Task-dependent inhomogeneous muscle activities within the bi-articular human rectus femoris muscle

Effect of tasks on intramuscular regional differences in rectus femoris elasticity during isometric contraction: An ultrasound shear wave elastography study

PURPOSE

This study aimed to investigate intramuscular regional differences and task specificity of rectus femoris (RF) elasticity during isometric contraction.

METHODS

Sixteen healthy males (aged 24.3 ± 4.1 years) participated in this study. The tasks included isometric hip flexion (HF) and knee extension (KE). The contractions were maintained at 0%, 30%, and 60% of their respective maximum voluntary isometric contraction (MVC) forces for 5 s each at 50˚ hip flexion and 90˚ knee flexion. RF elasticity was measured in two regions, proximal (33%) and distal (67%). The shear modulus (kPa), measured by shear wave elastography, was used to determine RF elasticity.

RESULTS

A significant interaction effect was observed (intensity × region × task) (P = 0.01). Regarding regional differences during contraction, the proximal region exhibited a greater shear modulus than the distal region at 60% MVC in the hip flexion task (P = 0.001). Conversely, no regional differences were observed at either 30% MVC (P = 0.625) or 60% MVC (P = 0.568) in the knee extension task.

DISCUSSION

Muscle shear modulus during contraction reflects active force, suggesting that mechanical stress can be applied primarily to the proximal region of the RF during the HF task.

Muscle morphological changes and enhanced sprint running performance: A 1-year observational study of well-trained sprinters

Numerous cross-sectional studies have attempted to identify the muscle morphology required to achieve high sprint velocity. Our longitudinal study addressed an unanswered question of cross-sectional studies: whether hypertrophy of the individual trunk and thigh muscles induced by daily training (e.g., sprint, jump, and resistance training) is linked to an improvement in sprint performance within well-trained sprinters. Twenty-three collegiate male sprinters (100-m best time of 11.36 ± 0.44 s) completed their daily training for 1 year without our intervention. Before and after the observation period, the sprint velocities at 0-100 m, 0-10 m, and 50-60 m intervals were measured using timing gates. The volumes of 14 trunk and thigh muscles were measured using magnetic resonance imaging. Muscle volumes were normalized to the participants' body mass at each time point. Sprint velocities increased at the 0-100 m (p < 0.001), 0-10 m (p = 0.019), and 50-60 m (p = 0.018) intervals after the observation period. The relative volumes of the tensor fasciae latae, sartorius, biceps femoris long head, biceps femoris short head, semitendinosus, and iliacus were increased (all p < 0.050). Among the hypertrophied muscles, only the change in the relative volume of the semitendinosus was positively correlated with the change in sprint velocity at the 50-60 m interval (p = 0.018 and ρ = 0.591). These findings suggest that semitendinosus hypertrophy seems to be associated with sprint performance improvement within well-trained sprinters during the maximal velocity phase.

Wearing a back-support exoskeleton alters lower-limb joint kinetics during single-step recovery following a forward loss of balance

We assessed the effects of a passive, back-support exoskeleton (BSE) on lower-limb joint kinetics during the initiation and swing phases of recovery from a forward loss of balance. Sixteen (8M, 8F) young, healthy participants were released from static forward-leaning postures and attempted to recover their balance with a single-step while wearing a BSE (backXTM) with different levels of support torque and in a control condition. The BSE provided ∼ 15-20 Nm of external hip extension torque on the stepping leg at the end of initiation and beginning of swing phases. Participants were unable to generate sufficient hip flexion torque, power, and work to counteract this external torque, although they sustained hip flexion torque for a more prolonged period, resulting in slightly increased hip contribution to positive leg work (compared to control). However, net positive leg work, and the net contribution of hip joint (human + BSE) to total leg work decreased with BSE use. While all participants had changes in hip joint kinetics, a significant compensatory increase in ankle contribution to positive leg work was observed only among females. Our results suggest that BSE use adversely affects reactive stepping by decreasing the stepping leg kinetic energy for forward propulsion, and that the relative contributions of lower-limb joints to total mechanical work done during balance recovery are altered by BSE use. BSEs may thus need to be implemented with caution for dynamic tasks in occupational settings, as they may impair balance recovery following a forward loss of balance.

