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Concept: Perimysium

27

The estimation of muscle fascicle behaviour is decisive in a Hill-type model as they are related to muscle force by the force-length-velocity relationship and the tendon force-strain relationship. This study was aimed at investigating the influence of subject-specific tendon force-strain relationship and initial fascicle geometry (IFG) on the estimation of muscle forces and fascicle behaviour during isometric contractions. Ultrasonography was used to estimate the in vivo muscle fascicle behaviour and compare the muscle fascicle length and pennation angle estimated from the Hill-type model. The calibration-prediction process of the electromyography-driven model was performed using generic or subject-specific tendon definition with or without IFG as constraint. The combination of subject-specific tendon definition and IFG led to muscle fascicle behaviour closer to ultrasound data and significant lower forces of the ankle dorsiflexor and plantarflexor muscles compared to the other conditions. Thus, subject-specific ultrasound measurements improve the accuracy of Hill-type models on muscle fascicle behaviour.

Concepts: Statistics, Mathematics, Muscle, Approximation, Estimation, Tendon, Perimysium, Muscle fascicle

9

The impact of using different resistance training (RT) kinematics, which therefore alters RT mechanics, and their subsequent effect on adaptations remain largely unreported. The aim of this study was to identify differences to training at a longer (LR) compared with a shorter (SR) range of motion, as well as the time-course of any changes during detraining. Recreationally active participants in LR (aged 19 ± 2.6 years; n=8) and SR (aged 19 ± 3.4 years; n=8) groups undertook 8 weeks of RT and 4 weeks detraining. Muscle size, architecture, subcutaneous fat and strength were measured at weeks 0, 8, 10 and 12 (repeated measures). A control group (aged 23 ± 2.4 years; n=10) was also monitored during this period. Significant (p>0.05) post-training differences existed in strength (on average 4±2% vs. 18±2%), distal anatomical cross-sectional area (59±15% vs. 16±10%), fascicle length (23±5% vs. 10±2%) and subcutaneous fat (22±8% vs. 5±2%), with LR exhibiting greater adaptations than SR. Detraining resulted in significant (p>0.05) deteriorations in all muscle parameters measured in both groups, with the SR group experiencing a more rapid relative loss of post-exercise increases in strength than LR (p>0.05). Greater morphological and architectural RT adaptations in LR (owing to higher mechanical stress) result in a more significant increase in strength compared to SR. The practical implications for this body of work follow that LR should be observed in resistance training where increased muscle strength and size are the objective, since we demonstrate here that ROM should not be compromised for greater external loading.

Concepts: Metabolism, Glucose, Muscle, Adipose tissue, Classical mechanics, Subcutaneous tissue, Result, Perimysium

3

The factors that drive variable aponeurosis behaviors in active versus passive muscle may alter the longitudinal stiffness of the aponeurosis during contraction, which may change the fascicle strains for a given muscle force. However, it remains unknown whether these factors can drive variable aponeurosis behaviors across different muscle-tendon unit (MTU) lengths and influence the subsequent fascicle strains during contraction. Here, we used ultrasound and elastography techniques to examine in vivo muscle fascicle behavior and central aponeurosis deformations of human tibialis anterior (TA) during force-matched voluntary isometric dorsiflexion contractions at three MTU lengths. We found that increases in TA MTU length increased both the length and apparent longitudinal stiffness of the central aponeurosis at low and moderate muscle forces (P< 0.01). We also found that increased aponeurosis stiffness was directly related to reduced magnitudes of TA muscle fascicle shortening for the same change in force (P< 0.01). The increase in slope and shift to longer overall lengths of the active aponeurosis force-length relationship as MTU length increased was likely due to a combination of parallel lengthening of aponeurosis and greater transverse aponeurosis strains. This study provides in vivo evidence that human aponeurosis stiffness is increased from low to moderate forces and that the fascicle strains for a given muscle force are MTU length dependent. Further testing is warranted to determine whether MTU length-dependent stiffness is a fundamental property of the aponeurosis in pennate muscles and evaluate whether this property can enhance muscle performance.

Concepts: Psychology, Behavior, Classical mechanics, Units of measurement, Tibialis anterior muscle, Height, Dorsiflexion, Perimysium

3

This study tested the hypothesis that the ratio of changes in muscle fascicle and tendon length that occurs with joint movement scales linearly with the ratio of the slack lengths of the muscle fascicles and tendons. We compared the contribution of muscle fascicles to passive muscle-tendon lengthening in muscles with relatively short and long fascicles. Fifteen healthy adults participated in the study. The medial gastrocnemius, tibialis anterior, and brachialis muscle-tendon units were passively lengthened by slowly rotating the ankle or elbow. Change in muscle fascicle length was measured with ultrasonography. Change in muscle-tendon length was calculated from estimated muscle moment arms. Change in tendon length was calculated by subtracting change in fascicle length from change in muscle-tendon length. The median (IQR) contribution of muscle fascicles to passive lengthening of the muscle-tendon unit, measured as the ratio of the change in fascicle length to the change in muscle-tendon unit length, was 0.39 (0.26-0.48) for the medial gastrocnemius, 0.51 (0.29-0.60) for tibialis anterior, and 0.65 (0.49-0.90) for brachialis. Brachialis muscle fascicles contributed to muscle-tendon unit lengthening significantly more than medial gastrocnemius muscle fascicles, but less than would be expected if the fascicle contribution scaled linearly with the ratio of muscle fascicle and tendon slack lengths.

