To examine the effect of long-term daily training on athletes' skeletal muscle, this study determined the relation between their muscle thickness and passive muscle stiffness, and compared the muscle thickness and muscle stiffness between athletes and non-athletes. Participants were elite Japanese athletes (278 men, 200 women) from various sports and non-athletes (35 men, 35 women). Rectus femoris (RF) muscle thickness was measured using B-mode ultrasonography and was normalized to the total body mass (muscle thickness/body mass(1/3)). RF passive muscle stiffness (shear modulus) was assessed by ultrasound shear-wave elastography. There was a negligibly significant correlation between muscle thickness and muscle stiffness in male athletes (p=0.003; r=-0.18) but not in female athletes (p=0.764; r=0.02). Among men, muscle thickness was significantly greater in athletes than non-athletes (p<0.001), whereas muscle stiffness was significantly less in athletes than non-athletes (p=0.020). Among women, muscle thickness was significantly greater in athletes than non-athletes (p <0.001), whereas muscle stiffness did not differ significantly between athletes and non-athletes (p =0.412). These results suggest that the effect of long-term daily training performed by athletes on muscle stiffness is more complicated than that on muscle thickness.

Abstract Kato, E, Nakamura, M, and Takahashi, H. Effect of compression garments on controlled force output after heel-rise exercise. J Strength Cond Res 32(4): 1174–1179, 2018—The purpose of this study was to elucidate the effects of compression garments (CG) on controlled force output after strenuous exercise. Sixteen healthy volunteers completed trials both with CG and without CG (control trial: CON) on 2 separate, random days which were at least 1 month apart. Both trials required participants to perform heel-rise exercises from maximal dorsiflexion to maximal plantar flexion 20 times with a single leg. The subjects repeated 3 sets of the exercise and took a rest for 2 minutes between sets. Before and after the heel-rise exercise, mechanical (stiffness) and architectural properties of the gastrocnemius medialis muscle were evaluated using the ultrasound method. Also, isometric maximal voluntary contraction (MVC) of plantar flexion was measured, and the subjects maintained 20% MVC of plantar flexion torque for 20 seconds as steadily as possible (steadiness task) as an index of force control. Repeated 2-way analysis of variance analysis (CG/CON × time) indicated that all the parameters declined immediately after heel-rise exercise in both CG and CON trials. Maximal voluntary contraction did not show different tendencies between CG and CON trials, but muscle stiffness and steadiness declined less in CG than CON (p < 0.05). In conclusion, CG are considered to positively benefit controlled force output after strenuous exercise.

Muscle strain injuries occur when the muscle is either stretched passively or activated during stretch. Calf strain injuries are most commonly found in the medial gastrocnemius (MG), especially at the distal musculotendinous junction. One of the reasons for the susceptibility of musculotendinous junction for strain injuries may be that the distal region of MG is stretched more than the medial belly region. Muscle tension during passive stretching can be estimated from shear elastic modulus measured by shear wave elastography. PURPOSE: To investigate the regional difference in MG tension during passive stretching, shear elastic modulus at the distal and medial belly regions of the MG was assessed by shear-wave elastography. METHODS: Twenty-two healthy men and 25 healthy women participated in this study. Shear elastic moduli at the distal and medial belly regions of the MG were measured with the ankle joint positions at 30° plantarflexion (PF), a neutral anatomical position (NE), and 20° dorsiflexion (DF). The regional differences in the shear elastic modulus were tested with a two-way analysis of variance (region of the MG × ankle joint position), followed by t-tests with Bonferroni correction, with the significance set at P < 0.05. RESULTS: Shear elastic modulus was not significantly different between the distal and medial belly regions at PF (distal region: 5.7 ± 2.0 kPa; medial belly region: 5.0 ± 1.4 kPa) and NE (distal region: 9.8 ± 1.6 kPa; medial belly region: 9.9 ± 1.8 kPa), but it was significantly higher at the distal region (40.7 ± 10.1 kPa) compared with the medial belly region (35.9 ± 9.1 kPa) at DF. CONCLUSIONS: Regional difference in MG tension during passive stretching was observed in the ankle joint position where the MG is sufficiently lengthened, which could lead to muscle strain injuries at the distal musculotendinous junction of the MG. The study was supported by JSPS KAKENHI (Grant Number 15K16500).