Modelling motor units in 3D: influence on muscle contraction and joint force via a proof of concept simulation

Functional heterogeneity is a skeletal muscle’s ability to generate diverse force vectors through localised motor unit (MU) recruitment. Existing 3D macroscopic continuum-mechanical finite element (FE) muscle models neglect MU anatomy and recruit muscle volume simultaneously, making them unsuitable for studying functional heterogeneity. Here, we develop a method to incorporate MU anatomy and information in 3D models. Virtual fibres in the muscle are grouped into MUs via a novel “virtual innervation” technique, which can control the units’ size, shape, position, and overlap. The discrete MU anatomy is then mapped to the FE mesh via statistical averaging, resulting in a volumetric MU distribution. Mesh dependency is investigated using a 2D idealised model and revealed that the amount of MU overlap is inversely proportional to mesh dependency. Simultaneous recruitment of a MU’s volume implies that action potentials (AP) propagate instantaneously. A 3D idealised model is used to verify this assumption, revealing that neglecting AP propagation results in a slightly less-steady force, advanced in time by approximately 20 ms, at the tendons. Lastly, the method is applied to a 3D, anatomically realistic model of the masticatory system to demonstrate the functional heterogeneity of masseter muscles in producing bite force. We found that the MU anatomy significantly affected bite force direction compared to bite force magnitude. MU position was much more efficacious in bringing about bite force changes than MU overlap. These results highlight the relevance of MU anatomy to muscle function and joint force, particularly for muscles with complex neuromuscular architecture.

Does Back Squat Exercise Lead to Regional Hypertrophy among Quadriceps Femoris Muscles?

The present study investigated effects of squat resistance training on intermuscular hypertrophy of quadriceps femoris muscles (i.e., rectus femoris, RF; vastus intermedius, VI; vastus medialis, VM; and vastus lateralis, VL). Eighteen university students (age: 24.1 ± 1.7 years, 9 females) underwent 7 weeks of parallel squat training (2 days/week) preceded by a 2-week familiarization period. Squat strength (1RM) and cross-sectional area (CSA) of four quadriceps muscles were assessed at baseline and at the end of the study. At posttest, 1RM and CSA of quadriceps muscles significantly increased (p < 0.01), with moderate-to-large effect (ES = 1.25−2.11) for 1RM (8.33 ± 6.64 kg), VM CSA (0.12 ± 0.08 cm2), and VL CSA (0.19 ± 0.09 cm2) and small effect (ES = 0.89−1.13) for RF CSA (0.17 ± 0.15 cm2) and VI CSA (0.16 ± 0.18 cm2). No significant differences were found in the changes of CSA between muscles (F = 0.638, p = 0.593). However, the squat 1RM gain was significantly associated only with the changes in CSA of the VL muscle (r = 0.717, p < 0.001). The parallel squat resulted in significant growth of all quadriceps muscles. However, the novelty of this study is that the increase in strength is associated only with hypertrophy of the VL muscle.

A biophysically guided constitutive law of the musculotendon-complex: modelling and numerical implementation in Abaqus

BACKGROUND AND OBJECTIVE

Many biomedical, clinical, and industrial applications may benefit from musculoskeletal simulations. Three-dimensional macroscopic muscle models (3D models) can more accurately represent muscle architecture than their 1D (line-segment) counterparts. Nevertheless, 3D models remain underutilised in academic, clinical, and commercial environments. Among the reasons for this is a lack of modelling and simulation standardisation, verification, and validation. Here, we strive towards a solution by providing an open-access, characterised, constitutive relation (CR) for 3D musculotendon models.

