Naoya Tsunoda

Activation of agonist and antagonist muscles at different joint angles during maximal isometric efforts

The purpose of this study was to investigate the influence of different angles of the knee joint on the activation level of an agonist (quadriceps femoris muscle) and antagonist (biceps femoris muscle) from electromyographic activities and activation levels (twitch interpolation). Isometric torque measurements were performed on 23 healthy subjects at 10° intervals between 40° and 110° of knee joint flexion. Superimposed twitches at maximal voluntary contraction were applied and the voluntary activation estimated. To quantify the antagonist muscle activity, we normalized its integrated EMG (iEMG) value at each joint angle with respect to its iEMG value at the same angle when acting as an agonist at maximal effort. The activation levels at the knee-flexed position (80–110°) were higher than that at the knee-extended position (40–70°). The co-activation levels at 90, 100, and 110° were significantly higher than that the other knee angle. These results suggest that the activation level of an agonist (quadriceps femoris) muscle and the co-activation level of an antagonist (biceps femoris) muscle were higher in longer muscles than in shorter muscles. It was also concluded that the risk of knee injuries could be reduced by applying these mutual relationships between activation levels of agonist and antagonist muscles.

Time Course of Changes in Muscle and Tendon Properties During Strength Training and Detraining

Kubo, K, Ikebukuro, T, Yata, H, Tsunoda, N, and Kanehisa, H. Time course of changes in muscle and tendon properties during strength training and detraining. J Strength Cond Res 24(2): 322-331, 2010-The purpose of this study was to investigate the time course of changes in mechanical and morphological properties of muscle and tendon during isometric training and detraining. Eight subjects completed 3 months of isometric knee extension training and detraining for another 3 months. At beginning and on every 1 month of training and detraining periods, muscle strength, neural activation level, muscle and tendon cross-sectional areas (CSA), and tendon stiffness were measured. Training increased muscle strength and neural activation level by 29.6 and 7.3% after 2 months and by 40.5 and 8.9% after 3 months (all ps < 0.05). Muscle CSA and tendon stiffness did not change until 2 months of training period, and afterward, the increases in muscle CSA and tendon stiffness reached statistical significance at the end of training period (both ps < 0.05). During detraining period, muscle strength and neural activation level did not change, although muscle CSA and tendon stiffness decreased to pre-training level at 1 and 2 months of detraining, respectively. These results suggest that the adaptations of tendon properties and muscle morphology to resistance training are slower than those of muscle function and inversely that the adaptations of former to detraining are faster than those of latter.

Effects of static and dynamic training on the stiffness and blood volume of tendon in vivo

The purpose of this study was to investigate the effects of static and dynamic training on the stiffness and blood volume of the human tendon in vivo. Ten subjects completed 12 wk (4 days/wk) of a unilateral training program for knee extensors. They performed static training on one side [ST; 70% of maximum voluntary contraction (MVC)] and dynamic training on the other side (DT; 80% of one repetition maximum). Before and after training, MVC, neural activation level (by interpolated twitch), muscle volume (by magnetic resonance imaging), stiffness of tendon-aponeurosis complex and patella tendon (by ultrasonography), and blood volume of patella tendon (by red laser lights) were measured. Both protocols significantly increased MVC (49% for ST, 32% for DT; both P < 0.001), neural activation level (9.5% for ST, 7.6% for DT; both P < 0.01), and muscle volume (4.5% for ST, 5.6% for DT; both P < 0.01). The stiffness of tendon-aponeurosis complex increased significantly after ST (55%; P = 0.003) and DT (30%; P = 0.033), while the stiffness of patella tendon increased significantly after ST (83%; P < 0.001), but not for DT (P = 0.110). The blood volume of patella tendon increased significantly after DT (47%; P = 0.016), but not for ST (P = 0.205). These results implied that the changes in the blood volume of tendon would be related to differences in the effects of resistance training on the tendon properties.

Strength and cross-sectional area of knee extensor muscles in children

Iksokinetic strength and cross-sectional area (CSA) of knee extensor muscles were examined in 60 boys and girls, aged 6–9 years, and in 71 young adult men and women to investigate the influences of age and gender on the strength capabilities in relation to muscle size. A single anatomical CSA of quadriceps femoris at the right mid-thigh was measured by using a B-mode ultrasonic apparatus. Isokinetic strength produced at three kinds of constant velocity, 1.05, 3.14, and 5.24 rad · s−1, was significantly correlated to the product of CSA and thigh length (TL) in separate samples by age and gender. In both genders, however, young adults had significantly higher ratios of strength to CSA · TL (ST · CSA−1 · TL−1) than children at all measurement velocities. Relative differences in ST · CSA−1 · TL−1 between children and young adults became larger with increasing test velocity. Within the same generation, men had significantly higher ST · CSA−1 · TL−1 than women at all measurement velocities except for the ratio in children at 1.05 rad · s−1. These results indicate that the ability to produce strength proportional to muscle size is lower in children than in young adults, regardless of gender, and lower in women than in men within the same generation.