The elasticity of skeletal muscle is a mechanical property of this tissue that can be non-invasively measured by shear-wave elastography. This technique is feasible for the monitoring and management of the changes in muscle condition that occur with fatigue, muscle damage, and neuromuscular disease. To utilize the measurement of muscle elasticity for the monitoring and management of muscle condition more effectively, it is important to understand the association of gender as well as the morphological and mechanical properties of adipose tissue and muscle with muscle elasticity. PURPOSE: To investigate the association of gender, tissue thickness, and tissue hardness with muscle elasticity measured by shear-wave elastography. METHODS: Twelve female and 18 male healthy subjects participated in this study. Tissue elasticity (Young’s modulus) of the rectus femoris (RF) was measured in the longitudinal plane at 50% of the thigh length using ultrasound shear-wave elastography. Subcutaneous adipose tissue thickness and muscle thickness at the same measurement site were measured as indicators of the morphological properties of these tissues using B-mode ultrasonography. Tissue hardness was measured as an indicator of the mechanical properties of adipose tissue and muscle by pressing on the skin overlying the RF with a tissue hardness meter. RESULTS: issue elasticity in female subjects (13.4 ± 2.9 kPa) was not significantly different from that in male subjects (15.3 ± 4.6 kPa) (P = 0.23). There was no significant correlation between tissue elasticity and adipose tissue thickness (r = -0.17) or between tissue elasticity and muscle thickness (r = 0.07) (P > 0.05). There was a significant correlation between tissue elasticity and tissue hardness (r = 0.51) (P < 0.01). CONCLUSIONS: Tissue elasticity of the RF measured by shear-wave elastography was not associated with gender or subcutaneous adipose tissue and muscle thicknesses, but was associated with tissue hardness, which reflects the mechanical properties of adipose tissue and muscle. Supported by JSPS KAKENHI Grant Number 24700695.

Background The skeletal muscle mass of the lower limb plays a role in its mobility during daily life. From the perspective of physical resources, leg muscle mass dominantly decreases after the end of the fifth decade. Therefore, an accurate estimate of the muscle mass is important for the middle-aged and older population. The present study aimed to clarify the validity of ultrasound muscle thickness (MT) measurements for predicting leg skeletal muscle mass (SM) in the healthy Japanese middle-aged and older population. Findings MTs at four sites of the lower limb and the bone-free lean tissue mass (LTM) of the right leg were determined using brightness-mode ultrasonography and dual-energy X-ray absorptiometry (DXA), respectively, in 44 women and 33 men, 52- to 78-years old. LTM was used as a representative variable of leg skeletal muscle mass. In the model-development group (30 women and 22 men), regression analysis produced an equation with R 2 and standard error of the estimate (SEE) of 0.958 and 0.3 kg, respectively: LTM (kg) = 0.01464 × (MT SUM ×L) (cm 2 ) - 2.767, where MT SUM is the sum of the product of MTs at four sites, and L is length of segment where MT is determined. The estimated LTM (7.0 ± 1.7 kg) did not significantly differ from the measured LTM (7.0 ± 1.7 kg), without a significant systematic error on a Bland-Altman plot. The application of this equation for the cross-validation group (14 women and 11 men) did not yield a significant difference between the measured (7.2 ± 1.6 kg) or estimated (7.2 ± 1.6 kg) LTM and systematic error. Conclusion The developed prediction equation may be useful for estimating the lean tissue mass of the lower extremity for the healthy Japanese middle-aged and older population.

