Association between elastography-assessed muscle mechanical properties and high-speed dynamic performance.

Clarifying the muscular factors that contribute to performance improvement can be beneficial for athletes and coaches. The present study examined the relationships between the resting muscle shear modulus and dynamic performance during the stretch-shortening cycle (SSC) and explosive exercises. To this end, we measured the jump height during three types of vertical jumps (squat jump [SJ], countermovement jump [CMJ], and rebound jump [RJ]) and the multi-joint leg extension power at three velocities (low, moderate, and high) of 30 healthy women. Using ultrasound elastography, the resting shear modulus of the vastus lateralis was assessed in the sitting position as an index of passive muscle mechanical properties. The results showed that the shear modulus was positively correlated with RJ height and multi-joint leg extension power at moderate and high velocities (r = 0.435-0.563, P < 0.05). There was no significant correlation between the shear modulus and SJ height, CMJ height, and multi-joint leg extension power at low velocity. Contact time (i.e. the time under force exertion against the ground) during RJ (161 ± 19 ms) was 19% of that during CMJ (869 ± 171 ms). The results suggest that passive muscle mechanical properties play an important role in high-speed SSCs and dynamic explosive performance.HighlightsThe resting shear modulus was related to performance in stretch-shortening cycle (SSC) exercise involving a series of rapidly performed SSCs.The resting shear modulus was associated with muscular power in high-velocity conditions but not in low-velocity conditions.This study suggests that the passive mechanical properties of an agonist muscle play an important role in high-speed dynamic performance.

Association between interindividual variability in training volume and strength gain.

This study aimed to examine the association between interindividual variability in strength changes and in training volume. A total of 26 untrained men completed 4-weeks of isometric knee extension (KE group, n = 12) and hip flexion (HF group, n = 14) training. Each training session comprised four sets of ten isometric contractions, 3-s contractions every 20 s. Training volume, which was defined as impulse during contractions, and maximal voluntary contraction (MVC) torque during KE and HF were evaluated. Based on the magnitude of MVC torque changes, the participants were divided into the high and low responders (n = 13; KE = 6 and HF = 7 per responders). The MVC torque changes (KE, 20.8%; HF, 22.4%) and total training volume did not significantly differ between the two groups. A higher training volume was demonstrated in the low responders than the high responders. The total training volume was positively associated with the MVC torque changes in low responders (r = 0.869%, 95% confidence interval [0.610, 0.960], p < 0.001), but not in high responders [r = 0.229, 95% confidence interval (-0.368, 0.693), p = 0.451], KE or HF group. Results showed that training volume was an important factor in determining the magnitude of strength gains in low responders, and MVC torque could improve by approximately 20% with the use of the study protocol regardless of joint actions involved during training.

Unique neuromuscular activation of the rectus femoris during concentric and eccentric cycling.

This study investigated neuromuscular activations of thigh muscles during concentric cycling (CONcycling) and eccentric cycling (ECCcycling). Eleven untrained men completed 30 s of CONcycling and ECCcycling each at 5 power outputs of 100-300 W (every 50-W interval). During cycling, root mean square of surface electromyographic signals (RMS-EMG) were obtained from the proximal and distal regions of the rectus femoris (RFp and RFd), vastus lateralis (VL), and biceps femoris (BF). The rating of perceived exertion (RPE) was evaluated using the 6-20 Borg Scale. The RMS-EMG of VL and BF were 21.6%-67.6% higher (P < 0.05) during CONcycling than ECCcycling at all power outputs, while those of RFp and RFd at 100-200 W were 29.6%-40.4% lower during CONcycling than ECCcycling. The RPE was similar between CONcycling at 150 W (10 ± 2) and ECC at 250 W (10 ± 2). There were no significant differences in the RMS-EMG for VL or BF between CONcycling at 150 W and ECCcycling at 250 W; however, the RF RMS-EMG was greater during ECCcycling as compared with CONcycling. There were no regional differences in RF activations. These results demonstrated the unique neuromuscular activation of RF as compared to those of other thigh muscles during CONcycling and ECCcycling.

Muscle Damage Indicated by Maximal Voluntary Contraction Strength Changes From Immediately to 1 Day After Eccentric Exercise of the Knee Extensors.