METHODS

The musculotendon complex is modelled following the state-of-the-art active stress approach and is treated as hyperelastic, transversely isotropic, and nearly incompressible. Furthermore, force-length and -velocity relationships are incorporated, and muscle activation is derived from motor-unit information. The CR was implemented within the commercial finite-element software package Abaqus as a user-subroutine. A masticatory system model with left and right masseters was used to demonstrate active and passive movement.

RESULTS

The CR was characterised by various experimental data sets and was able to capture a wide variety of passive and active behaviours. Furthermore, the masticatory simulations revealed that joint movement was sensitive to the muscle's in-fibre passive response.

CONCLUSIONS

This user-material provides a "plug and play" template for 3D neuro-musculoskeletal finite element modelling. We hope that this reduces modelling effort, fosters exchange, and contributes to the standardisation of such models.

Effects of growth on muscle architecture of knee extensors

This study aimed to investigate the effects of growth on the muscle architecture of knee extensors. The present study included 123 male children and adolescents. The muscle thicknesses of the rectus femoris (RF) and vastus intermedius (VI), and pennation angles and fascicle lengths of RF were measured in three regions using ultrasonography technique at rest. The relative muscle thickness was calculated by dividing the absolute muscle thickness by body mass1/3 . The years from age at peak height velocity were estimated for each participant, and used as a maturity index. The maturity index was significantly correlated with the relative muscle thicknesses of RF and VI in all regions. The slope of the relationship between the maturity index and the relative muscle thickness did not differ significantly between muscles within the same region or between regions within the individual muscles. The fascicle length and pennation angle of RF were significantly correlated with the absolute muscle thickness in all regions. In the proximal RF region, the coefficient of correlation between the muscle thickness and fascicle length was significantly greater than that between the muscle thickness and pennation angle. The present results showed that growth changes in muscle thickness were uniform between and within RF and VI. Our findings suggest that growth changes in the muscle thickness of RF depend on the increases in both pennation angle and fascicle length, but their contributions to the growth of muscle thickness differ among muscle regions.

Elite Rugby Players have Unique Morphological Characteristics of the Hamstrings and Quadriceps Femoris Muscles According to their Playing Positions

Rugby is a popular sport requiring high-intensity and maximal speed actions. Numerous studies have demonstrated that physical performance variables, such as strength, sprinting, and jumping, are different between the forwards and backs. However, there is little information about muscle morphological characteristics specific for each rugby playing position. This study aimed to clarify the morphological characteristics of the thigh muscles in forwards and backs. Ultrasound images were obtained from the proximal, middle, and distal regions of the thigh. Then, the anatomical cross-sectional areas of particular muscles in the hamstrings and quadriceps femoris were calculated for seven forwards, seven backs, and ten non-athletes. The anatomical cross-sectional areas were normalised by the two-third power of lean body mass, and the normalised values of the three regions were averaged as that of the individual muscle. In the hamstrings, the normalised anatomical cross-sectional areas of the biceps femoris long head were significantly greater in forwards than in non-athletes, whereas those of the semitendinosus were significantly greater in backs than in non-athletes. Furthermore, in the quadriceps femoris, the normalised anatomical cross-sectional areas of the rectus femoris and vastus intermedius were significantly greater in forwards than in backs and non-athletes. These results suggest that forwards have great muscularity of the biceps femoris long head and vastus intermedius which can generate large force, whereas backs possess great muscularity of the semitendinosus which can generate high contraction velocity. These findings allow coaches to design more effective training programs according to particular rugby playing positions.