The goal of this study was to examine the hypothesis that the relationships between passive ankle stiffness and leg stiffness would be different between males and females. 10 males and 10 females performed hopping in place on two legs at three frequencies of 2.0, 2.5, and 3.0 Hz. Based on a spring-mass model, leg stiffness, which is defined as the ratio of maximum ground reaction force to maximum center of mass displacement at the middle of the stance phase, was calculated using the vertical ground reaction force. Further, passive ankle stiffness was calculated as the slope of the passive ankle torque-angle relationship, which results from controlled passive ankle dorsiflexion. There was no significant difference in the leg stiffness between males and females at three hopping frequencies; however, females displayed less passive ankle stiffness than males. Further, significant positive linear relationships were found between the passive ankle stiffness and the leg stiffness for females at all hopping frequencies; however, there were no significant correlations between the passive ankle stiffness and the leg stiffness in males for all hopping frequencies. These results suggest that the relationship between the passive ankle stiffness and the leg stiffness are not the same between males and females, which may lead to a greater risk of anterior cruciate ligament injuries in females.

Although many athletic activities and plyometric training methods involve both unilateral and bilateral movement, little is known about differences in the leg stiffness ( K leg ) experienced during one-legged hopping (OLH) and two-legged hopping (TLH) in place. The purpose of this study was to investigate the effect of hopping frequencies on differences in K leg during OLH and TLH. Using a spring-mass model and data collected from 17 participants during OLH and TLH at frequencies of 2.0, 2.5, and 3.0 Hz, K leg was calculated as the ratio of maximal ground reaction force ( F peak ) to the maximum center of mass displacement (ΔCOM) at the middle of the stance phase measured from vertical ground reaction force. Both K leg and F peak were found to be significantly greater during TLH than OLH at all frequencies, but type of hopping was not found to have a significant effect on ΔCOM. These results suggest that K leg is different between OLH and TLH at a given hopping frequency and differences in K leg during OLH and TLH are mainly associated with differences in F peak but not ΔCOM.

Despite the fact that a stiffer leg spring is prerequisite for achieving a better performance during sports activities, effects of various types of warm-up on the leg stiffness is not well-known. The purpose of this study was to determine if static stretching influences the leg stiffness during two-legged hopping. Fourteen male subjects performed two-legged hopping at 2.2 Hz before and after a 3-min passive stretching of the triceps surae (dorsiflexion of 30°). Based on a spring-mass model, we calculated leg stiffness, which is defined as the ratio of maximal ground reaction force to maximum center of mass displacement at the middle of the stance phase. It was found that there was no significant difference in leg stiffness after passive static stretching. These results suggest that 3-min passive static stretching does not affect the leg-spring behavior and stiffness regulation during two-legged hopping. Finally, possible explanations for the invariant leg stiffness after the passive stretching are discussed.

This paper examines the acute effect of a bout of static stretches on torque fluctuation during an isometric torque-matching task that required subjects to sustain isometric contractions as steady as possible with the plantar flexor muscles at four intensities (5, 10, 15, and 20% of maximum) for 20 s. The stretching bout comprised five 60-s passive stretches, separated by 10-s rest. During the torque-matching tasks and muscle stretching, the torque (active and passive) and surface electromyogram (EMG) of the medial gastrocnemius (MG), soleus (Sol), and tibialis anterior (TA) were continuously recorded. Concurrently, changes in muscle architecture (fascicle length and pennation angle) of the MG were monitored by ultrasonography. The results showed that during stretching, passive torque decreased and fascicle length increased gradually. Changes in these two parameters were significantly associated ( r2= 0.46; P < 0.001). When data from the torque-matching tasks were collapsed across the four torque levels, stretches induced greater torque fluctuation ( P < 0.001) and enhanced EMG activity ( P < 0.05) in MG and TA muscles with no change in coactivation. Furthermore, stretching maneuvers produced a greater decrease (∼15%; P < 0.001) in fascicle length during the torque-matching tasks and change in torque fluctuation (CV) was positively associated with changes in fascicle length ( r2= 0.56; P < 0.001), MG and TA EMG activities, and coactivation ( r2= 0.35, 0.34, and 0.35, respectively; P < 0.001). In conclusion, these observations indicate that repeated stretches can decrease torque steadiness by increasing muscle compliance and EMG activity of muscles around the joint. The relative influence of such adaptations, however, may depend on the torque level during the torque-matching task.

