Coronal As Well As Sagittal Fascicle Dynamics Can Bring About a Gearing Effect in Muscle Elongation by Passive Lengthening.

PURPOSE: The amount of muscle belly elongation induced by passive lengthening is often assumed to be equal to that of fascicles. But these are different if fascicles shorter than the muscle belly rotate around their attachment sites. Such discrepancy between fascicles and muscle belly length changes can be considered as gearing. As the muscle fascicle arrangement is 3D, the fascicle rotation by passive lengthening may occur in the coronal as well as the sagittal planes. Here we examined the fascicle 3D dynamics and resultant gearing during passive elongation of human medial gastrocnemius in vivo . METHODS: For 16 healthy adults, we reconstructed fascicles three-dimensionally using diffusion tensor imaging and evaluated the change in fascicle length and angles in the sagittal and coronal planes during passive ankle dorsiflexion (from 20° plantar flexion to 20° dorsiflexion). RESULTS: Whole muscle belly elongation during passive ankle dorsiflexion was 38% greater than the fascicle elongation. Upon passive lengthening, the fascicle angle in the sagittal plane in all regions (-5.9°) and that in the coronal plane in the middle-medial (-2.7°) and distal-medial (-4.3°) regions decreased significantly. Combining the fascicle coronal and sagittal rotation significantly increased the gearing effects in the middle-medial (+10%) and distal-medial (+23%) regions. The gearing effect by fascicle sagittal and coronal rotations corresponded to 26% of fascicle elongation, accounting for 19% of whole muscle belly elongation. CONCLUSIONS: Fascicle rotation in the coronal and sagittal planes is responsible for passive gearing, contributing to the whole muscle belly elongation. Passive gearing can be favorable for reducing fascicle elongation for a given muscle belly elongation.

Fatigue-induced changes in hamstrings' active muscle stiffness: effect of contraction type and implications for strain injuries.

PURPOSE: Hamstring strain injuries may occur due to differential fatigue and compromised mechanical properties among the hamstring muscles. We examined (1) the effect of fatigue on hamstrings active muscle stiffness, and (2) whether contraction type affects active muscle stiffness changes during a submaximal fatiguing task. METHODS: Nine healthy males completed 99 submaximal knee flexions in isometric (ISO), concentric (CON), and eccentric (ECC) conditions. We measured the knee flexor maximal voluntary torque (MVT) (pre/post), shear wave velocity (SWV) during contraction and transverse relaxation times (T2) (pre/post) in biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) muscles. RESULTS: MVT decreased substantially after all conditions (- 18.4 to - 33.6%). The average relative torque sustained during the task was lower in CON than ISO and ECC, but absolute torque was similar. SWV interindividual responses were highly variable across muscles and contraction types. On average, BFlh SWV tended to increase in ISO (0.4 m/s, 4.5%, p = 0.064) but decreased in ECC condition (- 0.8 m/s, - 7.7%, p < 0.01). ST SWV decreased in CON (- 1.1 m/s, - 9.0%, p < 0.01), while it remained unchanged in ISO and ECC. SM SWV decreased in CON (- 0.8 m/s, - 8.1%, p < 0.01), but it was unaffected in ISO and variable in ECC. CONCLUSION: Fatigue has a differential effect on the mechanical properties of the constituent hamstring muscles, as measured with shear wave elastography, depending upon contraction type. We found preliminary evidence that BFlh is more fatigued than ST or SM during eccentric contractions, which may explain its susceptibility to strain injuries.

Three-dimensional architecture of human medial gastrocnemius fascicles in vivo: Regional variation and its dependence on muscle size.

Fascicle architecture (length and pennation angle) can vary regionally within a muscle. The architectural variability in human muscles has been evaluated in vivo, but the interindividual variation and its determinants remain unclear. Considering that within-muscle non-uniform changes in pennation angle are associated with change in muscle size by chronic mechanical loading, we hypothesized that the regional variation in fascicle architecture is dependent on interindividual variation in muscle size. To test this hypothesis, we reconstructed fascicles three-dimensionally along and across the whole medial gastrocnemius in the right lower leg of 15 healthy adults (10 males and 5 females, 23.7 ± 3.3 years, 165.8 ± 8.3 cm, 61.9 ± 11.4 kg, mean ± standard deviation) in neutral ankle joint position with the knee fully extended, using magnetic resonance diffusion tensor imaging and tractography. The 3D-reconstructed fascicles arose from the deep aponeurosis with variable lengths and angles both in sagittal and coronal planes. The fascicle length was significantly longer in the middle (middle-medial: 52.4 ± 6.1 mm, middle-lateral: 52.0 ± 5.1 mm) compared to distal regions (distal-medial: 41.0 ± 5.0 mm, distal-lateral: 38.9 ± 3.6 mm, p < 0.001). The 2D pennation angle (angle relative to muscle surface) was significantly greater in distal than middle regions, and medial than lateral regions (middle-medial: 26.6 ± 3.1°, middle-lateral: 24.1 ± 2.3°, distal-medial: 31.2 ± 3.6°, distal-lateral: 29.2 ± 3.0°, p ≤ 0.017), while only a proximo-distal difference was significant (p < 0.001) for 3D pennation angle (angle relative to line of action of muscle). These results clearly indicate fascicle's architectural variation in 3D. The magnitude of regional variation evaluated as standard deviation across regions differed considerably among individuals (4.0-10.7 mm for fascicle length, 0.9-5.0° for 2D pennation angle, and 3.0-8.8° for 3D pennation angle), which was positively correlated with the muscle volume normalized to body mass (r = 0.659-0.828, p ≤ 0.008). These findings indicate muscle-size dependence of the variability of fascicle architecture.

