates that the CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Mapping of spatial and temporal heterogeneity of plantar flexor muscle activity during isometric contraction: correlation of velocity-encoded MRI with EMG

The aim of this study was to assess the correlation between contraction-associated muscle kinematics as measured by velocity-encoded phase-contrast (VE-PC) magnetic resonance imaging (MRI) and activity recorded via electromyography (EMG), and to construct a detailed three-dimensional (3-D) map of the contractile behavior of the triceps surae complex from the MRI data. Ten axial-plane VE-PC MRI slices of the triceps surae and EMG data were acquired during submaximal isometric contractions in 10 subjects. MRI images were analyzed to yield the degree of contraction-associated muscle displacement on a voxel-by-voxel basis and determine the heterogeneity of muscle movement within and between slices. Correlational analyses were performed to determine the agreement between EMG data and displacements. Pearson's coefficients demonstrated good agreement (0.84 < r < 0.88) between EMG data and displacements. Comparison between different slices in the gastrocnemius muscle revealed significant heterogeneity in displacement values both in-plane and along the cranio-caudal axis, with highest values in the mid-muscle regions. By contrast, no significant differences between muscle regions were found in the soleus muscle. Substantial differences among displacements were also observed within slices, with those in static areas being only 17-39% (maximum) of those in the most mobile muscle regions. The good agreement between EMG data and displacements suggests that VE-PC MRI may be used as a noninvasive, high-resolution technique for quantifying and modeling muscle activity over the entire 3-D volume of muscle groups. Application to the triceps surae complex revealed substantial heterogeneity of contraction-associated muscle motion both within slices and between different cranio-caudal positions.

Variations in the spatial distribution of the amplitude of surface electromyograms are unlikely explained by changes in the length of medial gastrocnemius fibres with knee joint angle

This study investigates whether knee position affects the amplitude distribution of surface electromyogram (EMG) in the medial gastrocnemius (MG) muscle. Of further concern is understanding whether knee-induced changes in EMG amplitude distribution are associated with regional changes in MG fibre length. Fifteen surface EMGs were acquired proximo-distally from the MG muscle while 22 (13 male) healthy participants (age range: 23–47 years) exerted isometric plantar flexion at 60% of their maximal effort, with knee fully extended and at 90 degrees flexion. The number of channels providing EMGs with greatest amplitude, their relative proximo-distal position and the EMG amplitude averaged over channels were considered to characterise changes in myoelectric activity with knee position. From ultrasound images, collected at rest, fibre length, pennation angle and fat thickness were computed for MG proximo-distal regions. Surface EMGs detected with knee flexed were on average five times smaller than those collected during knee extended. However, during knee flexed, relatively larger EMGs were detected by a dramatically greater number of channels, centred at the MG more proximal regions. Variation in knee position at rest did not affect the proximo-distal values obtained for MG fibre length, pennation angle and fat thickness. Our main findings revealed that, with knee flexion: i) there is a redistribution of activity within the whole MG muscle; ii) EMGs detected locally unlikely suffice to characterise the changes in the neural drive to MG during isometric contractions at knee fully extended and 90 degrees flexed positions; iii) sources other than fibre length may substantially contribute to determining the net, MG activation.

Spatial variability of muscle activity during human walking: the effects of different EMG normalization approaches

Human leg muscles are often activated inhomogeneously, e.g. in standing. This may also occur in complex tasks like walking. Thus, bipolar surface electromyography (sEMG) may not accurately represent whole muscle activity. This study used 64-electrode high-density sEMG (HD-sEMG) to examine spatial variability of lateral gastrocnemius (LG) muscle activity during the stance phase of walking, maximal voluntary contractions (MVCs) and maximal M-waves, and determined the effects of different normalization approaches on spatial and inter-participant variability. Plantar flexion MVC, maximal electrically elicited M-waves and walking at self-selected speed were recorded in eight healthy males aged 24-34. sEMG signals were assessed in four ways: unnormalized, and normalized to MVC, M-wave or peak sEMG during the stance phase of walking. During walking, LG activity varied spatially, and was largest in the distal and lateral regions. Spatial variability fluctuated throughout the stance phase. Normalizing walking EMG signals to the peak value during stance reduced spatial variability within LG on average by 70%, and inter-participant variability by 67%. Normalizing to MVC reduced spatial variability by 17% but increased inter-participant variability by 230%. Normalizing to M-wave produced the greatest spatial variability (45% greater than unnormalized EMG) and increased inter-participant variability by 70%. Unnormalized bipolar LG sEMG may provide misleading results about representative muscle activity in walking due to spatial variability. For the peak value and MVC approaches, different electrode locations likely have minor effects on normalized results, whereas electrode location should be carefully considered when normalizing walking sEMG data to maximal M-waves.