Regional neuromuscular regulation within rectus femoris muscle following three-month limb-loaded walking in older adults

Limb-loaded walking using ankle weights has been widely applied to increase exercise intensity in older adults. Examining changes in the activation pattern between proximal (RFP) and distal (RFD) regions of the rectus femoris (RF) muscle is key to clarifying gait deficits in older adults. The aim of this study was to determine regional neuromuscular regulation within the RF muscle following three-month limb-loaded walking in older adults. The study participants were 22 healthy older adults (69 ​± ​6.3 years) who walked at least 160 ​min per month. Surface electromyography (EMG) and motion capture were used to measure the neuromuscular activities of RFP and RFD and generate kinematic data on the left lower extremity on walking for 240 ​s at the preferred gait speed on a treadmill at pre- and post-intervention, respectively. Averaged rectified values (ARV) of RFP and RFD were normalized by maximum values of ARV during a gait cycle within ten consecutive gait cycles. Normalized ARV of RFP was greater than RFD at 30%–40% and 70%–90% of the gait cycle and hip joint flexion at 0%–100%, and the walking speed and swing timing at post-were greater than at pre-intervention (p ​< ​0.05). No significant differences were noted in the RFP to RFD activity ratio (RFP/RFD ratio) between pre- and post-intervention, and there was no correlation between the RFP activity level and hip flexion angle in the swing phase (p > 0.05). The activity of RFP compared with RFD and hip joint flexion were increased following limb-loaded walking intervention in older adults.

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Hip joint flexion was increased after 3 months walking with load in older adults.

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The activity of proximal region of rectus femoris muscle was more than distal region.

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Swing timing and walking speed were increased after limb-loaded walking intervention.

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Changes of regional activity of rectus femoris muscle and hip flexion no correlated.

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Using load during walking can improve gait problems in older adults.

Central Tendon Injury Impairs Regional Neuromuscular Activation of the Rectus Femoris Muscle

We aimed to uncover which rectus femoris strain injury types affect regional activation within the rectus femoris. The rectus femoris has a region-specific functional role; the proximal region of the rectus femoris contributes more than the middle and distal regions during hip flexion. Although a history of strain injury modifies the region-specific functional role within the rectus femoris, it was not obvious which rectus femoris strain injury types affect regional activation within it. We studied 12 soccer players with a history of rectus femoris strain injury. Injury data were obtained from a questionnaire survey and magnetic resonance imaging. To confirm the region-specific functional role of the rectus femoris, surface multichannel electromyographic signals were recorded. Accordingly, eight legs had a history of central tendon injury, four had a history of myofascial junction injury, and four had a healed strain injury. When the injury was limited to the central tendon, the region-specific functional role disappeared. The region-specific functional role was confirmed when the injury was outside the central part. The neuromuscular function was also inhibited when the longitudinal range of the injured region was long. Our findings suggest that a central tendon injury with a long injury length impairs regional neuromuscular activation of the rectus femoris muscle.

Non-uniform excitation of the pectoralis major muscle during flat and inclined bench press exercises

Non-physiological sources may lead to equivocal interpretation on the degree of muscle excitation from electromyograms (EMGs) amplitude. This presumably explains the contradictory findings regarding the effect of the bench press inclination on the pectoralis major (PM) activation pattern. To contend with these issues, herein we used high-density surface EMG to investigate whether different PM regions are excited during the flat and 45° inclined bench press exercises. Single-differential EMGs were collected from 15 regions along the PM cranio-caudal axis, while 8 volunteers performed a set of the flat and 45° inclined bench press at 50% and 70% of 1 repetition maximum. The coefficient of variation, the range of motion, and the cycle duration were calculated from the barbell vertical position to assess the within-subject consistency across cycles. The number of channels detecting the largest EMGs amplitude (active channels), their interquartile range, and their barycentre coordinate were assessed to characterize the EMG amplitude distribution within PM. No significant differences in the range of motion (p > 0.11), cycle duration (p > 0.28), number of active channels (p > 0.05), and interquartile range of active channels (p > 0.39) were observed between the two bench press inclinations. Conversely, the barycentre shifted toward the PM clavicular region (p < 0.001) when the bench press changed from flat to 45°. Our results revealed that greatest EMG amplitudes were concentrated at the PM sternocostal and clavicular heads when exercising in the flat and 45° inclined bench press, respectively. Performing the bench press exercise, with different postures, seem to demand the excitation of different PM regions.