The present study examined if the magnitude of changes in indirect muscle damage markers could be predicted by maximal voluntary isometric contraction (MVIC) torque changes from immediately to 1 day after eccentric exercise. Twenty-eight young men performed 100 maximal isokinetic (60°/s) eccentric contractions of the knee extensors. MVIC torque, potentiated doublet torque, voluntary activation (VA) during MVIC, shear modulus of rectus femoris (RF), vastus medialis and lateralis, and muscle soreness of these muscles were measured before, immediately after, and 1-3 days post-exercise. Based on the recovery rate of the MVIC torque from immediately to 1-day post-exercise, the participants were placed to a recovery group that showed an increase in the MVIC torque (11.3-79.9%, n = 15) or a no-recovery group that showed no recovery (-71.9 to 0%, n = 13). No significant difference in MVIC torque decrease immediately post-exercise was found between the recovery (-33 ± 12%) and no-recovery (-32 ± 9%) groups. At 1-3 days, changes in MVIC torque (-40 to -26% vs. -22 to -12%), potentiated doublet torque (-37 to -22% vs. -20 to -9%), and proximal RF shear modulus (29-34% vs. 8-15%) were greater (p < 0.05) for the no-recovery than recovery group. No significant group differences were found for muscle soreness. The recovery rate of MVIC torque was correlated (p < 0.05) with the change in MVIC torque from baseline to 2 (r = 0.624) or 3 days post-exercise (r = 0.526), or peak change in potentiated doublet torque at 1-3 days post-exercise from baseline (r = 0.691), but not correlated with the changes in other dependent variables. These results suggest that the recovery rate of MVIC torque predicts changes in neuromuscular function but not muscle soreness and stiffness following eccentric exercise of the knee extensors.

Muscle length influence on rectus femoris damage and protective effect in knee extensor eccentric exercise.

This study tested the hypothesis that the magnitude of rectus femoris (RF) damage and the repeated bout effect (RBE) would be greater after knee extensor eccentric exercise performed in a supine (long RF lengths) than a sitting (short RF lengths) position, and the muscle length effects would be more prominent at the proximal than distal RF. Young untrained men were placed to one of the two groups (n = 14 per group). S group performed the knee extensor eccentric exercise in the sitting position for the first bout and the supine position for the second bout, and L group performed the exercise in the supine position for two bouts, with 4 weeks between bouts. Dependent variables included evoked and maximal voluntary isometric contraction (MVC) torque, electromyography (EMG) during MVC, muscle soreness, and shear modulus, which were measured before and 1-3 days after each exercise bout. After the first bout, L group in comparison with S group showed greater (P < .05) changes in hip flexor MVC torque (average of 1-3 days post-exercise: -11.1 ± 9.4% vs -5.0 ± 7.5%), proximal RF EMG (-22.4 ± 16% vs -9.0 ± 21.9%), and proximal RF shear modulus (33.2 ± 22.8% vs 16.9 ± 13.5%). No significant differences between groups were evident for any of other variables after the first bout including knee extensor MVC torque, and for the changes in all variables after the second bout. These results supported the hypothesis that RF damage would be greater for the spine than sitting position especially at the proximal region, but did not support the hypothesis about the RBE.

Age-Related Differences in the Effect of Prolonged Vibration on Maximal and Rapid Force Production and Balance Ability.

We tested the hypothesis that older adults would not likely experience deficits in maximal and explosive plantar flexion strength and standing balance performance induced by prolonged Achilles tendon vibration compared with young adults. Fifteen older men (OM, 73 ± 5 years) and 15 young men (YM, 24 ± 4 years) participated in two interventions on different days: lying in a quiet supine position for 30 min with or without prolonged vibration to the Achilles tendon. Before and after the interventions, maximal voluntary contraction (MVC) torque during plantar flexion, rate of torque development (RTD), and center of pressure (COP) speed during single-leg standing were measured. The root mean square of the electromyogram (RMS-EMG) during performance and V-wave and voluntary activation during MVC were assessed. The MVC torque (7 ± 7%) and RTD (16 ± 15%) of YM but not OM significantly decreased after vibration. In addition, the relative changes observed in YM positively correlated with changes in RMS-EMG of the medial gastrocnemius (MG) (MVC torque and RTD) and in MG V-wave and voluntary activation (MVC torque). COP speed significantly increased (16 ± 20%) in YM only after vibration and was accompanied by increased activation of the lateral gastrocnemius. This is the first study to show that the effects of prolonged Achilles tendon vibration on strength and balance performances were apparent in young adults only. The differences between the age groups may be related to the attenuated gastrocnemius neuromuscular function in older adults.