The purpose of this study was to investigate changes in ankle joint stiffness and the associated changes in the gastrocnemius muscle and tendon due to static stretching. Seven healthy male participants lay supine with the hip and knee joints fully extended. The right ankle joint was rotated into dorsiflexion from a 30 degrees plantar flexed position and the torque measured by a dynamometer. The ankle joint was maintained in a dorsiflexed position for 20 min (static stretching of the calf muscles). We performed surface electromyography of the medial and lateral gastrocnemii, the soleus, and the tibialis anterior of the right leg to confirm no muscle activity throughout static stretching and the passive test (passive dorsiflexion). During static stretching, the ankle joint angle and elongation of the gastrocnemius were recorded by goniometry and ultrasonography, respectively. Tendon elongation of the gastrocnemius was calculated based on the changes in the ankle joint angle and muscle elongation. In addition, the relationships between passive torques and ankle joint angles, and elongation of muscle and tendon, were examined before and after static stretching. The ankle dorsiflexion angle and tendon elongation increased significantly by 10 min after the onset of static stretching, while there was no further increase in muscle length. In addition, ankle dorsiflexion angle and tendon elongation at an identical passive torque (30 N center dot m) increased significantly (from 24 +/- 7 degrees to 33 +/- 5 degrees and from 17 +/- 2 mm to 22 +/- 1 mm, respectively) after static stretching. However, muscle elongation was unchanged. In conclusion, the current results suggest that an increase in the ankle joint dorsiflexion angle due to static stretching is attributable to a change in tendon not muscle stiffness.

The present study examined the age-related changes in muscle thickness (MT) and volume (MV) of elbow flexors and developed a prediction equation of the MV based on the MT applicable to men and women with a wide range of age. The MT and MV were determined from a single ultrasonographic image and multiple magnetic resonance imaging scans, respectively, in 72 men and 75 women aged 19-77 year. As a result of examining the age-MT and age-MV relationships by calculation of partial correlation coefficients with the control variable of gender, MV was decreased with aging whereas the corresponding decline in MT was not significant. The subjects were randomly separated into either a validation (38 men and 42 women) or a cross-validation (34 men and 33 women) group, and a multiple regression equation to estimate MV using not only MT but also upper arm length (L), age and gender as independent variables [MV (cm(3)) = 60.8 x MT (cm) + 6.48 x L (cm) - 0.709 x age (year) + 51.4 x gender (0 women, 1 men) - 187.4] was validated and cross-validated. Thus, the prediction equation for MV of elbow flexors newly developed was shown to be applicable to men and women with a wide range of age.

The purpose of this study was to determine the interindividual variability of the upper and lower limb muscle size in young men. Subjects were 655 Japanese men aged 18-39 years. They were sedentary and mildly to highly active individuals, including college athletes of various sports. Muscle thicknesses at each of the anterior and posterior portions of the upper arm, thigh, and lower leg were measured using B-mode ultrasonography. Interindividual variability of muscle thickness was evaluated by coefficients of variation (CVs). The CVs of muscle thicknesses were found to be in the order of upper arm posterior (17.7%), thigh anterior (14.8%), thigh posterior (12.6%), upper arm anterior (12.2%), lower leg anterior (9.8%), and lower leg posterior (9.4%). The CVs were significantly different between each pair of measurement sites except for those of upper arm anterior-thigh posterior and lower leg anterior-posterior. These differences remain significant even when the muscle thicknesses were normalized to the segment length. The observed differences in the size variability can be interpreted as muscle-related differences in hypertrophic responsiveness to resistance training. The muscle-dependent size variability may be related to the differences in the fiber-type composition and/or muscle usage in daily life among examined muscle groups.