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.

Changes in supramaximal M-wave amplitude at different regions of biceps brachii following eccentric exercise of the elbow flexors

PURPOSE

Previous evidence from surface electromyograms (EMGs) suggests that exercise-induced muscle damage (EIMD) may manifest unevenly within the muscle. Here we investigated whether these regional changes were indeed associated with EIMD or if they were attributed to spurious factors often affecting EMGs.

METHODS

Ten healthy male subjects performed 3 × 10 eccentric elbow flexions. Maximal voluntary contraction (MVC), muscle soreness and ultrasound images from biceps brachii distal and proximal regions were measured immediately before (baseline) and during each of the following 4 days after the exercise. Moreover, 64 monopolar surface EMGs were detected while 10 supramaximal pulses were applied to the musculocutaneous nerve. The innervation zone (IZ), the number of electrodes detecting largest M-waves and their centroid longitudinal coordinates were assessed to characterize the spatial distribution of the M-waves amplitude.

RESULTS

The MVC torque decreased (~ 25%; P < 0.001) while the perceived muscle soreness scale increased (~ 4 cm; 0 cm for no soreness and 10 cm for highest imaginable soreness; P < 0.005) across days. The echo intensity of the ultrasound images increased at 48 h (71%), 72 h (95%) and 96 h (112%) for both muscle regions (P < 0.005), while no differences between regions were observed (P = 0.136). The IZ location did not change (P = 0.283). The number of channels detecting the greatest M-waves significantly decreased (up to 10.7%; P < 0.027) and the centroid longitudinal coordinate shifted distally at 24, 48 and 72 h after EIMD (P < 0.041).

CONCLUSION

EIMD consistently changed supramaximal M-waves that were detected mainly proximally from the biceps brachii, suggesting that EIMD takes place locally within the biceps brachii.

The Effect of a Previous Strain Injury on Regional Neuromuscular Activation Within the Rectus Femoris

The rectus femoris (RF) has a region-specific functional role; that is, the proximal region of the RF contributes more than the middle and distal regions during hip flexion. This study aimed to investigate whether RF strain injury affected the region-specific functional role of the muscle. We studied seven soccer players with a history of unilateral RF strain injury. Injury data were obtained from a questionnaire survey and magnetic resonance imaging (MRI). Multichannel surface electromyographic (SEMG) signals were recorded from the proximal to distal regions of the RF with 24 electrodes during isometric knee extension and hip flexion. The SEMG signals of each channel during hip flexion were normalised by those during knee extension for the injured and non-injured RF (HF/KE), and compared among the proximal, middle, and distal regions. Six RF strain injuries showed a low signal area in MRI. There was no significant difference in muscle strength between the injured and non-injured RF. While the HF/KE in the proximal region was significantly higher than those in the middle and distal regions in the non-injured RF, a difference in the HF/KE was seen only between the proximal and distal regions of the injured RF. Furthermore, the HF/KE of the most proximal channel in the injured RF was significantly lower than that in the non-injured RF. However, there was no significant difference between injured and non-injured areas in the HF/KE. Our findings suggest that the region-specific functional role of the RF muscle is partly affected by RF strain injury.

Early Functional and Morphological Muscle Adaptations During Short-Term Inertial-Squat Training

Purpose: To assess early changes in muscle function and hypertrophy, measured as increases in muscle cross-sectional areas (CSAs) and total volume, over a 4 weeks inertial resistance training (RT) program.