Plantar flexor strength at different knee positions in older and young males and females.

INTRODUCTION: This study examined the effects of age and knee position (fully extended, K0; 90° flexed, K90) on plantar flexor maximal voluntary contraction (MVC) torque and the rate of torque development (RTD) in both sexes. METHODS: The following parameters were measured in 32 older (66-81 yr, 17 males and 15 females) and 37 young (20-30 yr, 18 males and 19 females) adults: evoked peak twitch torque, time to peak twitch torque, RTD of the twitch torque, MVC torque, RTD at early (0-50 ms, RTD0-50) and later (100-200 ms, RTD100-200) time intervals during explosive contractions, voluntary activation (VA%) during MVC, root mean square of the electromyogram (RMS-EMG) during MVC and explosive contractions, thickness of the triceps surae, and pennation angle of the medial gastrocnemius. The magnitudes of the differences were interpreted based on Cohen's d (d). RESULTS: Age-related difference in RTD0-50 was greater for females (d = 1.36) than males (d = 1.03) and vice versa for MVC torque and RTD100-200. For young adults, MVC torque, RTDs, and RMS-EMGs of the gastrocnemius but not the soleus were significantly higher in K0 than in K90. For older adults, no differences in voluntary RTDs were observed between K0 and K90, and RMS-EMGs of the gastrocnemius were higher in K90 than in K0, except for that of the lateral gastrocnemius in the early time intervals during explosive contraction. The age-related difference in the effect of knee position for RTD0-50 was higher in females than males, and vice versa for MVC torque and RTD100-200. CONCLUSION: The results suggested that the effects of age and knee joint angle on the plantar flexor performance were more prominent in the early phase of force production for females and were more apparent in the later phase and maximal force for males.

Eight-Week Low-Intensity Squat Training at Slow Speed Simultaneously Improves Knee and Hip Flexion and Extension Strength.

Considering that the squat exercise requires flexion and extension of the knee and hip joints, a resistance training program based on squat exercises should efficiently increase the flexion and extension strength of both the knee and hip. To our knowledge, however, no study has simultaneously investigated the effects of squat training on both flexion and extension strength in both the knee and hip. Low-intensity squat exercises at slow speeds can be expected to effectively and safely improve knee and hip flexion and extension strength in a wide range of individuals. This study aimed to clarify whether knee and hip flexion and extension strength improved after an 8-week low-intensity squat training program at slow speed. Twenty-four untrained young men were randomly assigned to a training or control group. Participants in the training group performed 40% one-repetition maximum parallel squats at slow speed (4 s for concentric/eccentric actions), 3 days per week for 8 weeks. Before and after the intervention, isometric peak torque of the knee and hip flexors and extensors during maximal voluntary contraction (MVC) was determined. For the knee flexors and extensors, muscle volume was also measured. There were significant training-induced increases in peak torque (P < 0.05). The training effects on knee and hip extension torque (effect size = 0.36-0.38) were higher than those on knee and hip flexion torque (effect size = 0.09-0.13). The squat training used here increased both knee and hip flexion and extension strength, but the training effects on the flexion strength were less than those on the extension strength. Regarding the knee extensors, a significant training-related increase in muscle volume was found (P < 0.05) without neuromuscular adaptations. In addition, there were significant correlations between the training-induced increases in muscle volume and peak torque of KE. These results suggest that muscle hypertrophy may be responsible for increased muscle strength of the knee extensors after an 8-week low-intensity squat training program at slow speed.

Sex difference in fatigability of knee extensor muscles during sustained low-level contractions.