High-field (11.75T) multimodal MR imaging of exercising hindlimb mouse muscles using a non-invasive combined stimulation and force measurement device

We have designed and constructed an experimental set-up allowing electrical stimulation of hindlimb mouse muscles and the corresponding force measurements at high-field (11.75T). We performed high-resolution multimodal MRI (including T2 -weighted imaging, angiography and diffusion) and analysed the corresponding MRI changes in response to a stimulation protocol. Mice were tested twice over a 1-week period to investigate the reliability of mechanical measurements and T2 changes associated with the stimulation protocol. Additionally, angiographic images were obtained before and immediately after the stimulation protocol. Finally, multislice diffusion imaging was performed before, during and immediately after the stimulation session. Apparent diffusion coefficient (ADC) maps were calculated on the basis of diffusion weighted images (DWI). Both force production and T2 values were highly reproducible as illustrated by the low coefficient of variation (<8%) and high intraclass correlation coefficient (≥0.75) values. Maximum intensity projection angiographic images clearly showed a strong vascular effect resulting from the stimulation protocol. Although a motion sensitive imaging sequence was used (echo planar imaging) and in spite of the strong muscle contractions, motion artifacts were minimal for DWI recorded under exercising conditions, thereby underlining the robustness of the measurements. Mean ADC values increased under exercising conditions and were higher during the recovery period as compared with the corresponding control values. The proposed experimental approach demonstrates accurate high-field multimodal MRI muscle investigations at a preclinical level which is of interest for monitoring the severity and/or the progression of neuromuscular diseases but also for assessing the efficacy of potential therapeutic interventions.

Nonuniform muscle hypertrophy: its relation to muscle activation in training session

PURPOSE

Muscle hypertrophy in response to resistance training has been reported to occur nonuniformly along the length of the muscle. The purpose of the present study was to examine whether the regional difference in muscle hypertrophy induced by a training intervention corresponds to the regional difference in muscle activation in the training session.

METHODS

Twelve young men participated in a training intervention program for the elbow extensors with a multijoint resistance exercise for 12 wk (3 d · wk(-1)). Before and after the intervention, cross-sectional areas of the triceps brachii along its length were measured with magnetic resonance images. A series of transverse relaxation time (T2)-weighted magnetic resonance images was recorded before and immediately after the first session of training intervention. The T2 was calculated for each pixel within the triceps brachii. In the images recorded after the session, the number of pixels with a T2 greater than the threshold (mean + 1 SD of T2 before the session) was expressed as the ratio to the whole number of pixels within the muscle and used as an index of muscle activation (percent activated area).

RESULTS

The percent activated area of the triceps brachii in the first session was significantly higher in the middle regions than that in the most proximal region. Similarly, the relative change in cross-sectional area induced by the training intervention was also significantly greater in the middle regions than the most proximal region.

CONCLUSION

The results suggest that nonuniform muscle hypertrophy after training intervention is due to the region-specific muscle activation during the training session.

Plantarflexor muscle function in healthy and chronic Achilles tendon pain subjects evaluated by the use of EMG and PET imaging

BACKGROUND

Achilles tendon pathologies may alter the coordinative strategies of synergistic calf muscles. We hypothesized that both surface electromyography and positron emission tomography would reveal differences between symptomatic and asymptomatic legs in Achilles tendinopathy patients and between healthy controls.

METHODS

Eleven subjects with unilateral chronic Achilles tendon pain (28 years) and eleven matched controls (28 years) were studied for triceps surae and flexor hallucis longus muscle activity in response to repetitive isometric plantarflexion tasks performed at 30% of maximal voluntary contraction using surface electromyography and glucose uptake using positron emission tomography. Additionally, Achilles tendon glucose uptake was quantified.

