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Purpose: This study aimed to investigate hamstring architectural, strength, and morphological adaptations after an eccentric or isometric hip extension exercise intervention. Methods: Twenty-four recreationally active males performed either an eccentric (n = 12) or an isometric hip extension (n = 12) exercise intervention, twice per week for 6 wk, followed by a 4-wk detraining period. Biceps femoris long head (BFlh) architecture was assessed pre-intervention, mid-intervention, post-intervention, and post-detraining via two-dimensional ultrasound. Strength was assessed pre-intervention, post-intervention, and post-detraining during an isokinetic knee flexion, an isometric hip extension, a Nordic hamstring exercise, and a single-leg hamstring bridge repetition to fatigue test. Hamstring muscle morphology was assessed via magnetic resonance imaging before strength testing sessions. Results: The eccentric hip extension exercise intervention significantly lengthened BFlh fascicles (+19.7%, P < 0.001, d = 1.57), increased eccentric knee flexion torque (ECC 60°·s−1, +12%, P < 0.005, d = 0.66; ECC 180°·s−1, +8.3%, P < 0.05, d = 0.41), and increased BFlh (+13.3%, P < 0.001, d = 1.96) and semimembranosus (SM) muscle volume (+12.5%, P < 0.001, d = 2.25). After 4 wk of detraining, BFlh fascicles were significantly shortened in the eccentric group (−14.8%, P < 0.005, d = −1.25), whereas eccentric knee flexion torque and BFlh and SM volumes were unchanged. The isometric hip extension exercise intervention significantly increased isometric knee flexion torque (+10.4%, P < 0.05, d = 0.54), isometric hip extension force (+12.4%, P < 0.05, d = 0.41), and semitendinosus volume (+15%, P = 0.054, d = 1.57). All other outcome measures saw no significant changes. After 4 wk of detraining, no significant changes to any variables were observed in the isometric group. Conclusions: The eccentric but not isometric hip extension exercise intervention significantly increased BFlh fascicle length. Both exercise interventions demonstrated contraction mode–specific increases in strength. However, the eccentric hip extension exercise intervention resulted in preferential hypertrophy of BFlh and SM, and the isometric hip extension exercise intervention led to selective hypertrophy of semitendinosus. [ABSTRACT FROM AUTHOR]