The purpose of the present study was to examine the influence of tendon elasticity, muscle strength and muscle activities on the amount of mechanical work enhancement associated with a counter movement. Twenty-one athletes performed a unilateral maximal jump using only the ankle joint with (CMJ) and without (no-CMJ) a counter movement on a sledge apparatus. Mechanical work done by the ankle joint was calculated from the ground reaction force and ankle joint kinematic parameters, and the difference between CMJ and no-CMJ conditions (ΔWork) was determined. During the exercise, electromyographic (EMG) activities were recorded from the triceps surae muscles. The maximal isometric plantar flexion torque and Achilles tendon stiffness were also determined using a torque meter and ultrasonogram. No significant correlation was found between ΔWork and either tendon stiffness or the maximal torque. In addition, neither the difference between CMJ and no-CMJ in iEMG nor mEMG was correlated with ΔWork. On the other hand, ΔWork was significantly correlated with the integrated EMG during the braking phase (r=0.52, p<0.05) and both the integrated (r=0.55, p<0.01) and mean (r=0.53, p<0.05) EMG during the push-off phase of CMJ. These results suggest that individual differences in ΔWork are influenced not by differences in the mechanical properties of the muscle-tendon unit, but by the individuality of muscle activities during CMJ.

Akagi, R, Takai, Y, Kato, E, Fukuda, M, Wakahara, T, Ohta, M, Kanehisa, H, Kawakami, Y, and Fukunaga, T. Relationships between muscle strength and indices of muscle cross-sectional area determined during maximal voluntary contraction in middleaged and elderly individuals. J Strength Cond Res 23(4): 1258– 1262, 2009—The present study examined how muscle crosssectional area (CSA) indices determined at rest and during maximal voluntary contraction (MVC) are related to muscle strength in middle-aged and elderly individuals (22 men and 36 women, 51–77 years). The muscle thickness (MT) of elbow flexors and circumference (C) at the level 60% distal to the upper arm was measured by ultrasonography and a measuring tape, respectively, both at rest and during isometric MVC of elbow flexion. The muscle strength (F) of elbow flexors was calculated by dividing the torque developed during MVC by the forearm length of each subject. The product of MT and C (MT3C) and the square of MT (MT) were defined as the muscle CSA indices. The F was significantly correlated with MT3C during MVC (r = 0.905, p # 0.001) and at rest (r = 0.778, p # 0.001), with the former relationship significantly stronger than the latter (p # 0.001). Similarly, F was significantly correlated with MT both during MVC (r = 0.896, p # 0.001) and at rest (r = 0.780, p # 0.001), and there was also a significant difference between the correlation coefficients (p # 0.01). These findings show that, in middle-aged and elderly individuals, muscle strength is more closely related to muscle CSA indices during MVC than at rest. It is concluded that the present muscle CSA indices taken during MVC enable easy and practical evaluation of the muscle strength per size of elbow flexors in middle-aged and elderly individuals.

Akagi, R, Takai, Y, Kato, E, Fukuda, M, Wakahara, T, Ohta, M, Kanehisa, H, Kawakami, Y, and Fukunaga, T. Relationships between muscle strength and indices of muscle cross-sectional area determined during maximal voluntary contraction in middle-aged and elderly individuals. J Strength Cond Res 23(4): 1258-1262, 2009-The present study examined how muscle cross-sectional area (CSA) indices determined at rest and during maximal voluntary contraction (MVC) are related to muscle strength in middle-aged and elderly individuals (22 men and 36 women, 51-77 years). The muscle thickness (MT) of elbow flexors and circumference (C) at the level 60% distal to the upper arm was measured by ultrasonography and a measuring tape, respectively, both at rest and during isometric MVC of elbow flexion. The muscle strength (F) of elbow flexors was calculated by dividing the torque developed during MVC by the forearm length of each subject. The product of MT and C (MT×C) and the square of MT (MT2) were defined as the muscle CSA indices. The F was significantly correlated with MT×C during MVC (r = 0.905, p ≤ 0.001) and at rest (r = 0.778, p ≤ 0.001), with the former relationship significantly stronger than the latter (p ≤ 0.001). Similarly, F was significantly correlated with MT2 both during MVC (r = 0.896, p ≤ 0.001) and at rest (r = 0.780, p ≤ 0.001), and there was also a significant difference between the correlation coefficients (p ≤ 0.01). These findings show that, in middle-aged and elderly individuals, muscle strength is more closely related to muscle CSA indices during MVC than at rest. It is concluded that the present muscle CSA indices taken during MVC enable easy and practical evaluation of the muscle strength per size of elbow flexors in middle-aged and elderly individuals.