The hamstrings to quadriceps functional ratio expressed over the full angle-angular velocity range using a limited number of data points.

The hamstring to quadriceps (H : Q) strength ratio is widely used to identify individuals at risk of sustaining hamstring strain injuries. However, its efficacy is not supported by the current evidence. Current methods for the calculation of the H : Q ratio provide only a one- or two-dimensional ratio, often ignoring fundamental muscle mechanical properties. Based on isokinetic torque measurements of the knee flexors and extensors (0-400° s-1) in 25 young, physically active males, we derived a model equation that creates a three-dimensional H : Q functional ratio profile. The model robustness was tested against a different number of input torque data (8, 11, 14 and 17 pairs of points) and small perturbation of the knee joint angle data (5°). The model was consistent and behaved well under all conditions apart from the eight pairs of points (R 2 = 0.84-0.96; RMSE = 0.14-0.25; NRMSE = 0.12-0.27), and the H : Q functional ratio was successfully described even at angles and velocities that cannot be normally assessed with isokinetic dynamometry. Overall, our results suggest that the model can provide a fast and accurate three-dimensional description of the knee joint muscle strength balance using as few as 11 experimental data points and this could be an easy-to-employ screening tool.

Characteristics of inhomogeneous lower extremity growth and development in early childhood: a cross-sectional study.

BACKGROUND: Early childhood is a transferring stage between the two accelerated growth periods (infant and adolescent). Body dimensions are related to physical growth and development. The purpose of this study was to investigate physical growth in terms of anthropometry, muscle growth of the lower extremity, and functional development over early childhood. METHODS: A cross-sectional study was carried out on 29 preschool children (PS: 3-5 years), 21 school children (SC: 6-8 years), and 22 adults (AD: 20-35 years). Lower extremity characteristics (segmental dimensions, muscle and adipose tissue thicknesses of the thigh and lower leg), and voluntary joint torque (knee and ankle) were measured. Correlations between parameters and group comparisons were performed. RESULTS: All the parameters except for body mass index (BMI) and subcutaneous adipose tissue thickness were correlated with age for PS and SC combined (r = 0.479-0.920, p < 0.01). Relative thigh and shank lengths to body height were greatest in AD and smallest in PS (p < 0.05) but the relative foot dimensions were significantly larger in PS and SC than in AD (p < 0.05). Relative subcutaneous adipose tissue thickness was largest in PS and lowest in AD. Muscle thickness and the muscle volume measure (estimated from muscle thickness and limb length) were significantly larger in older age groups (p < 0.05). All groups showed comparable muscle thickness when normalized to limb length. Joint torque normalized to estimated muscle volume was greatest for AD, followed by SC and PS (p < 0.05). CONCLUSIONS: Relative lower extremity lengths increase with age, except for the foot dimensions. Muscle size increases with age in proportion to the limb length, while relative adiposity decreases. Torque-producing capacity is highly variable in children and rapidly develops toward adulthood. This cross-sectional study suggests that children are not a small scale version of adults, neither morphologically nor functionally.

Hamstrings load bearing in different contraction types and intensities: A shear-wave and B-mode ultrasonographic study.

The main aim was to examine the load bearing of individual hamstring muscles in different contraction types and intensities, through local stiffness measurement by shear wave elastography (SWE). A secondary aim was to examine the relationship between the SWE stiffness measure and hamstrings morphology. Ten healthy males (age 22.1±4.1 years; height 173.7±5.9 cm; body mass 68.6±12.4 kg; mean ± SD) performed knee flexions on an isokinetic dynamometer at different intensities (20-70%MVC, random order) in three separate, randomized conditions: isometric (ISO), concentric (CON) and eccentric (ECC). SWE was used to measure muscle shear wave velocity (SWV) in biceps femoris long head (BFlh), semitendinosus (ST) and semimembranosus (SM) during contraction. Muscle anatomical cross-sectional area (ACSA) was measured with magnetic resonance imaging and muscle architecture with B-mode ultrasonography. Muscle SWV increased linearly with contraction intensity, but at a varying rate among muscles and contraction types. ST exhibited greater SWV than BFlh and SM in all contraction types, however, there was an upward shift in the SM SWV-torque relationship in ECC compared to ISO and CON. Strong negative correlations were found between peak ISO SWV and ST ACSA (r = -0.81, p = 0.005) and BFlh pennation angle (r = -0.75, p = 0.012). These results suggest that ST has a primary role in hamstrings load bearing in all contraction types, likely due to its morphology; however, there is evidence of increased contribution from SM in eccentric muscle actions.