This study investigated whether the sex difference in fatigability of the knee extensors (KE) is explained by the sex difference in fatigue-induced changes in the shear modulus of one or more muscles of KE in 18 young men and 23 young women. The shear moduli of the resting rectus femoris and medial and lateral vastus muscles (VL) were measured before and after a sustained contraction at 20% peak torque during a maximal voluntary isometric contraction of KE until the endurance limit, in addition to evoked torque and voluntary activation (VA%). The fatigue-induced decrease in maximal muscle strength was more prominent in men than in women. Only the VL shear modulus for men increased after the fatiguing task, and a sex difference was observed in the percentage change in the VL shear modulus before and after the fatiguing task. The fatigue-induced decreased ratio was greater for men than for women in evoked torque, but not in VA%. These results suggest that although peripheral and central fatigue both influenced the fatigue-induced decrease in maximal muscle strength regardless of sex, the sex difference in KE fatigability is explained by that in peripheral fatigue, particularly the degree of peripheral VL fatigue.

Effects of sex and joint action on voluntary activation.

The current study tested the hypothesis that voluntary activation during maximal voluntary contraction (MVC) conditionally depends on sex and joint action. Twenty-eight healthy adults (14 of each sex) performed knee extensor MVC and plantar flexor MVC at extended and flexed knee positions. Voluntary activation during MVC was assessed using a twitch interpolation technique. The voluntary activation during plantar flexor MVC at the extended knee position was significantly lower (P = 0.020, 95% confidence interval 1.4 to 14.6, Cohen's d for between-subject design = 0.94) in women (88.3% ± 10.0%) than in men (96.2% ± 6.6%). In contrast, no significant sex differences were shown in the voluntary activation during knee extensor MVC (93.7% ± 5.9% (women) vs. 95.0%  ± 3.9% (men)) and during plantar flexor MVC at the flexed knee position (90.4% ± 12.2% (women) vs. 96.8% ± 5.6% (men)). The voluntary activation during knee extensor MVC was significantly higher (P = 0.001, 95% confidence interval 2.1 to 8.8, Cohen's d for within-subject design = 0.69) than that during plantar flexor MVC at the extended knee position in women, whereas the corresponding difference was not observed in men. The results revealed that the existence of sex difference in the voluntary activation during MVC depends on joint action and joint angle.

Thigh and Psoas Major Muscularity and Its Relation to Running Mechanics in Sprinters.

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.

Muscle size-strength relationship including ultrasonographic echo intensity and voluntary activation level of a muscle group.

This study aimed to investigate the relationship between muscle volume (MV) and joint torque for the plantar flexors (PF) in 40 young (20 men and 20 women) and 33 elderly (19 men and 14 women) individuals in consideration of the voluntary activation (VA) of PF and ratio of intramuscular adipose tissue within PF assessed by ultrasonographic echo intensity (EI). MV was estimated from the thickness of PF on ultrasonography and the lower leg length using a prediction equation previously reported. The maximal voluntary contraction torque of isometric plantar flexion was measured as TQMVC. VA (%) was assessed using the twitch interpolation technique, and maximal joint torque calculated by TQMVC/VA × 100 was adopted as TQ100%. The correlation coefficients between MV and TQMVC (r = 0.518) and between MV and TQ100% (r = 0.602) were both significant, with the latter being significantly higher than the former. When a stepwise multiple regression analysis using MV and EI as independent variables and TQ100% as the dependent variable was performed, MV (ß = 0.554) and EI (ß = -0.203) were both selected as significant contributors for estimating TQ100%. Additionally, the residual errors of TQ100% using the multiple regression equation (independent variables: MV and EI; 18.6 ±â€¯14.4 Nm) were significantly lower than those using the simple regression equation (independent variable: MV; 36.6 ±â€¯28.0 Nm). These results suggest that the consideration of VA and EI with muscle size results in a closer muscle size-strength relationship than previously achieved.

Effect of hip joint angle on concentric knee extension torque.

This study tested the hypothesis that the effect of hip joint angle on concentric knee extension torque depends on knee joint angle during a single knee extension task. Twelve men performed concentric knee extensions in fully extended and 80° flexed hip positions with maximal effort. The angular velocities were set at 30°â€¯s-1 and 180°â€¯s-1. The peak torque and torques attained at 30°, 50°, 70° and 90° (anatomical position = 0°) of the knee joint were compared between the two hip positions. Muscle activations of the vastus lateralis, medialis, rectus femoris and biceps femoris were determined using surface electromyography. The peak torque was significantly greater in the flexed than in the extended hip position irrespective of angular velocity. The torques at 70° and 90° of the knee joint at both angular velocities and at 50° at 180°â€¯s-1 were significantly greater in the flexed than in the extended hip position, whereas corresponding differences were not found at 30° (at either angular velocity) and 50° (at 30°â€¯s-1) of the knee joint. No effect of hip position on muscle activation was observed in any muscle. These results supported our hypothesis and may be related to the force-length and force-velocity characteristics of the rectus femoris.