Introduction: A paucity of research exists examining the importance of muscle morphological and functional characteristics for elite female sprint performance. Purpose: This study aimed to compare lower body muscle volumes and vertical jumping power between elite and subelite female sprinters and assess the relationships of these characteristics with sprint race and acceleration performance. Methods: Five elite (100 m seasons best [SBE100], 11.16 ± 0.06 s) and 17 subelite (SBE100, 11.84 ± 0.42 s) female sprinters underwent: 3T magnetic resonance imaging to determine the volume of 23 individual leg muscles/compartments and five functional muscle groups; countermovement jump and 30 m acceleration tests. Results: Total absolute lower body muscle volume was higher in elite versus subelite sprinters (+15%). Elite females exhibited greater muscle volume of the hip flexors (absolute, +28%; relative [to body mass], +19%), hip extensors (absolute, +22%; relative, +14%), and knee extensors (absolute, +21%), demonstrating pronounced anatomically specific muscularity, with relative hip flexor volume alone explaining 48% of sprint performance variability. The relative volume of five individual muscles (sartorius, gluteus maximus, adductor magnus, vastus lateralis, illiopsoas) were both distinct between groups (elite > subelite) and related to SBE100 (r = 0.553–0.639), with the combination of the sartorius (41%) and the adductor magnus (17%) explaining 58% of the variance in SBE100. Elite female sprinters demonstrated greater absolute countermovement jump power versus subelite, and absolute and relative power were related to both SBE100 (r = −0.520 to −0.741) and acceleration performance (r = 0.569 to 0.808). Conclusions: This investigation illustrates the distinctive, anatomically specific muscle volume distribution that facilitates elite sprint running in females, and emphasizes the importance of hip flexor and extensor relative muscle volume. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to determine the best muscle size index of muscle strength by establishing if incorporating muscle architecture measurements improved the human muscle size-strength relationship. The influence of calculating muscle force and the location of anatomical cross-sectional area (ACSA) measurements on this relationship were also examined. Methods: Fifty-two recreationally active men completed unilateral isometric knee extension strength assessments and magnetic resonance imaging scans of the dominant thigh and knee to determine quadriceps femoris size variables (ACSA along the length of the femur, maximum ACSA (ACSAMAX), and volume (VOL)) and patellar tendon moment arm. Ultrasound images (two sites per constituent muscle) were analyzed to quantify muscle architecture (fascicle length, pennation angle) and, when combined with VOL (from magnetic resonance imaging), facilitated calculation of quadriceps femoris effective PCSA (EFFPCSA) as potentially the best muscle size determinant of strength. Muscle force was calculated by dividing maximum voluntary torque by the moment arm and addition of antagonist torque (derived from hamstring EMG). Results: The associations of EFFPCSA (r = 0.685), ACSAMAX (r = 0.697), or VOL (r = 0.773) with strength did not differ, although qualitatively VOL explained 59.8% of the variance in strength, ~11%-13% greater than EFFPCSA or ACSAMAX. All muscle size variables had weaker associations with muscle force than maximum voluntary torque. The association of strength--ACSA at 65% of femur length (r = 0.719) was greater than for ACSA measured between 10%-55% and 75%-90% (r = -0.042-0.633) of femur length. Conclusions: In conclusion, using contemporary methods to assess muscle architecture and calculate EFFPCSA did not enhance the muscle strength--size association. For understanding/monitoring muscle size, the major determinant of strength, these findings support the assessment of muscle volume, which is independent of architecture measurements and was most highly correlated with strength. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to identify the relationships between lower limb muscle characteristics and mechanical variables derived from the vertical (jumping) and horizontal (sprinting) force-- velocity--power (FVP) profiles. Methods: Nineteen subelite male rugby league players performed a series of squat jumps and linear 30-m sprints to derive the vertical and horizontal FVP profiles, respectively. The theoretical maximal force (F0), velocity (V0), and power (Pmax) were derived from both the vertical (i.e., vF0, vV0, and vPmax) and the horizontal (i.e., hF0, hV0, and hPmax) FVP profiles. Vastus lateralis (VL), biceps femoris long head, and gastrocnemius medialis (GM) and lateralis muscle fascicle length, pennation angle, and thickness were measured using B-mode ultrasonography.Magnetic resonance imaging was used to calculate volumes of major lower limb muscles, whereas proton magnetic resonance spectroscopy was used to quantify the carnosine content of the GM to estimate muscle fiber typology. Results: Variation in vPmax was best explained by GM muscle fiber typology (i.e., greater estimated proportion of Type II fibers) and VL volume (adjusted r² = 0.440, P = 0.006), whereas adductor and vastus medialis volume and GM muscle fiber typology explained the most variation in hPmax (adjusted r² = 0.634, P = 0.032). Rectus femoris and VL volume explained variation in vF0 (r² = 0.430, P = 0.008), whereas adductor and vastus medialis volume explained variation in hF0 (r² = 0.432, P = 0.007). Variations in vV0 and hV0 were best explained by GM muscle fiber typology (adjusted r² = 0.580, P < 0.001) and GM muscle fiber typology and biceps femoris short head volume (adjusted r² = 0.590, P < 0.001), respectively. Conclusion: Muscle fiber typology and muscle volume are strong determinants of maximal muscle power in jumping and sprinting by influencing the velocity- and force-oriented mechanical variables. [ABSTRACT FROM AUTHOR]