FINDINGS

Normalized myoelectric activity of soleus was higher (P<0.05) in the symptomatic leg versus the contralateral and control legs despite lower absolute force level maintained (P<0.005). Electromyography amplitude of flexor hallucis longus was also greater on the symptomatic side compared to the healthy leg (P<0.05). Both the symptomatic and asymptomatic legs tended to have higher glucose uptake compared to the control legs (overall effect size: 0.9 and 1.3, respectively). Achilles tendon glucose uptake was greater in both legs of the patient group (P<0.05) compared to controls. Maximal plantarflexion force was ~14% greater in the healthier leg compared to the injured leg in the patient group.

INTERPRETATIONS

While the electromyography showed greater relative amplitude in the symptomatic leg, the results based on muscle glucose uptake suggested relatively similar behavior of both legs in the patient group. Higher glucose uptake in the symptomatic Achilles tendon suggests a higher metabolic demand.

Influence of knee flexion angle and age on triceps surae muscle fatigue during heel raises

The triceps surae (TS) muscle-tendon unit is 1 of the most commonly injured in elite and recreational athletes, with a high prevalence in middle-aged adults. The performance of maximal numbers of unilateral heel raises is used to assess, train, and rehabilitate TS endurance and conventionally prescribed in 0° knee flexion (KF) for the gastrocnemius and 45° for the soleus (SOL). However, the extent of muscle selectivity conferred through the change in the knee angle is lacking for heel raises performed to volitional fatigue. This study investigated the influence of knee angle on TS muscle fatigue during heel raises and determined whether fatigue differed between middle-aged and younger-aged adults. Forty-eight healthy individuals aged 18-25 and 35-45 years performed maximal numbers of unilateral heel raises in 0° and 45° KF. Median frequencies and linear regression slopes were calculated from the SOL, gastrocnemius medialis (GM), and gastrocnemius lateralis (GL) surface electromyographic signals. Stepwise mixed-effect regressions were used for analysis. The subjects completed an average of 45 and 48 heel raises in 0° and 45° KF, respectively. The results indicated that the 3 muscles fatigued during testing as all median frequencies decreased, and regression slopes were negative. Consistent with muscle physiology and fiber typing, fatigue was greater in the GM and GL than in the SOL (p < 0.001). However, knee angle did not influence TS muscle fatigue parameters (p = 0.814), with similar SOL, GM, and GL fatigue in 0° and 45° KF. These findings are in contrast with the traditionally described clinical use of heel raises in select knee angles for the gastrocnemius and the SOL. Furthermore, no difference in TS fatigue between the 2 age groups was able to be determined, despite the reported higher prevalence of injury in middle-aged individuals.

How much does the human medial gastrocnemius muscle contribute to ankle torques outside the sagittal plane?

Ankle movements in the frontal plane are less prominent though not less relevant than movements in the plantar or dorsal flexion direction. Walking on uneven terrains and standing on narrow stances are examples of circumstances likely imposing marked demands on the ankle medio-lateral stabilization. Following our previous evidence associating lateral bodily sways in quiet standing to activation of the medial gastrocnemius (MG) muscle, in this study we ask: how large is the MG contribution to ankle torque in the frontal plane? By arranging stimulation electrodes in a selective configuration, current pulses were applied primarily to the MG nerve branch of ten subjects. The contribution of populations of MG motor units of progressively smaller recruitment threshold to ankle torque was evaluated by increasing the stimulation amplitude by fixed amounts. From smallest intensities (12–32 mA) leading to the firstly observable MG twitches in force-plate recordings, current pulses reached intensities (56–90 mA) below which twitches in other muscles could not be observed from the skin. Key results showed a substantial MG torque contribution tending to rotate upward the foot medial aspect (ankle inversion). Nerve stimulation further revealed a linear relationship between the peak torque of ankle plantar flexion and inversion, across participants (Pearson R > .81, p < .01). Specifically, regardless of the current intensity applied, the peak torque of ankle inversion amounted to about 13% of plantar flexion peak torque. Physiologically, these results provide experimental evidence that MG activation may contribute to stabilize the body in the frontal plane, especially under situations of challenged stability.

Lumbar muscle dysfunction during remission of unilateral recurrent nonspecific low-back pain: evaluation with muscle functional MRI

OBJECTIVES

After cessation of a low-back pain (LBP) episode, alterations in trunk muscle behavior, despite recovery from pain, have been hypothesized to play a pathogenic role in the recurrence of LBP. This study aimed to identify the presence of lumbar muscle dysfunction during the remission of recurrent LBP, while performing a low-load trunk-extension movement.