Methods: Ten young RT-naive volunteers (age 23.4 ± 4.1 years) underwent 10 training sessions (2–3 per week) consisting of five sets of 10 flywheel squats (moment of inertia 900 kg⋅cm²). Magnetic resonance imaging (MRI) scans of both thighs were performed before (PRE), and after 2 (IN) and 4 (POST) weeks of training to compute individual muscle volumes and regional CSAs. Scans were performed after ≥96 h of recovery after training sessions, to avoid any influence of acute muscle swelling. PRE and POST regional muscle activation was assessed using muscle functional MRI (mfMRI) scans. Concentric (CON) and eccentric (ECC) squat force and power, as well as maximal voluntary isometric contraction force (MVIC) of knee extensors and flexors, were measured in every training session.

Results: Significant quadriceps hypertrophy was detected during (IN: 5.5% ± 1.9%) and after (POST: 8.6% ± 3.6%) the training program. Increases in squat force (CON: 32% ± 15%, ECC: 31 ± 15%) and power (CON: 51% ± 30%, ECC: 48% ± 27%) were observed over the training program. Knee extensor MVIC significantly increased 28% ± 17% after training, but no changes were seen in knee flexor MVIC. No correlation was found between regional muscular activation in the first session and the % of increase in regional CSAs (r = -0.043, P = 0.164).

Conclusion: This study reports the earliest onset of whole-muscle hypertrophy documented to date. The process initiates early and continues in response to RT, contributing to initial increases in force. The results call into question the reliability of mfMRI as a tool for predicting the potential hypertrophic effects of a given strengthening exercise.

Effect of prolonged vibration to synergistic and antagonistic muscles on the rectus femoris activation during multi-joint exercises

PURPOSE

Unique neuromuscular activation of the quadriceps femoris is observed during multi-joint leg extensions: lower activation of the biarticular rectus femoris (RF) than monoarticular vasti muscles. As one of the potential mechanisms for the lower RF activation, Ia afferent-mediated inhibitory connections between synergistic muscles and/or between agonist and antagonist muscles have been proposed. If this is the major factor, it is hypothesized that RF activation during multi-joint leg extensions increases after prolonged vibration to synergistic and/or antagonist muscles. This study tested the hypothesis.

METHODS

Fourteen men exerted maximal voluntary isometric knee extension and flexion and performed submaximal parallel squat before and after one of the following three interventions on different days: prolonged vibration to the vastus lateralis (VL, synergist) or biceps femoris (BF, antagonist), or quiet sitting for 30 min. Muscle activations of the quadriceps femoris and hamstrings were determined using surface electromyography.

RESULTS

After prolonged VL or BF vibration, VL (21%) or BF (30%) activation during isometric contractions significantly decreased, which was significantly correlated with the reduction of the maximal isometric knee extension or flexion strength. The magnitude of RF activation during squat was significantly lower than those of VL and the vastus medialis. No significant increase in RF activation during squat was observed after vibrations.

CONCLUSION

The findings suggest that lower biarticular RF activation compared with the monoarticular vasti muscles during multi-joint exercises does not result from the modulation by peripheral inhibitory input from Ia afferents originating from synergist and/or antagonist muscles.

Is myoelectric activity distributed equally within the rectus femoris muscle during loaded, squat exercises?

Recent evidence suggests different regions of the rectus femoris (RF) muscle respond differently to squat exercises. Such differential adaptation may result from neural inputs distributed locally within RF, as previously reported for isometric contractions, walking and in response to fatigue. Here we therefore investigate whether myoelectric activity distributes evenly within RF during squat. Surface electromyograms (EMGs) were sampled proximally and distally from RF with arrays of electrodes, while thirteen healthy volunteers performed 10 consecutive squats with 20% and 40% of their body weight. The root mean square (RMS) value, computed separately for thirds of the concentric and eccentric phases, was considered to assess the proximo-distal changes in EMG amplitude during squat. The channels with variations in EMG amplitude during squat associated with shifts in the muscle innervation zone were excluded from analysis. No significant differences were observed between RF regions when considering squat phases and knee joint angles individually (P>0.16) while a significant interaction between phase and knee joint angle with detection site was observed (P<0.005). For the two loads considered, proximal RMS values were greater during the eccentric phase and for the more flexed knee joint position (P<0.001). Our results suggest inferences on the degree of RF activation during squat must be made cautiously from surface EMGs. Of more practical relevance, there may be a potential for the differential adaption of RF proximal and distal regions to squat exercises.