It has been recognized that there are intramuscular site-specific differences in muscle injury. Based on the accumulating evidence of inter-individual variation in the mechanical properties of human tendinous tissues(TT), we investigated how the differences in Young's modulus of TT affect the strain experienced by muscle and TT during forced lengthening of muscle-tendon unit, with computer simulation. PURPOSE: We tested the effect of different Young's modulus of TT on the magnitude and distribution of strain in human triceps surae muscle-tendon complex under eccentric contraction. Our hypothesis was that the different Young's modulus influences the strain not only in TT but also in muscle tissues. METHODS: Finite element analysis was used for computation of strain in tissues. A hexahedral constitutive model based on MR images was constructed. Young's modulus and Poisson's ratio of muscle tissues during maximal eccentric contraction were estimated from our experimental data and previous reports. The variation of TT stiffness was directly measured by human experiments with ultrasonography (n=30), and the calculated maximal (STIF: 2439.0MPa) and minimal (COMP: 272.4MPa) Young's modulus were used for analysis. Anatomical origins of muscle were fully fixed. A 20mm lengthening was applied to simulate eccentric contraction by displacing the end point of Achilles tendon. RESULTS: The change in Young's modulus of TT altered the magnitude and distribution of strain in both muscle and TT. In both conditions, strain of muscle tissues was concentrated on the origin of soleus muscle. The stain was larger in STIF than in COMP (COMP: 0.068+/-0.068 and STIF: 0.173+/-0.149). The strain at the muscle-tendon junction did not show significant difference. In COMP, larger strain was determined at the portion with smallest cross-sectional area in Achilles tendon (COMP: 0.144+/-0.042 and STIF: 0.047+/-0.014). CONCLUSIONS: The elongation of compliant TT decreased strain at the origin of soleus muscle by 61%, suggesting the role of compliant tendon in the reduction of muscle damage by decreasing the amplitude and distribution of sizable strains in muscle tissues.

Flexibility is often evaluated from the joint range of motion (ROM), but the mechanisms underlying gender differences in joint ROM have not been elucidated. The purpose of this study was to investigate the factors influencing the difference in ankle joint ROM between men and women with respect to the extensibility of muscle-tendon complex. Eighteen men (21-26 yr, 66.4±6.0kg, 173.7±7.4cm mean±SD) and 12 women (19-27yr, 52.9±4.8kg, 163.4±4.3cm) participated in this study. Each subject was seated with the knee extended, and the ankle joint was attached to a foot plate, by which the ankle joint was passively dorsiflexed with torque gradually increasing from zero to a value at which the passive loading to the ankle joint was just tolerable for each subject. During the passive loading, real-time ultrasonogram was taken to track the movement of MTJ (muscle-tendon junction of the gastrocnemius medialis and Achilles tendon) as the elongation of muscle belly (dMus). The change of MTC (muscle-tendon complex) length (dMTC) during the passive dorsiflexion was estimated from changes in ankle joint angle. Tendon elongation (dTen) was calculated by subtracting dMus from dMTC. There was no significant difference in normalized passive torque during passive dorsiflexion between men and women. Women were more flexible, i.e., they demonstrated greater dMTC, which was accompanied by greater dTen at lower torque levels. However, dMus was not different between men and women. It was concluded that gender difference in the joint ROM at the ankle reflects more compliant Achilles tendon in women than in men.

The present study aimed to determine the differences in the length-force relationship between muscle fiber and muscle tendon complex (MTC) and to relate the shape of the length-force relationship to the architectural changes in MTC (the elongation of tendon structures and pennation angle changes). In six male subjects, tetanic contractions (2s at 50Hz) of the tibialis anterior muscle were induced electrically. At a steady state of tetanic contraction, the muscle fiber length and pennation angle were measured from ultrasonic images, and the muscle fiber force, tendon force, MTC length and elongation of tendon structures were calculated. The measured and normalized length-force relationships were clearly different between muscle fiber and MTC, especially on the length axis. The differences could be attributed to the existence of compliant tendon structures and increment of pennation angles related to elongation of tendon structures, suggesting that compliant tendon structures in human muscles influence the length-dependent force generation in both muscle fiber and MTC.