METHODS

Thirteen participants with unilateral recurrent LBP were tested at least 1 month after cessation of the previous LBP episode and were compared with a healthy control group without any history of LBP (n=13). Also, differences between previously painful and nonpainful sides were examined. Muscle functional magnetic resonance imaging, based on quantitative T2-imaging, was used to examine muscle tissue characteristics (T2 rest) and muscle recruitment (T2 shift) during prone trunk extension. The lumbar multifidus, erector spinae, quadratus lumborum, and psoas were bilaterally visualized on 2 lumbar levels using a T2-weighted (spin-echo multicontrast) magnetic resonance imaging sequence.

RESULTS

Linear mixed model analysis revealed a significantly lower T2 rest (P=0.044) and a significantly higher T2 shift (P=0.034) solely for the multifidus in the LBP group compared with the control group. No significant differences between pain sides were found.

DISCUSSION

Lower T2-rest values have been suggested to correlate with a conversion of the multifidus' fiber typing toward the glycolytic muscle spectrum. Elevated T2 shifts correspond with increased levels of metabolic activity in the multifidus in the LBP group, for which several hypotheses can be put forward. Taken together, these findings provide evidence of concurrent alterations in the multifidus structure and activity in individuals with unilateral recurrent LBP, despite being pain free and functionally recovered.

Heterogeneous atrophy occurs within individual lower limb muscles during 60 days of bed rest

To better understand disuse muscle atrophy, via magnetic resonance imaging, we sequentially measured muscle cross-sectional area along the entire length of all individual muscles from the hip to ankle in nine male subjects participating in 60-day head-down tilt bed rest (2nd Berlin BedRest Study; BBR2-2). We hypothesized that individual muscles would not atrophy uniformly along their length such that different regions of an individual muscle would atrophy to different extents. This hypothesis was confirmed for the adductor magnus, vasti, lateral hamstrings, medial hamstrings, rectus femoris, medial gastrocnemius, lateral gastrocnemius, tibialis posterior, flexor hallucis longus, flexor digitorum longus, peroneals, and tibialis anterior muscles (P ≤ 0.004). In contrast, the hypothesis was not confirmed in the soleus, adductor brevis, gracilis, pectineus, and extensor digitorum longus muscles (P ≥ 0.20). The extent of atrophy only weakly correlated (r = -0.30, P < 0.001) with the location of greatest cross-sectional area. The rate of atrophy during bed rest also differed between muscles (P < 0.0001) and between some synergists. Most muscles recovered to their baseline size between 14 and 90 days after bed rest, but flexor hallucis longus, flexor digitorum longus, and lateral gastrocnemius required longer than 90 days before recovery occurred. On the basis of findings of differential atrophy between muscles and evidence in the literature, we interpret our findings of intramuscular atrophy to reflect differential disuse of functionally different muscle regions. The current work represents the first lower-limb wide survey of intramuscular differences in disuse atrophy. We conclude that intramuscular differential atrophy occurs in most, but not all, of the muscles of the lower limb during prolonged bed rest.

Muscle functional MRI as an imaging tool to evaluate muscle activity

Muscle functional magnetic resonance imaging (mfMRI) is an innovative technique that offers a noninvasive method to quantify changes in muscle physiology following the performance of exercise. The mfMRI technique is based on signal intensity changes due to increases in the relaxation time of tissue water. In contemporary practice, mfMRI has proven to be an excellent tool for assessing the extent of muscle activation following the performance of a task and for the evaluation of neuromuscular adaptations as a result of therapeutic interventions. This article focuses on the underlying mechanisms and methods of mfMRI, discusses the validity and advantages of the method, and provides an overview of studies in which mfMRI is used to evaluate the effect of exercise and exercise training on muscle activity in both experimental and clinical studies.

Are the myoelectric manifestations of fatigue distributed regionally in the human medial gastrocnemius muscle?