Supplemental digital content is available in the text. The biarticular hamstrings are lengthened more in a seated (hip-flexed) than prone (hip-extended) position. Purpose: We investigated the effects of seated versus prone leg curl training on hamstrings muscle hypertrophy and susceptibility to eccentric exercise-induced muscle damage. Methods: Part 1: Twenty healthy adults conducted seated leg curl training with one leg (Seated-Leg) and prone with the other (Prone-Leg), at 70% one-repetition maximum (1RM), 10 repetitions per set, 5 sets per session, 2 sessions per week for 12 wk. Magnetic resonance imaging (MRI)–measured muscle volume of the individual and whole hamstrings was assessed pre- and posttraining. Part 2: Nineteen participants from part 1 and another 12 untrained controls (Control-Leg) performed eccentric phase-only leg curl exercise at 90% 1RM, 10 repetitions per set, 3 sets for each of the seated/prone conditions with each leg. MRI-measured transverse relaxation time (T2) and 1RM of seated/prone leg curl were assessed before, 24, 48, and 72 h after exercise. Results: Part 1: Training-induced increases in muscle volume were greater in Seated-Leg versus Prone-Leg for the whole hamstrings (+14% vs +9%) and each biarticular (+8%–24% vs +4%–19%), but not monoarticular (+10% vs +9%), hamstring muscle. Part 2: After eccentric exercise, Control-Leg had greater increases in T2 in each hamstring muscle (e.g., semitendinosus at 72 h: +52%) than Seated-Leg (+4%) and Prone-Leg (+6%). Decreases in 1RM were also greater in Control-Leg (e.g., seated/prone 1RM at 24 h: −12%/−24%) than Seated-Leg (0%/−3%) and Prone-Leg (+2%/−5%). None of the changes significantly differed between Seated-Leg and Prone-Leg at any time points. Conclusion: Hamstrings muscle size can be more effectively increased by seated than prone leg curl training, suggesting that training at long muscle lengths promotes muscle hypertrophy, but both are similarly effective in reducing susceptibility to muscle damage. [ABSTRACT FROM AUTHOR]

Supplemental digital content is available in the text. The influence of muscle morphology and strength characteristics on sprint running performance, especially at elite level, is unclear. Purpose: This study aimed to investigate the differences in muscle volumes and strength between male elite sprinters, sub-elite sprinters, and untrained controls and to assess the relationships of muscle volumes and strength with sprint performance. Methods: Five elite sprinters (100-m season's best equivalent [SBE100], 10.10 ± 0.07 s), 26 sub-elite sprinters (SBE100, 10.80 ± 0.30 s), and 11 untrained control participants underwent 1) 3-T magnetic resonance imaging scans to determine the volume of 23 individual lower limb muscles/compartments and 5 functional muscle groups and 2) isometric strength assessment of lower body muscle groups. Results: Total lower body muscularity was distinct between the groups (controls < sub-elite +20% < elite +48%). The hip extensors exhibited the largest muscle group differences/relationships (elite, +32% absolute and +15% relative [per kg] volume, vs sub-elite explaining 31%–48% of the variability in SBE100), whereas the plantarflexors showed no differences between sprint groups. Individual muscle differences showed pronounced anatomical specificity (elite vs sub-elite absolute volume range, +57% to −9%). Three hip muscles were consistently larger in elite vs sub-elite (tensor fasciae latae, sartorius, and gluteus maximus; absolute, +45%–57%; relative volume, +25%–37%), and gluteus maximus volume alone explained 34%–44% of the variance in SBE100. The isometric strength of several muscle groups was greater in both sprint groups than controls but similar for the sprint groups and not related to SBE100. Conclusions: These findings demonstrate the pronounced inhomogeneity and anatomically specific muscularity required for fast sprinting and provides novel, robust evidence that greater hip extensor and gluteus maximus volumes discriminate between elite and sub-elite sprinters and are strongly associated with sprinting performance. [ABSTRACT FROM AUTHOR]

Purpose: We aimed to examine the morphological characteristics of the thigh and psoas major muscles in sprinters as well as interrelations among their muscularity, hip joint mechanics, and running speed during maximal running. Methods: T1-weighted magnetic resonance images of the thigh and trunk were obtained from 15 male sprinters (best 100-m sprint times, 10.63–11.57 s) and 12 untrained men. From the magnetic resonance images, the volumes of each of the quadriceps femoris and hamstrings, total adductors, sartorius, tensor fasciae latae, gracilis, and psoas major were determined. For sprinters, the kinetic and kinematic data were measured using a three-dimensional motion capture system and force plates during maximal running. The data for one step from the foot strike of the stance leg to that of the swing leg were analyzed. The center of gravity (CG) velocity and hip joint kinetics were quantified. Results: Compared with untrained men, sprinters had significantly greater thigh muscle volumes of the hip flexors and extensors, total adductors, gracilis, and psoas major, whereas the monoarticular knee extensor and flexor thigh muscle volumes were similar between the two groups. The CG velocity was positively correlated with the hip flexion angular impulse. Only the rectus femoris volume was significantly correlated with the CG velocity and peak hip flexion moment for the swing leg (r = 0.66–0.69). Conclusions: The sprinters in this study presented greater muscularity of the thigh and psoas major but not the monoarticular knee extensor or flexor muscles. The rectus femoris, in particular, may play an important role during the swing phase of sprinting. [ABSTRACT FROM AUTHOR]