Muscle Shear Moduli Changes and Frequency of Alternate Muscle Activity of Plantar Flexor Synergists Induced by Prolonged Low-Level Contraction.

During prolonged low-level contractions, synergist muscles are activated in an alternating pattern of activity and silence called as alternate muscle activity. Resting muscle stiffness is considered to increase due to muscle fatigue. Thus, we investigated whether the difference in the extent of fatigue of each plantar flexor synergist corresponded to the difference in the frequency of alternate muscle activity between the synergists using muscle shear modulus as an index of muscle stiffness. Nineteen young men voluntarily participated in this study. The shear moduli of the resting medial and lateral gastrocnemius muscles (MG and LG) and soleus muscle (SOL) were measured using shear wave ultrasound elastography before and after a 1-h sustained contraction at 10% peak torque during maximal voluntary contraction of isometric plantar flexion. One subject did not accomplish the task and the alternate muscle activity for MG was not found in 2 subjects; therefore, data for 16 subjects were used for further analyses. The magnitude of muscle activation during the fatiguing task was similar in MG and SOL. The percent change in shear modulus before and after the fatiguing task (MG: 16.7 ± 12.0%, SOL: -4.1 ± 13.9%; mean ± standard deviation) and the alternate muscle activity during the fatiguing task (MG: 33 [20-51] times, SOL: 30 [17-36] times; median [25th-75th percentile]) were significantly higher in MG than in SOL. The contraction-induced change in shear modulus (7.4 ± 20.3%) and the alternate muscle activity (37 [20-45] times) of LG with the lowest magnitude of muscle activation during the fatiguing task among the plantar flexors were not significantly different from those of the other muscles. These results suggest that the degree of increase in muscle shear modulus induced by prolonged contraction corresponds to the frequency of alternate muscle activity between MG and SOL during prolonged contraction. Thus, it is likely that, compared with SOL, the alternate muscle activity of MG occurs more frequently during prolonged contraction due to the greater increase in fatigue of MG induced by the progression of a fatiguing task.

Effect of knee alignment on the quadriceps femoris muscularity: Cross-sectional comparison of trained versus untrained individuals in both sexes.

Knee alignment is suggested to be a factor affecting each quadriceps femoris muscle size, and knee alignment such as Q-angle differs between men and women. Also, training can induce inhomogeneous hypertrophy among the quadriceps femoris, thereby leading to different component characteristics of the muscles. If Q-angle is a major determinant of the quadriceps femoris muscularity, it is hypothesized that the sex-related difference in the quadriceps femoris muscularity, if any, is further highlighted in trained individuals, being associated with Q-angle. We tested this hypothesis. Magnetic resonance images of the right thigh were obtained from 26 varsity rowers as trained subjects (13 for each sex) and 34 untrained individuals as controls (17 for each sex). From the images, muscle volume of each constituent of the quadriceps femoris (vastus lateralis, VL; medialis, VM; intermedius; rectus femoris) was determined. The Q-angle was measured during quiet bilateral standing with hand support as needed. Percent volume of VM to the total quadriceps femoris was greater in female rowers than male rowers and female controls, and that of VL was greater in male rowers than male controls. There were no correlations between Q-angle and percent muscle volume in any muscles regardless of rowing experience or sex. The current study revealed that well-trained rowers have sex-related quadriceps femoris muscularity but no significant correlations between percent muscle volume in any muscles and Q-angle. Our findings suggest that Q-angle is not a major determinant of the quadriceps femoris muscularity in either well-trained or untrained individuals.

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.

Effect of calf-raise training on rapid force production and balance ability in elderly men.