Myoelectric fatigue typically manifests as variations in the amplitude and spectrum of surface electromyograms (EMGs). Interestingly, these variations seem to be represented locally in different muscles. In this study, we ask whether such a regional distribution of myoelectric fatigue extends to the medial gastrocnemius (MG) muscle. If the MG muscle is activated locally during fatiguing contractions, or if the most fatigable MG fibers are located at distinct muscle regions, then, the myoelectric manifestations of MG fatigue are expected to appear locally in a grid of surface electrodes. With a matrix of surface electrodes (7×15 single-differential EMGs) we show that myoelectric fatigue, indeed, manifests regionally in the MG muscle of 12 subjects, who exerted intermittent, fatiguing plantar flections at 50% of their maximal effort. Contrary to the root mean square amplitude, the median frequency of surface EMGs varied consistently across subjects throughout the plantar flections (P=0.002). On average, changes in EMG spectrum were represented at 78-93 (interquartile interval) out of the 105 channels in the matrix, though with different degrees across channels. For all participants, about 29% of the channels detected significantly greater reductions in median frequency when compared to all channels in the matrix (P<0.003). Strikingly, these channels were not sparsely distributed; they rather occupied localized skin regions across subjects. Physiologically, our results suggest that, during sub-maximal fatiguing tasks, myoelectric manifestations of MG fatigue are represented in spatially localized muscle regions. Technically, the possibility of studying myoelectric fatigue in the MG muscle appears to depend on the electrode location.

Isometric fatigue patterns in time and time-frequency domains of triceps surae muscle in different knee positions

The occurrence of fatigue in triceps surae (TS) muscles during sustained plantar flexion contraction is investigated by means of the RMS electromyogram (EMG) and the instantaneous median frequency (IMF) of the short time Fourier transform (STFT). Six male subjects realized a 40% maximal plantar flexion isometric voluntary contraction until fatigue in two knee positions. Electrodes were positioned on gastrocnemius medialis, gastrocnemius lateralis and soleus muscles. The torque (TO) and EMG signals were synchronized. The RMS and the median of the IMF values were obtained, respectively, for each 250 ms and 1s windows of signal. Each signal was segmented into 10 epochs, from which the mean values of IMF, RMS and TO were obtained and submitted to linear regressions to determine parameter trends. Friedman test with the Dunn's post hoc were used to test for differences among muscles activation for each knee position and among slopes of regression curves, as well as to observe changes in TS RMS values over time. The results indicate different activation strategies with the knee extended (KE) in contrast to knee flexed (KF). With the KE, the gastrocnemii showed typical fatigue behavior with significant (p<0.05) IMF reductions and RMS increases over time, while soleus showed concomitant RMS and IMF increases (p<0.05) suggesting an increased soleus contribution to the torque production. With KF, the gastrocnemii were under activated, increasing the role of soleus. Thus, time-frequency analysis represented an important tool for TS muscular fatigue evaluation, allowing differentiates the role of soleus muscle.

Evidence that a higher ATP cost of muscular contraction contributes to the lower mechanical efficiency associated with COPD: preliminary findings

Impaired metabolism in peripheral skeletal muscles potentially contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). We used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine the energy cost and skeletal muscle energetics in six patients with COPD during dynamic plantar flexion exercise compared with six well-matched healthy control subjects. Patients with COPD displayed a higher energy cost of muscle contraction compared with the controls (control: 6.1 ± 3.1% of rest·min(-1)·W(-1), COPD: 13.6 ± 8.3% of rest·min(-1)·W(-1), P = 0.01). Although, the initial phosphocreatine resynthesis rate was also significantly attenuated in patients with COPD compared with controls (control: 74 ± 17% of rest/min, COPD: 52 ± 13% of rest/min, P = 0.04), when scaled to power output, oxidative ATP synthesis was similar between groups (6.5 ± 2.3% of rest·min(-1)·W(-1) in control and 7.8 ± 3.9% of rest·min(-1)·W(-1) in COPD, P = 0.52). Therefore, our results reveal, for the first time that in a small subset of patients with COPD a higher ATP cost of muscle contraction may substantially contribute to the lower mechanical efficiency previously reported in this population. In addition, it appears that some patients with COPD have preserved mitochondrial function and normal energy supply in lower limb skeletal muscle.

Quantitative analysis of masticatory activity during unilateral mastication using muscle fMRI

OBJECTIVE

Quantitative analysis of the activities of all masticatory muscles is required to elucidate the mechanism of stomatognathic dysfunction. Electromyography can be used to record the activity of masticatory muscles, but quantification of the overall activity of every masticatory muscle has not been accomplished because of methodological limitations. In this study, we used muscle functional magnetic resonance imaging for simultaneous quantification of the overall activities of the masseter, medial pterygoid and lateral pterygoid muscles during unilateral gum chewing.