Introduction: Previous studies have reported inhomogeneous changes in quadriceps femoris (QF) cross-sectional area (CSA) in response to strength training. It is assumed that these differential changes in muscle shape influence the muscle's functional capacity during high-force and high-power movements. The purpose of the current study was to compare intermuscular and intramuscular QF adaptations to high-load strength training and fast-speed power training. Methods: Thirty-six non-strength-trained men were randomly assigned to four groups and completed 8 wk of parallel-depth heavy squat-lift training (HS-P), parallel-depth jump squat training (JS-P), volitional-depth jump squat training (JS-V), or no training (C). Quadriceps femoris, vastus lateralis (VL), intermedius (VI), medialis (VM), and rectus femoris (RF) CSA were measured in distal-, mid-, and proximal-thigh regions using extended field-of-view ultrasonography and compared using a 3 x 2 mixed-model MANOVA with Bonferroni post hoc tests (P < 0.05). Results: Parallel-depth heavy squat-lift training and JS-P elicited similar changes in mid-CSAQF as well as summed CSA of the QF, VL, VI, and VM. Cross-sectional area of the VL (CSAVL) and CSAVI increased in both HS-P and JS-P at mid-thigh, but only JS-P significantly increased CSA proximally, and only HS-P significantly increased CSA distally. Cross-sectional area of the VM (CSAVM) increased in HS-P and JS-P distally, but only HS-P increased at mid-thigh. No hypertrophy was observed in RF at any location and no significant differences were observed between JS-P and JS-V. Parallel-depth heavy squat-lift training elicited greater proximal hypertrophy in each of the vasti muscles, whereas only JS-P elicited distal VL and VI hypertrophy. Conclusions: These observed inhomogeneous changes in CSA may alter the thigh's moment of inertia and moment arms of muscle ''compartments,'' and the influence of elastic component force transmission on the muscular force expression. Such selective hypertrophy is speculated to be biomechanically beneficial to high-force or high-power movements used in training. [ABSTRACT FROM AUTHOR]

The article reports on research which was conducted to investigate hamstring strength and morphology at the time of return to sport after hamstring injury and six months after returning to sport (RTS). Researchers evaluated 25 athletes who experienced an acute hamstring injury and had completed a controlled rehabilitation program. They found that the injured limb showed a peak torque deficit of 9.6% compared to the uninjured limb at RTS but not 6 months after, that the knee flexion angle of peak torque decreased over time for both limbs and that residual edema and deficits in isokinetic knee flexion strength were present at RTS but resolved during the subsequent 6 months.

The article presents the study which sought to examine whether the relationship between muscle size and maximum isometric strength changed during growth and maturation. Researchers have assessed the maximal isometric handgrip force and forearm muscle size of 14 prepubertal boys, 16 adolescents and 16 men. Results showed that the maximal isometric strength exerted by the forearm muscles in humans was proportional to the size whatever the age, and that muscle volume was the best index of muscle size during growth.

The article presents the study that examines the influence of sex and race on the effects of strength training (ST) on thigh muscle volume (MV), midthigh subcutaneous fat (SCF), and intermuscular fat. It focuses on the 181 previously inactive healthy Caucasian and African American men and women aged 50-85 years old who underwent about 10 weeks of unilateral knee extension ST. The results of the study revealed that ST does not alter subcutaneous or intermuscular fat regardless of sex or racial differences.