This study examined whether home-based, high-speed calf-raise training changes the rate of torque development (RTD) during plantar flexion contractions and balance performance in elderly men. Thirty-four healthy elderly men (73 ± 5 yr) were randomly assigned to a training or control group (n = 17 in each group). The subjects in the training group completed 8 wk (3 times/wk) of home-based bilateral calf-raise training using body mass. Before and after the intervention, RTD during plantar flexion contractions and center-of-pressure (COP) displacement during single-leg standing were measured. Surface electromyographic amplitude of the triceps surae and tibialis anterior during the strength and single-leg standing was measured. Clinical magnitude-based inferences were used to interpret the training effect, with the smallest worthwhile effect assumed to be 0.2 of the baseline SD. The peak RTD increased 21% (90% confidence limits, ±19%) relative to the control group, which was accompanied by corresponding changes of the medial gastrocnemius (MG) and soleus (SOL) activations. The effect on COP displacement was possibly trivial (0%, ±13%), whereas substantial reduction in the MG (-19%, ±15%) and SOL (-25%, ±13%) activations during standing was observed. Our findings indicate that calf-raise training at home, performed without special equipment or venue, induces a substantial increase in the plantar flexors' rapid force-generating capability and triceps surae activations. Although the training effect on standing balance performance was not substantial, observed changes in the triceps surae activations during standing are expected to contribute to future balance performance improvement.NEW & NOTEWORTHY Calf-raise training with the intent to move rapidly, without special equipment or venue, induces an improvement of explosive plantar flexion force, which is attributable to neuromuscular rather than musculotendinous adaptations. Although the training effect on balance performance was trivial, we found a sign of improvement (i.e., neuromuscular adaptations during standing). In conclusion, functional neuromuscular capacity can be enhanced by home-based calf-raise exercise in elderly men, which may protect against mobility loss with aging.

Plyometric Training Favors Optimizing Muscle-Tendon Behavior during Depth Jumping.

The purpose of the present study was to elucidate how plyometric training improves stretch-shortening cycle (SSC) exercise performance in terms of muscle strength, tendon stiffness, and muscle-tendon behavior during SSC exercise. Eleven men were assigned to a training group and ten to a control group. Subjects in the training group performed depth jumps (DJ) using only the ankle joint for 12 weeks. Before and after the period, we observed reaction forces at foot, muscle-tendon behavior of the gastrocnemius, and electromyographic activities of the triceps surae and tibialis anterior during DJ. Maximal static plantar flexion strength and Achilles tendon stiffness were also determined. In the training group, maximal strength remained unchanged while tendon stiffness increased. The force impulse of DJ increased, with a shorter contact time and larger reaction force over the latter half of braking and initial half of propulsion phases. In the latter half of braking phase, the average electromyographic activity (mEMG) increased in the triceps surae and decreased in tibialis anterior, while fascicle behavior of the gastrocnemius remained unchanged. In the initial half of propulsion, mEMG of triceps surae and shortening velocity of gastrocnemius fascicle decreased, while shortening velocity of the tendon increased. These results suggest that the following mechanisms play an important role in improving SSC exercise performance through plyometric training: (1) optimization of muscle-tendon behavior of the agonists, associated with alteration in the neuromuscular activity during SSC exercise and increase in tendon stiffness and (2) decrease in the neuromuscular activity of antagonists during a counter movement.

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.

Association between rapid force production by the plantar flexors and balance performance in elderly men and women.

Plantar flexion strength and balance ability are considered to be crucial for avoiding falls. However, no clear relationship has been established between these two factors in elderly population. This study aimed to examine the association between plantar flexion strength and balance performance in elderly men and women. Forty-three men and 35 women aged over 65 years performed isometric plantar flexion as fast and hard as possible. From the time-torque curve, the rate of torque development in time intervals of 30, 50, 100, 150, and 200 ms from the onset of contraction was determined and normalized to peak torque. In addition, the center of pressure displacement during single-leg standing was calculated and normalized to height. When the data were collapsed over sexes, the normalized rate of torque development was negatively correlated with the normalized center of pressure displacement, except for the time interval of 200 ms. By sex, regardless of the time interval, there was a negative correlation between the normalized rate of torque development and the normalized center of pressure displacement in the elderly men but not in the elderly women. No correlation was seen between the peak torque and normalized center of pressure displacement in either pooled or separated data. The findings suggest that the capability of rapid force production rather than maximal force production of the plantar flexion is important for balance ability in elderly men, but this capability may not be relevant in elderly women.