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.

Unique activation of the quadriceps femoris during single- and multi-joint exercises

PURPOSE

This study aimed to examine whether muscle activation of the quadriceps femoris differs between single- and multi-joint exercises, and to explore the factors resulting in muscle and exercise specificity in activation.

METHODS

Eleven adults developed isometric hip extension torque gradually while maintaining submaximal isometric knee extension torque (Experiment 1). In Experiment 2, 15 men performed knee extension and leg press separately at intensities of 20, 40, 60 and 80 % of their one repetition maximum (1RM) load, and 14 men conducted leg press at intensities of 40 and 80 % of 1RM until exhaustion (Experiment 3). Muscle activation during exercises was measured using surface electromyography from the rectus femoris, vastus lateralis and medialis.

RESULTS

The addition of isometric hip extension torque significantly decreased rectus femoris activation (Experiment 1). In Experiment 2, the rectus femoris activation was significantly higher during knee extension than during leg press, whereas no differences were observed in the vasti. The rectus femoris activation was not significantly different between leg press at 80 % and knee extension at 20 % of 1RM. The results of Experiment 3 showed significant increases in vasti activation at both intensities, whereas rectus femoris activation did not change at 80 % of 1RM.

CONCLUSION

The results revealed that even at high intensity, the rectus femoris activation during multi-joint exercise is low and does not increase with fatigue, unlike the vasti, and that the inter-muscle and inter-exercise differences in activation depend on whether hip extension torque is exerted in the exercise.

Inter- and intramuscular differences in training-induced hypertrophy of the quadriceps femoris: association with muscle activation during the first training session

The purpose of this study was to examine whether inter- and intramuscular differences in hypertrophy induced by resistance training correspond to differences in muscle activation during the first training session. Eleven young men completed 12 weeks of training intervention for knee extension. Before and after the intervention, T1-weighted magnetic resonance (MR) images were recorded to determine the volume and anatomical cross-sectional area (CSA) along the length of the individual muscles of the quadriceps femoris. The T2-weighted MR images were also acquired before and immediately after the first training session. The T2 was calculated for each pixel within the quadriceps femoris, from which the muscle activation was evaluated as %activated volume and area. The results showed that the %activated volume after the first training session was significantly higher in the vastus intermedius than the vastus medialis. However, the relative change in muscle volume after the training intervention was significantly greater in the rectus femoris than the vasti muscles (vastus lateralis, intermedius and medialis). Within the rectus femoris, both the %activated area and relative increase in CSA were significantly greater in the distal region than the proximal region. In contrast, the %activated area and relative increase in CSA of the vasti were nearly uniform along each muscle. These results suggest that the muscle activation during the first training session is associated with the intramuscular difference in hypertrophy induced by training intervention, but not with the intermuscular difference.

Neuromuscular Activation of the Vastus Intermedius Muscle during Isometric Hip Flexion

Although activity of the rectus femoris (RF) differs from that of the other synergists in quadriceps femoris muscle group during physical activities in humans, it has been suggested that the activation pattern of the vastus intermedius (VI) is similar to that of the RF. The purpose of present study was to examine activation of the VI during isometric hip flexion. Ten healthy men performed isometric hip flexion contractions at 25%, 50%, 75%, and 100% of maximal voluntary contraction at hip joint angles of 90°, 110° and 130°. Surface electromyography (EMG) was used to record activity of the four quadriceps femoris muscles and EMG signals were root mean square processed and normalized to EMG amplitude during an isometric knee extension with maximal voluntary contraction. The normalized EMG was significantly higher for the VI than for the vastus medialis during hip flexion at 100% of maximal voluntary contraction at hip joint angles of 110° and 130° (P < 0.05). The onset of VI activation was 230–240 ms later than the onset of RF activation during hip flexion at each hip joint angle, which was significantly later than during knee extension at 100% of maximal voluntary contraction (P < 0.05). These results suggest that the VI is activated later than the RF during hip flexion. Activity of the VI during hip flexion might contribute to stabilize the knee joint as an antagonist and might help to smooth knee joint motion, such as in the transition from hip flexion to knee extension during walking, running and pedaling.