METHODS

Seven healthy male volunteers participated in the study. We evaluated changes in the mean proton transverse relaxation time in the bilateral masseter, medial pterygoid and lateral pterygoid muscles before and after unilateral gum chewing, and to quantify the overall activity of these muscles simultaneously during unilateral gum chewing.

RESULTS

After 5 min of chewing, the activity of the ipsilateral masseter was highest among the six muscles, followed by the ipsilateral medial pterygoid, contralateral lateral pterygoid and contralateral masseter muscles.

CONCLUSION

These results affirm the importance of the ipsilateral masseter muscle and quantitatively demonstrate the important contribution of the ipsilateral medial pterygoid and contralateral lateral pterygoid muscles during unilateral mastication.

Postural activation of the human medial gastrocnemius muscle: are the muscle units spatially localised?

In cat medial gastrocnemius (MG), fibres supplied by individual motoneurones (muscle units) distribute extensively along the muscle longitudinal axis. In the human MG, the size of motor unit territory is unknown. It is uncertain if the absolute size of muscle unit territory or the size relative to the whole muscle is most comparable with the cat. By comparing intramuscular and surface electromyograms we tested whether muscle units extend narrowly or widely along the human MG muscle. Due to the pennation of the MG, if individual motoneurones supply fibres scattered along the muscle, then action potentials of single motor units are expected to appear sparsely on the surface of the skin. In nine healthy subjects, pairs of wire electrodes were inserted in three locations along the MG muscle (MG60%, MG75% and MG90%). A longitudinal array of 16 surface electrodes was positioned alongside the intramuscular electrodes. While subjects stood quietly, 55 motor units were identified, of which, significantly more units were detected in the most distal sites. The surface action potentials had maximum amplitude at 4.40 ±1.67 (mean±S.D.), 8.02±2.16 and 11.63±2.09 cm (P <0.001) from the most proximal surface electrode, for motor units in the MG60%, MG75% and MG90% locations, respectively. Single motor unit potentials were recorded by five consecutive surface electrodes, at most, indicating that muscle units extend shortly along the MG longitudinal axis. It is concluded that relative to the whole muscle, and compared with the cat, muscle units in human MG are localised. The localisation of muscle units might have implications for the regional control of muscle activity.

Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles

Studies using animal models have been unable to determine the mechanical stimuli that most influence muscle architectural adaptation. We examined the influence of contraction mode on muscle architectural change in humans, while also describing the time course of its adaptation through training and detraining. Twenty-one men and women performed slow-speed (30 degrees /s) concentric-only (Con) or eccentric-only (Ecc) isokinetic knee extensor training for 10 wk before completing a 3-mo detraining period. Fascicle length of the vastus lateralis (VL), measured by ultrasonography, increased similarly in both groups after 5 wk (Delta(Con) = +6.3 +/- 3.0%, Delta(Ecc) = +3.1 +/- 1.6%, mean = +4.7 +/- 1.7%; P < 0.05). No further increase was found at 10 wk, although a small increase (mean approximately 2.5%; not significant) was evident after detraining. Fascicle angle increased in both groups at 5 wk (Delta(Con) = +11.1 +/- 4.0%, Delta(Ecc) = +11.9 +/- 5.4%, mean = 11.5 +/- 3.2%; P < 0.05) and 10 wk (Delta(Con) = +13.3 +/- 3.0%, Delta(Ecc) = +21.4 +/- 6.9%, mean = 17.9 +/- 3.7%; P < 0.01) in VL only and remained above baseline after detraining (mean = 13.2%); smaller changes in vastus medialis did not reach significance. The similar increase in fascicle length observed between the training groups mitigates against contraction mode being the predominant stimulus. Our data are also strongly indicative of 1) a close association between VL fascicle length and shifts in the torque-angle relationship through training and detraining and 2) changes in fascicle angle being driven by space constraints in the hypertrophying muscle. Thus muscle architectural adaptations occur rapidly in response to resistance training but are strongly influenced by factors other than contraction mode.