Concepts: Muscle, Units of measurement, Tibialis anterior muscle, Gastrocnemius muscle, Soleus muscle, Tendon, Perimysium, Muscle fascicle

3

The architectural characteristics of muscle (fascicle length, pennation angle muscle thickness) respond to varying forms of stimuli (eg, training, immobilisation and injury). Architectural changes following injury are thought to occur in response to the restricted range of motion experienced during rehabilitation and the associated neuromuscular inhibition. However, it is unknown if these differences exist prior to injury, and had a role in injury occuring (prospectively), or if they occur in response to the incident itself (retrospectively). Considering that the structure of a muscle will influence how it functions, it is of interest to understand how these architectural variations may alter how a muscle acts with reference to the force-length and force-velocity relationships.

Concepts: Acetylcholine, Neuromuscular disease, Perimysium

3

The present study examined the muscle-tendon interaction of ten international level Kenyan runners. Ultrasonography and kinematics were applied together with EMG recordings of lower limb muscles during repetitive hopping performed at maximal level. The ten Kenyans had longer gastro Achilles tendon at rest (p < 0.01) as compared with ten control subjects matched in height. Conversely, the stretching and shortening amplitudes of the tendinous tissues of the medial gastrocnemius (MG) muscle were significantly smaller in the Kenyans than in controls during the contact phase of hopping. This applied also to the fascicle length changes, which were smaller and more homogeneous among Kenyans. These limited musculo-tendinous changes resulted in higher maximal hopping height and in larger power despite their reduced body weight. The associated finding of a greater shortening to stretching ratio of the MG tendinous tissues during contact could imply that the Kenyan MG muscle-tendon unit is optimized to favor efficient storage and recoil of elastic energy, while operating at optimal muscle fascicle working range (plateau region).

Concepts: Energy, Muscle, Electromyography, Classical mechanics, Tendon, Length, Human leg, Perimysium

2

Eccentric strength training alters muscle architecture, but it is also an important factor for the prevention of hamstring injuries. The purpose was to determine the architectural adaptations of the Biceps Femoris long head (BFlh) after eccentric strength training with Nordic Hamstring exercise (NHE), followed by a subsequent detraining period. The participants in this intervention (n = 23) completed a period of 13 weeks consisting of a first week of control and prior training, followed by 8 weeks of eccentric strength training with NHE, and concluding with a 4-week period of detraining. The architectural characteristics of the BFlh were measured at rest using two-dimensional ultrasound before (M1 - week 1) and after (M2 - week 9) the eccentric strength training, and at the end of the detraining period (M3 - week 13). The muscle fascicle length significantly increased (t = -7.73, d = 2.28, P < .001) in M2 compared to M1, as well as the muscle thickness (t = -5.23, d = 1.54, P < .001), while the pennation angle presented a significant decrease (t = 7.81, d = 2.3, P < .001). The muscle fascicle length decreased significantly (t = 6.07, d = 1.79, P < .001) in M3 compared to M2, while the pennation angle showed a significant increase (t = -4.63, d = 1.36, P < .001). The results provide evidence that NHE may cause alterations in the architectural conditions of the BFlh and may have practical implications for injury prevention and rehabilitation programs. This article is protected by copyright. All rights reserved.

Concepts: Muscle, Physical exercise, Biceps brachii muscle, Flexion, All rights reserved, Copyright, Perimysium, Muscle fascicle

1

The primary human ankle plantarflexors, soleus (SO), medial gastrocnemius (MG) and lateral gastrocnemius (LG), are typically regarded as synergists and play a critical role in running. However, due to differences in muscle-tendon architecture and joint articulation, the muscle fascicles and tendinous tissue of the plantarflexors may exhibit differences in their behaviour and interactions during running. We combined in-vivo dynamic ultrasound measurements with inverse dynamics analyses to identify and explain differences in muscle fascicle, muscle-tendon unit (MTU) and tendinous tissue (SEE) behaviour of the primary ankle plantarflexors across a range of steady-state running speeds. Consistent with their role as a force generator, the muscle fascicles of the uni-articular SO shortened less rapidly than the fascicles of the MG during early stance. Furthermore, the MG and LG exhibited delays in tendon recoil during the stance phase, reflecting their ability to transfer power and work between the knee and ankle via tendon stretch and storage of elastic strain energy. Our findings add to the growing body of evidence surrounding the distinct mechanistic functions of uni- and bi-articular muscles during dynamic movements. This article is protected by copyright. All rights reserved.

Concepts: Critical thinking, Muscle, Knee, Gastrocnemius muscle, All rights reserved, Copyright, Perimysium, Muscle fascicle

1

Whilst neural and morphological adaptations following resistance training (RT) have been investigated extensively at a group level, relatively little is known about the contribution of specific physiological mechanisms, or pre-training strength, to the individual changes in strength following training. This study investigated the contribution of multiple underpinning neural [agonist EMG (QEMGMVT), antagonist EMG (HEMGANTAG)] and morphological variables [total quadriceps volume (QUADSVOL), and muscle fascicle pennation angle (QUADSθ p)], as well as pre-training strength, to the individual changes in strength after 12 weeks of knee extensor RT.

Concepts: Thermodynamics, Knee, Receptor antagonist, Agonist, Extension, Adaptation, Perimysium, Muscle fascicle

1

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (three-dimensional ultrasound imaging) to represent the muscles' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r = 0.68) and moderate (r = 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.

Concepts: Medical imaging, Magnetic resonance imaging, Ratio, Knee, Electromyography, Ultrasound, Vastus lateralis muscle, Perimysium