Eletromiográfia do reto femoral em diferentes equipamentos proprioceptivos no meio aquático

INTRODUÇÃO: A propriocepção é uma variável imprescindível para prevenção e reabilitação das lesões do joelho, podendo ser estimulada por diferentes equipamentos, os quais ainda não foram testados em meio aquático.OBJETIVO: Avaliar a atividade eletromiográfica (EMG) do músculo reto femoral de atletas no meio aquático frente a três diferentes equipamentos proprioceptivos (cama elástica, disco proprioceptivo e balancim).MÉTODOS: A amostra foi composta por dez jogadores de futsal profissional, sem histórico de lesões musculoesqueléticas (últimos três meses), com 23,1 (±1,5) anos e índice de massa corporal 25,2 (±0,5) kg/m². A aquisição do sinal EMG do reto femoral do membro dominante foi adquirido por eletrodos de superfície, aproximadamente 2,5 cm da posição distal do ponto motor. O nível da água foi ajustado individualmente (entre a região umbilical e o processo xifoide) e a temperatura mantida a 32 °C. As avaliações compreenderam o repouso, a contração voluntária máxima (CVM) antes e depois dos experimentos e os estímulos proprioceptivos (cama elástica, disco proprioceptivo e balancim) em apoio unipodal. Os dados (média ± erro padrão) foram comparados pelo teste-tpareado e pela ANOVA para medidas repetidas seguida de testeBonferrroni (post hoc).RESULTADOS: A EMG da CVM antes (221,0 ± 134 RMS/µVolts) e depois (243,0 ± 154,0 RMS/µVolts) foi semelhante (p = 0,129). No meio aquático, a cama elástica, o balancim e o disco apresentaram respectivamente 24,5 (±4,3), 33,9 (±4,3) e 32,5 (±6,7) %CVM. A atividade EMG do reto femoral na cama elástica foi 8% menor que o balancim e 9,5% que o disco proprioceptivo (p < 0,001).CONCLUSÃO: No meio aquático os equipamentos proprioceptivos promovem a ativação do reto femoral. Entretanto, a cama elástica apresenta menor atividade que o disco e o balancim, sugerindo-se que este equipamento deva ser utilizado no início da estimulação proprioceptiva.

Non-uniform muscle oxygenation despite uniform neuromuscular activity within the vastus lateralis during fatiguing heavy resistance exercise

Previous studies have reported for the vastus lateralis (VL) that the extent of muscle hypertrophy in response to resistance training is greater in the distal than in the middle region, despite uniform muscle fibre composition within VL along its length. In the present study, to investigate mechanism(s) for such non-uniform muscle hypertrophy, we simultaneously measured neuromuscular activity and muscle oxygenation state at the middle and distal regions of VL during fatiguing heavy resistance exercise. Twelve males performed unilateral knee extension exercise which consisted of 4 sets of 8 repetitions at intensity of 80% of the individual one repetition maximum. During the resistance exercise, neuromuscular activities and muscle oxygenation status at the middle and distal regions (50% and 70% of the thigh length, respectively) of VL were measured by using electromyography and near-infrared spectroscopy, respectively. Neuromuscular activities were similar between the distal and middle regions of VL, whereas muscle tissue oxygenation saturation was significantly lower at the distal than at the middle region of VL. These results suggest a possibility that the regional difference in muscle oxygenation but not in neuromuscular activity during fatiguing heavy resistance exercise is responsible for the regional difference in hypertrophy within a muscle.