Leg muscles differ in spatial activation patterns with differing levels of voluntary plantarflexion activity in humans

BACKGROUND/AIMS

The purpose of this study was to investigate the differential activity between and within individual muscles commonly grouped as plantarflexors. Much of the previous information gathered on plantarflexor activity has been attained using electromyographic recordings. In this study, we used magnetic resonance imaging which allowed us to look at spatial differences in activation.

METHODS

Twenty-two human subjects exercised under four different conditions - combinations of loads of 25 or 65% of maximum voluntary contraction (MVC) and the direction of plantarflexion at a sagittal and off-sagittal angle. Before and after each exercise condition, T2-weighted magnetic resonance images were collected. Regions of interest were drawn around the lateral gastrocnemius (LG), medial gastrocnemius (MG), soleus (SOL), peroneus longus (PER) and tibialis anterior (TA) muscles and analyzed for differences.

RESULTS

Significant increases in T2 relaxation times during 25% MVC conditions were found for PER and, during the 65% MVC, for all four muscles considered plantarflexors (LG, MG, SOL, PER). No significant differences were found between sagittal and off-sagittal conditions. Within LG and MG, greater increases in T2 times with exercise were found in proximal regions compared with distal regions.

CONCLUSION

These results are consistent with suggestions that individual members of muscle groups are capable of differential activity and that for at least some muscles, such differential activity may exist within subvolumes of individual muscles.

Mapping activation levels of skeletal muscle in healthy volunteers: an MRI study

PURPOSE

To use muscle functional MRI (mfMRI) to compare activation levels within and among triceps surae (TS) muscles.

MATERIALS AND METHODS

Seven healthy males performed five sets of 10 repetitions of a unilateral heel-raise exercise. T2-weighted images were obtained before and immediately after the exercise. Pixels that showed T2 greater than the mean +1 SD of the region of interest (ROI) in pre-exercise images and lower than the mean +1 SD of the ROI in post-exercise images were identified. The remaining T2 values in the post-exercise images were assigned to five categories indicated by color: red (highest level of activation), yellow, green, sky blue, and blue (lowest level of activation). The images were then used to construct three-dimensional (3D) images from which the volumes at each level of activation were determined.

RESULTS

Within each of the TS muscles the % activated volumes with low and moderate levels of activation were larger than those with a high level of activation (P < 0.05). The % activated volumes with a high level of activation were larger in the medial gastrocnemius than the soleus (Sol; P < 0.05). The Sol had a larger % activated volume with a low level of activation than the lateral gastrocnemius (P < 0.05). Each activation level was nonuniformly distributed along the length within each TS muscle.

CONCLUSION

There is substantial variation in the level of activation within and among the TS muscles; however, the activation level is mainly in the moderate to low range in all three muscles.

Oxygen cost of dynamic or isometric exercise relative to recruited muscle mass

BACKGROUND

Oxygen cost of different muscle actions may be influenced by different recruitment and rate coding strategies. The purpose of this study was to account for these strategies by comparing the oxygen cost of dynamic and isometric muscle actions relative to the muscle mass recruited via surface electrical stimulation of the knee extensors.

METHODS

Comparisons of whole body pulmonary delta VO2 were made in seven young healthy adults (1 female) during 3 minutes of dynamic or isometric knee extensions, both induced by surface electrical stimulation. Recruited mass was quantified in T2 weighted spin echo magnetic resonance images.

RESULTS

The delta VO2 for dynamic muscle actions, 242 +/- 128 ml x min(-1) (mean +/- SD) was greater (p = 0.003) than that for isometric actions, 143 +/- 99 ml x min(-1). Recruited muscle mass was also greater (p = 0.004) for dynamic exercise, 0.716 +/- 282 versus 0.483 +/- 0.139 kg. The rate of oxygen consumption per unit of recruited muscle (VO2(RM)) was similar in dynamic and isometric exercise (346 +/- 162 versus 307 +/- 198 ml x kg(-1) x min(-1); p = 0.352), but the VO2(RM) calculated relative to initial knee extensor torque was significantly greater during dynamic exercise 5.1 +/- 1.5 versus 3.6 +/- 1.6 ml x kg(-1) x Nm(-1) x min(-1) (p = 0.019).

CONCLUSION

These results are consistent with the view that oxygen cost of dynamic and isometric actions is determined by different circumstances of mechanical interaction between actin and myosin in the sarcomere, and that muscle recruitment has only a minor role.