The article compares the effectiveness of exclusive strength training (ST), conventional endurance (ET) and combined ET-ST training (CT) in three groups of 15 patients with severe chronic heart failure (CHF). The subjects underwent ST, ET or CT three times a week, 45 minutes each, for 40 sessions. The subjects' left ventricular ejection fraction (LVEF), peak oxygen consumption, peak workload, thigh muscle volume, knee extensor strength, endurance and quality of life (QoL) were evaluated before and after intervention. Findings show that none of the training modalities is more effective than the other modalities. The study concludes that cardiac function, exercise capacity, peripheral muscle function and QoL of patients with CHF can be improved with intensive exercise training.

The article reports on knee-flexion torque and morphology of the semitendinosus after anterior cruciate ligament (ACL) reconstruction. The study targeted to shed light on the relationship between deficits in knee-flexion torque and morphological changes in the semitendinosus muscle-tendon complex after gathering the semitendinosus tendon for ACL reconstruction. The isometric knee-flexion torque at 45 and 90 degrees of knee flexion was gauged in limbs of 23 patients who underwent ACL reconstruction using ipsilateral semitendinosus tendon. Magnetic resonance imaging scans were employed in this study to measure muscle volume and strength.

Purpose: This study was conducted to describe tower extremity muscle morphology (volume and peak cross-sectional area (CSA)) in young athletes and compare these with previously reported values. A second aim was to determine if muscle morphological values differ significantly between sides, implying that unilateral measurements cannot represent both limbs accurately. Methods: Axial spin-echo T1-weighted magnetic resonance (MR) images were obtained between the ankle mortise and iliac crest in 10 athletes (age 18.8 ± 3.7 yr). Subsequently, each subject's three-dimensional anatomy was digitally reconstructed. Muscle volume, peak CSA, and length were calculated for 13 muscles. Results: The mean volumes and CSA for the current sample of athletes were larger than previously reported values (primarily from cadaver studies of nonathletes). The ratio of total quadriceps volume to total hamstrings volume averaged nearly 3: 1 (2.9 ± 0.2), whereas previous reports have been closer to 2:1 (2.1 ± 0.2). The relative contribution of each muscle to the muscle group (hamstrings or quadriceps) volume was also different for these athletes. Significant differences in side-to-side muscle morphology were observed in several knee muscles (P < 0.03). Vastus medialis muscle volume was larger in the dominant leg (difference between sides: 59 ± 25 cm³, representing 15.9 ± 6.7% of its average volume), whereas vastus lateralis muscle volume was larger in the nondominant leg (difference: 54 ± 47 cm³ representing 9.6 ± 8.3% of its average). Despite this, total quadriceps volumes were similar between sides. Conclusions: Findings suggest that the morphology data presented in this study should be used instead of data from cadavers when studying young athletic people. These data should improve the accuracy of biomechanical modeling in the athletic population. [ABSTRACT FROM AUTHOR]

Purpose: The magnitude of atrophy and strength loss induced by partial or complete disuse of skeletal muscles varies greatly among individuals. Factors predisposing some individuals to more extreme responses have not been identified. The purpose of this investigation was to determine whether 1) level of activity before disuse or 2) sex differences influence the magnitude of atrophy and changes in muscle strength, endurance, and sense of muscular effort in response to 21 d of arm suspension. Methods: Thirty-one individuals (18 women, 13 men) completed either 8 wk of resistance training (TRAINED group) or no training (UNTRAINED group) before 21 d of elbow muscle arm suspension achieved by having one arm in a sling tethered to the body by a swathe. Muscle volume was measured using serial magnetic resonance imaging (MRI) cross-sectional images. Functional measurements included maximal isometric force (MIF) for elbow flexion, one-repetition maximum (1 RM) for biceps curl, number of repetitions to fatigue at 50% of 1 RM, and sense of muscular effort measured using a force-matching task. Results: Flexor muscle volume decreased (P < 0.001) by -7.7 ± 7.3% across all subjects. The decrease (P < 0.001) in flexor muscle volume was significant in men but not women. Arm suspension induced decreases (P < 0.001) in MIF and 1 RM that did not differ across sex or training groups. The number of repetitions to fatigue decreased (P < 0.05) in the UNTRAINED but not TRAINED groups. No changes in sense of muscular effort were measured. Conclusion: The smaller initial muscle size or sex-specific factors attenuated muscle loss but not strength or endurance losses in females during disuse. Resistance training before disuse may attenuate the loss in muscular endurance. [ABSTRACT FROM AUTHOR]