The functional significance of hamstrings composition: is it really a "fast" muscle group?

Hamstrings muscle fiber composition may be predominantly fast-twitch and could explain the high incidence of hamstrings strain injuries. However, hamstrings muscle composition in vivo, and its influence on knee flexor muscle function, remains unknown. We investigated biceps femoris long head (BFlh) myosin heavy chain (MHC) composition from biopsy samples, and the association of hamstrings composition and hamstrings muscle volume (using MRI) with knee flexor maximal and explosive strength. Thirty-one young men performed maximal (concentric, eccentric, isometric) and explosive (isometric) contractions. BFlh exhibited a balanced MHC distribution [mean ± SD (min-max); 47.1 ± 9.1% (32.6-71.0%) MHC-I, 35.5 ± 8.5% (21.5-60.0%) MHC-IIA, 17.4 ± 9.1% (0.0-30.9%) MHC-IIX]. Muscle volume was correlated with knee flexor maximal strength at all velocities and contraction modes (r = 0.62-0.76, P < 0.01), but only associated with late phase explosive strength (time to 90 Nm; r = -0.53, P < 0.05). In contrast, BFlh muscle composition was not related to any maximal or explosive strength measure. BFlh MHC composition was not found to be "fast", and therefore composition does not appear to explain the high incidence of hamstrings strain injury. Hamstrings muscle volume explained 38-58% of the inter-individual differences in knee flexor maximum strength at a range of velocities and contraction modes, while BFlh muscle composition was not associated with maximal or explosive strength.

Strength and size relationships of the quadriceps and hamstrings with special reference to reciprocal muscle balance.

PURPOSE: This study examined the association of muscle size and strength for the quadriceps and hamstrings, the relationship between the size of these muscles, and whether the H:Q size ratio influenced reciprocal strength balance-widely regarded as a risk factor for hamstrings injury. METHODS: Knee extensor and flexor isometric, concentric and eccentric (50 and 350° s(-1)) strength were measured in 31 healthy, recreationally active young men. Muscle volume was measured with magnetic resonance imaging. RESULTS: The knee flexors achieved higher concentric and eccentric torques (normalised to isometric values) than the extensors. Muscle volume explained a significant part of the inter-individual differences in strength in both extensors (isometric 71%, concentric 30-31%) and flexors (isometric 38%, concentric 50-55%). Notably, muscle size was related to knee flexor eccentric strength (r = 0.69-0.76; R (2) = 48-58%) but not extensor eccentric strength. Quadriceps and hamstrings volumes were moderately correlated (r = 0.64), with the majority of the variance in the size of one muscle (59%) not explained by the size of the other muscle. The hamstrings-to-quadriceps (H:Q) volume ratio was correlated with the isometric (r = 0.45) and functional strength ratios (350° s(-1), r = 0.56; 50° s(-1), r = 0.34). CONCLUSIONS: Muscle size exhibited a differential influence on knee extensor and flexor eccentric strength. Quadriceps and hamstrings muscle size was related, and the H:Q size ratio contributed to their strength ratios. Muscle size imbalances contribute to functional imbalances and these findings support the use of hamstrings strength training with an emphasis on hypertrophic adaptations for reducing injury risk.

Biceps Femoris Aponeurosis Size: A Potential Risk Factor for Strain Injury?

PURPOSE: A disproportionately small biceps femoris long head (BFlh) proximal aponeurosis has been suggested as a risk factor for hamstring strain injury by concentrating mechanical strain on the surrounding muscle tissue. However, the size of the BFlh aponeurosis relative to BFlh muscle size, or overall knee flexor strength, has not been investigated. This study aimed to examine the relationship of BFlh proximal aponeurosis area with muscle size (maximal anatomical cross-sectional area (ACSAmax)) and knee flexor strength (isometric and eccentric). METHODS: Magnetic resonance images of the dominant thigh of 30 healthy young males were analyzed to measure BFlh proximal aponeurosis area and muscle ACSAmax. Participants performed maximum voluntary contractions to assess knee flexion maximal isometric and eccentric torque (at 50° s and 350° s). RESULTS: BFlh proximal aponeurosis area varied considerably between participants (more than fourfold, range = 7.5-33.5 cm, mean = 20.4 ± 5.4 cm, coefficient of variation = 26.6%) and was not related to BFlh ACSAmax (r = 0.04, P = 0.83). Consequently, the aponeurosis/muscle area ratio (defined as BFlh proximal aponeurosis area divided by BFlh ACSAmax) exhibited sixfold variability, being 83% smaller in one individual than another (0.53 to 3.09, coefficient of variation = 32.5%). Moreover, aponeurosis size was not related to isometric (r = 0.28, P = 0.13) or eccentric knee flexion strength (r = 0.24, P ≥ 0.20). CONCLUSION: BFlh proximal aponeurosis size exhibits high variability between healthy young men, and it was not related to BFlh muscle size or knee flexor strength. Individuals with a relatively small aponeurosis may be at increased risk of hamstring strain injury.