Effects of isometric training at different knee angles on the muscle–tendon complex in vivo

The purpose of this study was to investigate the influences of isometric training at different joint angles on the muscle size and function of the human muscle–tendon complex in vivo. Furthermore, we tried to gain a better understanding of the mechanisms involved in angle specificity after isometric training from the aspect of neuromuscular adaptation and the changes in the properties of the muscle–tendon complex. Nine males completed 12‐week unilateral training program (70% of maximal voluntary contraction (MVC) × 15 s × six sets) on the knee extensors at 50° (shorter muscle length: ST) and 100° (longer muscle length: LT). The internal muscle force (mechanical stress) is higher at 100° than at 50° because of the difference in the moment arm length, although there were no difference in the relative torque level, contraction and relaxation times, and repetition between ST and LT. Before and after training, isometric strength at eight angles and muscle volume were determined. Tendon elongation of knee extensors was measured by ultrasonography. There was no significant difference in the rate of increment of muscle volume between the protocols. Tendon stiffness increased significantly for LT, but not for ST. Although significant gain was limited to angles at or near the training angle for ST, increases in MVC at all angles were found for LT. These results suggest that only mechanical stress (internal muscle force imposed on muscle and tendon) contributes to adaptation in the tendon stiffness, although metabolic (relative torque level, etc.) and mechanical stress relate to muscle hypertrophy. Furthermore, increment of tendon stiffness for LT might contribute to increase torque output at smaller angles other than the training angle.

Relation between grip strength and radial bone mineral density in young athletes

In this study, we evaluated the relationship between bone mineral density (BMD) and muscle strength in young athletes who had not yet experienced age-related bone loss. Radial BMD and grip strength were measured in 10 male college wrestlers, 16 female college basketball players, and 12 female college tennis players. Radial BMD was measured in the distal and middle radius by dual energy x-ray absorptiometry (DEXA). Isometric grip strength was assessed with a hand-held dynamometer. The dominant forearm was examined in the amateur wrestlers and basketball players for grip strength and BMD. Both forearms were examined in the tennis players. A significant positive correlation was found between radial BMD and grip strength in the dominant forearm, and between radial BMD and body weight. Moreover, to eliminate a possible effect of body weight on radial BMD, we compared radial BMD with grip strength in both the dominant and nondominant arm of 12 college tennis players. Grip strength in the dominant forearm was significantly greater than in the nondominant forearm. The midradial BMD of the dominant forearm was also significantly higher than in the nondominant forearm. Based on these findings, we conclude that grip strength is one of the determinant factors of radial BMD in the dominant forearm of young college athletes.

Effects of 6 months of walking training on lower limb muscle and tendon in elderly

The purpose of this study was to investigate the effects of 6 months of walking training on muscle strength, muscle thickness and tendon stiffness on various parts of the lower limbs in the elderly. Subjects were assigned to training (n=35) and control (n=10) groups. Maximal isometric torque (MVC) and muscle thickness for knee extensors (KE), knee flexors (KF), dorsi flexors (DF) and plantar flexors (PF) were measured. Tendon stiffness for KE and PF was measured using ultrasonography while subjects performed isometric contraction. No significant changes occurred in any measured variables in the control group. In the training group, muscle thickness increased significantly for KF and DF, but not for PF. For KE, significant increases of muscle thickness at the proximal and medial sides were observed, although mean relative increase of the eight measured sites for KE was not significant. MVC increased significantly for KF, DF, and PF, but not for KE. In addition, tendon stiffness for KE and PF did not change after training. These results indicated that walking training brought about increments of muscle thickness and strength in most of the lower limbs in the elderly, but it did not result in any changes in tendon stiffness.

Muscle force per cross-sectional area is inversely related with pennation angle in strength trained athletes

The present study aimed to examine the effect of pennation angle on the force per cross-sectional area for elbow extensor muscles in strength-trained athletes. A total of 52 male bodybuilders (n = 32) and Olympic weightlifters (n = 20) did maximal isometric elbow extension on an isokinetic dynamometer. Muscle cross-sectional area (CSA) and muscle-fiber pennation angle (PA) of the triceps brachii muscles were measured by ultrasonography. Bodybuilders had significantly greater isometric elbow extension force (F), CSA and PA than weightlifters. The ratio of force to CSA (F/CSA) of bodybuilders was significantly lower than that of weightlifters. A significant positive correlation was observed between CSA and PA in both groups (r = 0.832, P < 0.001, and r = 0.682, P < 0.001, for bodybuilders and weightlifters, respectively). The F/CSA was negatively correlated to PA both for bodybuilders (r = -0.408, P < 0.05) and weightlifters (r = -0.465, P < 0.05). Thus present study indicates that the larger pennation angle is associated with the lower force relative to muscle CSA in strength-trained athletes.

Noninvasive measures of blood volume and oxygen saturation of human Achilles tendon by red laser lights

Aim:  Spectroscopic measurement using near‐infrared spectroscopy and red laser lights was performed to investigate the blood volume (total haemoglobin; THb) and oxygen saturation (oxygenated haemoglobin saturation; StO2) of the human Achilles tendon during single and repetitive isometric contractions.

Muscle and tendon properties during menstrual cycle.

The present study aimed to investigate the changes in the mechanical properties of human muscle and tendon during the menstrual cycle in vivo. The subjects were young healthy women (n=8, age 22.5+/-0.9 years) with a normal menstrual cycle. Cycle phases were divided into the menstrual (when estradiol and progesterone concentrations were low), ovulatory (when estradiol was elevated and progesterone was low), and luteal (when progesterone was elevated). Measurements included maximal isometric voluntary contraction (MVC), muscle activation level (using interpolated twitch method), and tendon properties (using ultrasonography) in knee extensors and plantar flexors. No significant changes in MVC and muscle activation level were found during the menstrual cycle. Similarly, there were no significant differences in the maximal elongation and stiffness of tendons among the three phases. These results suggested that the changes in female steroid hormones during the menstrual cycle did not affect the mechanical properties of human muscle and tendon.